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WeatherWise

ARIANE 5 SUPPLEMENT INSIDE!

AUTOMOTIVEReverse FlowCatalytic ConverterHeats Up

POLYMERPROCESSINGA ComputationalCure for Tires

PRODUCT NEWSFLUENT for CATIA:Rapid FlowModeling for PLM

SPORTSDr. Ice and hisSkeleton Crew

URBAN CANYONS. The phrase brings to mindimages of city streets lined with skyscrapers and eitherraging winds funneling through narrow passagewaysor excessive heat that cannot escape. Architects andcity planners pay close attention to these phenomena,and try to assess the role that building geometries andsitings play on the local climate. While the weather canbe used to augment the internal HVAC system of abuilding, excessive wind can create hazards for passersby and be responsible for spreading the exhaustfrom automobiles and smokestacks to more distantareas. In this issue, we focus on weather and its impacton buildings and their surroundings in a series of arti-cles. The series starts on page 5, where the combinedeffects of wind and solar radiation on a college build-ing are analyzed. A simulation of the wind patternsnear a Belgian landmark appears on page 7, and therole of wind in dispersing pollutants from cooling towers is reviewed on page 8. The measurement ofwind and airborne particulates by the French nationalweather service is the focus of an article on page 10,and the impact of wind on measurements of rainfallappears on page 12.

With the 2006 Winter Olympics around the corner, weare pleased to report on two cases where sportsequipment is being improved through the use of CFD(p. 16-17). The Sports Engineering Research Group(SERG) in the UK has used FLUENT to simulate theflow around a skeleton rider, and the Institute forResearch and Development of Sporting Equipment(FES) in Germany has used it to study the four-manbobsled. Watch for these teams when the events takeplace in Turin, Italy in February. A number of other

interesting applications of CFD also appear in thenewsletter, and include waste water treatment (p. 14),a novel catalytic converter (p. 20), and the curing of arubber tire (p. 26).

We are pleased to introduce two new products in thisissue: FLUENT for CATIA (p. 27) and studentFLUENT (p. 36). The benefits brought by an existing product,FloWizard, to a companys product design cycle arereported on page 28. The Support Corner takesanother look at drag laws in FLUENT (p. 32), follow-ing an introductory article on the same topic in thesummer issue of Fluent News.

In early November, the heavy capacity Ariane 5 ECAlauncher delivered two communications satellites intoorbit. The launch was celebrated by the EuropeanSpace Agency and the dozens of companies who havecontributed to this program. As a tribute to Ariane 5,our entire supplement is devoted to stories fromEuropean companies who have worked towards thesuccess of this program. The stories show how FLUENThas been used to simulate cryogenic flows (p. s4, s8,and s10), heat transfer (p. s4, s8, s12), free surfacephenomena (p. s4, s10), and instabilities (p. s10, s14).

As the seasons change, seek shelter from the weatherand enjoy this issue of Fluent News. We look forward,as always, to hearing about your work.

LIZ MARSHALL

[email protected]

Fluent News is published by

10 Cavendish Court Lebanon, NH 03766 USA1-603-643-2600 www.fluent.com

2005 Fluent Inc. All rights reserved.

Editor: Liz MarshallAssistant Editor: Susan Wheeler

Contributing Editors: Erik Ferguson and Keith Hanna Design: Lufkin Graphic Designs

FLUENT, FiDAP, GAMBIT, POLYFLOW, G/Turbo, MixSim,FlowLab, Icepak, Airpak, and FloWizard are trademarks of Fluent Inc. Icepak and Airpak are joint developments ofFluent Inc. and ICEM-CFD Engineering. All other productsor name brands are trademarks of their respective holders.

EDITORS NOTE

ON THE COVER:Pathlines and surface temperatures on a building atMichigan Technological University, computed usingRadTherm and Fluent; temperatures from a thermalimaging camera are shown across the middleCourtesy of ThermoAnalytics and Monte Consulting

ON THE SUPPLEMENT COVER:Pressure oscillations in the combustionproducts inside a solid rocket motorCourtesy of Avio SpaPhoto copyright ESA/CNES/ARIANESPACE Service Optique CSG

Fluent News Fall 2005 3

FEATURES APPLICATIONS

5 ENVIRONMENTALSolar Loads inNorthern Climates

7 FloWizard Conjuresup the Atomium

8 Cooling TowerDrift

10 Research Activitiesat Mto-France

12 Gauging Rainfall

14 WATER TREATMENTWaste Water TreatmentGets an Oxygen Boost

16 SPORTSBerlins OlympicGoldsmiths

17 Dr. Ice and hisSkeleton Crew

18 OIL & GASDistilling ExergySavings

19 AUTOMOTIVEShape Optimization ofa Defroster Duct

20 Reverse Flow CatalyticConverter Heats Up

22 Incipient Cavitation ina Steering Rotary Valve

24 POWER GENERATIONLithium Jet Hydraulics

26 POLYMER PROCESSINGA Computational Curefor Radial Tires

CONTENTS

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4 Fluent News Fall 2005

CONTENTS

27 PRODUCT NEWSFLUENT for CATIA: Rapid FlowModeling for PLM

28 FloWizard at MMA

30 PARTNERSHIPSQuality & Reliability inEngineering CFD Simulations

32 SUPPORT CORNERDrag Laws 102

33 ACADEMIC NEWSCFD for Future Engineers

34 Airfoil Noise in a Turbulent Jet

36 studentFLUENT Goes to College

DEPARTMENTS

s2 INTRODUCTIONAriane 5 Reaches for the Skies

s4 CRYOGENICSSpace EngineeringActivities atCRYOSPACE and AIR LIQUIDE

s8 Cryogenic Flows in Rocket Engines

s10 The Path toPassivation

s12 HEAT TRANSFERAriane 5 InternalCavities Beat theHeat

s14 SOLID ROCKETSPressure Oscillations in Solid RocketMotors

ARIANE 5 SUPPLEMENT

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ENVIRONMENTAL


ADVANCED SIMULATION OFTEN REQUIRES theuse of multiple analysis codes to account for alldesired phenomena. As an example, an approximaterepresentation of the Raymond L. Smith MechanicalEngineering Building on the campus of MichiganTechnological University in Houghton, Michigan wasrecently used for an environmental simulation in thepresence of transient solar radiation. The modeldemonstrated the combined use of FLUENT andRadTherm software for architectural analysis.

Traditional HVAC and energy design makes use of empirical methods based on one- and two-dimensional approaches. These methods, while verypowerful and established, often neglect wind effectsand many other relevant situational factors, such asthe surrounding terrain and neighboring structures.By combining FLUENTs accurate prediction of windflow around a building with RadTherms compre-hensive environmental analysis, transient diurnalthermal results can be generated with minimal com-putation time. Wind flow patterns and eddy currenteffects are thus captured in FLUENT, while accurate

solar shadowing, solar loading through glass, andinfrared band heat transfer are rapidly computed inRadTherm. Interior heat loads, infiltration and venti-lation effects are also accounted for in the thermalcalculation. This combined approach allows archi-tects and building designers to more accuratelyimprove the thermal performance of each zone of aplanned structure, to test the efficacy of energy-saving devices like low-e windows, or to developpassive thermal heating and cooling designs.

The R. L. Smith Building geometry was created inRhinoceros and meshed with ANSA for thermal analysis in RadTherm. The thermal model geometryconsisted of 32,000 surface quads, including interiorwalls and floors. A preliminary thermal analysis wasperformed using RadTherm's built-in wind model.The building exterior wall temperatures were thenexported to FLUENT to be used as a boundary condition profile in the next phase of the solution.

For the FLUENT calculation, a high resolution CFDmesh of 1.5 million tetrahedral cells was generated in

A cutting plane through the fluid domain shows the wind velocity magnitude; the building geometries arecolored by surface temperature with exterior walls ghosted to show internal rooms Postprocessed in EnSight

Solar Loads in Northern Climates

By Craig Makens and Amit Shah, ThermoAnalytics, Calumet, Michigan, USAand Matthew Monte, Monte Consulting Company, Houghton, Michigan, USA

ENVIRONMENTAL


GAMBIT. A steady-state analysis was performed using a bulk airtemperature of 6.8C and wind speed of 5 m/s from the north,representing a cold north wind a common occurrence forautumn in this location. The flow analysis in FLUENT capturedadvective effects as the wind moved around the structure.

After the flow analysis was completed, the standard FLUENTmenu export command was used and "RadTherm" chosen asthe file type. This command exported a Patran Neutral file containing surface mesh geometry and convection data (convection coefficients and fluid temperatures on an element-level basis). These data were then imported into RadTherm andmapped onto the lower resolution geometry as a boundarycondition for the external surfaces of the buildings. Interiorbuilding surfaces retained the 1D fluid nodes used for naturaland forced convection computation in RadTherm. A completemultimode thermal analysis was then carried out, includingtransient solar conditions with loading through the windows.The solar model considers global position, time of day, cloudconditions for predicting direct and diffuse solar loads, surfacecharacteristics and glass characteristics. Post processing wascarried out in EnSight, courtesy of CEI.

This model illustrates the comprehensive in-situ analysis thatFLUENT and RadTherm can provide to buildings and down-town areas where complex flow and radiation effects rendertraditional empirical methods inadequate. The approach usedin this model provides architects and building designers withmore accurate heating and cooling requirements, which leadsto proper and efficient HVAC sizing for each zone. Theincreased efficiency yields fast returns on the engineeringinvestment, especially during high cost energy markets.

Particle trace of wind pathways colored by velocity magnitude. Note the high speed windcanyon between the R.L. Smith Mechanical Engineering Building and the ChemicalSciences Building to the east (right). In November 1994, several windows in the northwestarea of the Chemical Sciences building were cracked by high speed winds. The windowshad no mechanism to be opened, so it was not the result of them slamming shut, butrather the intense flexing within their rigid frames. The Smith building was constructedin the early 70s, and no wind analysis was done at the time. Wind canyons between thesebuildings can be quite severe, as many students and professors can testifyPostprocessed in Ensight

[email protected]

Diurnal surface temperatures of buildings and terrain,viewed from the southeast; varying environmentalparameters include diffuse and direct solar loads,direct solar angle, cloud cover, and apparent skytemperature Postprocessed in EnSight

ENVIRONMENTAL


FloWizard Conjures up the

Atomium By Corine Chauvin, Fluent Benelux

IN URBAN ARCHITECTURE, a studyof the flow around a proposed newbuilding is an important step prior toconstruction. In particular, the intensi-ty of the wind at human height (2m)near the base of the building is a keycomponent to pedestrian comfort.

FloWizard now makes it easy to accom-plish such simulations quickly. Thebuilding geometry can be taken fromthe architects CAD software, loadedinto FloWizard, and the flow regionaround the building will be automati-cally created and meshed. By specifyingthe wind speed and direction, the flowconditions in various regions can bereadily determined.

As an example, the airflow around oneof the most famous European monu-ments, the Atomium in Brussels,Belgium, has been computed. Thisstructure, with a total height of 100meters, was built in 1958 for theBrussels Worlds Fair. It is a representa-tion of iron atoms in a unit cell (arepeating structure in a solid). At thetime of its construction, experimentalstudies were performed in a wind tun-nel to test the wind loads for a variety ofconditions. What took months of physi-cal testing back then can now be doneusing FloWizard in only a few hours!

The challenge for the CFD simulationwas to capture the airflow around the

arms of the monument, and behindthe spheres. At the inlet to the simula-tion domain, a large rectangular vol-ume containing the monument, anormal constant velocity of 10m/s (22mph) was applied. A fine meshwith a boundary layer near the wallswas used along with the realizable k-turbulence model. After a few hoursof run time, a solution was generated.It showed a low velocity regionbehind the building and local recircu-lation zones under the building andbehind the spheres. As the millions ofvisitors who have visited the monu-ment over the years would agree, thewind currents near the base were notfound to cause extreme discomfort.

Mesh on the Atomiumbuilding and contours ofpressure on the surface

The Atomium monument in Brussels

Velocity vectors on thecentral cutting plane show

regions of recirculation

ENVIRONMENTAL


Cooling Tower DriftBy Robert N. Meroney, Emeritus Professor of Civil Engineering, Colorado State University, Fort Collins, Colorado, USA

THE DRIFT OF SMALL WATER DROPLETS frommechanical and natural draft cooling tower installa-tions can contain water treatment chemicals thatcan be hazardous if they make contact with plants,building surfaces, or human activity. Prediction ofdrift accretion is generally provided by analyticmodels as found in the US EPA-approved ISCST3 orSACTI codes. However, these codes are not suitablewhen cooling towers are located in the midst oftaller structures and buildings. A CFD calculationincluding a Lagrangian prediction of the stochastic,gravity-driven trajectory descent of droplets is a better approach in this kind of environment. Onesuch calculation has been performed and comparedto data from the 1977 Chalk Point Dye TracerExperiment in preparation for using such methodsin more complex building configurations. Thenumerical analysis predicts plume rise, surface con-centrations, plume centerline concentrations, and

surface drift accretion within the bounds of fieldexperimental accuracy.

Estimation of the impact of cooling tower drift onthe downwind deposition of droplet-born toxins isdifficult. A few field studies performed between1965 and 1984 examined cooling tower plume rise, visibility, and downwind concentrations.Unfortunately, only a couple of these actually meas-ured deposition rates downwind. Despite limitedfield data, concern about drift and deposition led tothe development of more than a dozen separateanalytic models to predict downwind ground-levelconcentrations and accretion rates. Chen [1] com-pared ten drift deposition models using a set ofstandard input conditions for a natural-draft coolingtower, and found that most of the models agreedwithin a factor of three. However, when all ten mod-els were compared, the predicted maximum drift

Particle-laden exhaust flows in a typical urban setting where cooling towers emit 300 micron particles in an 8.5m/sec exhaust stream, using reference wind speeds of 5m/s at an angle of 240 from true North; pathlines are shown at left, and particle tracks are shown at right

Chalk Point Coal Fired Power Station (2640 MW), MarylandCourtesy of Power Plant Research Program, Department of Natural Resources, Maryland

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ENVIRONMENTAL


deposition differed by two orders of magnitude,and the downwind locations of the maximum dif-fered by one order of magnitude. These compar-isons occurred before improved sets of field datafrom the Chalk Point Dye Tracer Experimentsbecame available (after 1977). Policastro et al. [2]compared most of the same drift deposition modelsto the new Chalk Point experimental data, and con-cluded that None of the existing models per-formed well. A number of researchers have usedCFD previously to calculate cooling tower plumebehavior, but none of the CFD calculations found inthe literature predicted deposition levels downwindof cooling towers.

Results from the 1977 Chalk Point Dye TracerExperiment are described in papers and reports byHanna [3]. These experiments are considered tohave produced the best single source of coolingtower deposition data available. Two natural drafthyperbolic cooling towers are located on the ChalkPoint site in Maryland, on a peninsula that extendsinto the local bay and wetlands. The two towers andthe turbine building are located along an east-westline, and are separated from one another by about500ft. The hyperbolic cooling towers are 400ft(124m) tall, 374ft (114m) in diameter at the base,and 180ft (54.8m) in diameter at the exit.Instruments to measure drift deposition were placedat 5 intervals on 35 arcs at distances of 0.5 and 1.0km north of the cooling towers. The average deposition rate of the dye-tagged sodium dropletson the 0.5 and 1.0km arcs was 1080 and 360kg/km2/month, respectively. Drift droplet sizesat the measurement stations had a mass mediandiameter of 340 and 260m on the 0.5 and 1.0kmarcs, respectively. Most of the drop sizes werebetween 250 and 450m on the 0.5km arc and 200and 400m on the 1.0km arc.

Calculations for the Chalk Point Cooling Tower simulation were performed using FLUENT on adomain 2000m long, 1000m wide and 500m high,using 165,000 tetrahedral cells. The simulatedhyperbolic cooling tower height was 124m, with adiameter of 54.8m at the tower exit. The plume ver-tical exhaust speed was set to 4.5m/sec, and meanwind speed profiles were set to field values of 5m/sat a height of 100m. Rather than specify the actualtemperatures, virtual temperatures were used toaccount for the water vapor content in the plumemixed with the ambient humidity of the back-ground atmosphere. The plume virtual ambient

temperature was set to 295.3K, and the virtualexhaust temperature was set to 315.3K. Buoyancywas included in the calculation.

Once the overall flow and turbulence fields werecalculated, the Lagrangian discrete phase model(DPM) used a sample of this data to predict thedownwind distribution of a phase distributionequivalent to measured field cooling tower exit values. Ground level accretion of the particles wasnoted at the 0.5 and 1.0km distances downwind ofthe cooling tower.

The height of the centerline of the cooling towerplume was determined based on the height of themaximum in the water vapor and temperature pro-files downwind of the cooling tower. The calculatedpoints agreed very well with the predictions of theBriggs plume-rise formula calculated by Hanna [3] aswell as with the trend of the visual observations forplume height recorded during the experiment.Predictions of ground level and plume centerlinewater vapor concentration were compared to valuespredicted by the ISCST3 program, and the agree-ment was within 25%. The calculations were done interms of log K factors, where K is the dimensionlesswater vapor concentration: CUref / Qsource, where C isthe actual concentration, Uref is the approachingwind velocity at the cooling tower release height, andQsource is the water vapor content of the exhaustemissions at the cooling tower exit. Particle tracks fora typical Rosin-Rammler particle distribution releasewith a mean diameter of 0.09mm and spreadparameter, n, of 0.65 were also examined. The calculated deposition accretion magnitudes werecompared to observed and analytic values predictedby ISCST-3 and Hanna [3]. The CFD grid face valuesfor the specified inlet profile and Rosin-Rammler representation of the Chalk Point source droplet distribution agreed within factors of 0.75 and 0.5 at0.5 and 1.0km, respectively.

References1 Chen, N.C.J.: A Review of Cooling Tower Drift

Deposition Models; Oak Ridge National Laboratory,ORNL/TM-5357, 1977.

2 Policastro, A.J.; Dunn, W.E.; Breig, M.; Ziebarth, J.:Comparison of Ten Drift Deposition Models to FieldData Acquired in the Chalk Point Dry TracerExperiment; Symposium on Environmental Effects ofCooling Tower Plumes, U. of Maryland, May 2-4, 1978.

3 Hanna, S.R: A Simple Drift Deposition Model Applied tothe Chalk Point Dye Tracer Experiment; Symposium onEnvironmental Effects of Cooling Tower Plumes, U. ofMaryland, PPSP CPCTP-22, WRRC Special Report No.9, May 2-4, 1978.

Cooling tower plume rise comparison

Predicted plume centerline concentration

Particle tracks downwind of the modeled Chalk Pointcooling tower and deposition regions located at 500and 1000 m downwind

Deposition observed and predicted

ENVIRONMENTAL


Research Activities at By Philippe Nacass, Mto-France, Toulouse, France

WITHIN MTO-FRANCE (the French Weather Service), the Centre National de RecherchesMtorologiques (CNRM, or The National Center forMeteorological Research) is the department responsi-ble for conducting most of the organizations mete-orological research activities. The center is primarilyoriented towards the needs of the public in the areasof meteorology, weather forecasts, and the physicsand dynamics of the atmosphere. Its work also coversrelated fields, such as atmospheric chemistry (acidrain and ozone), surface oceanography, the physicsand dynamics of snow cover (avalanches), surfacehydrology (floods), and urban pollution.

At CNRM, the development of new-generationatmospheric models is an ongoing effort. To carryout this mission, CNRM hosts approximately 225permanent staff (one-third being research scien-tists), and 45 students and visitors. At the nationallevel, the research is conducted in close cooperationwith many universities and atmospheric laborato-ries, such as the Centre National de RechercheScientifique (CNRS, the National Center forScientific Research). At the international level,CNRM collaborates with the European Center forMedium-Range Weather Forecasts (ECMWF) in theUnited Kingdom and the National Center forAtmospheric Research (NCAR) in the USA, amongothers. The CNRM also participates in internationallarge-scale field experiments and multidisciplinaryresearch programs, such as the InternationalGeosphere-Biosphere Program and the WorldClimate Research Program.

Some of the data collection at Mto-France is doneusing atmospheric research ships and aircraft, whichdeploy a variety of sensors for state parameters,atmospheric chemistry, cloud physics, and remotesensing. Instruments used for these measurements

have fundamental sources of uncertainty that canoften be quantified. One source of error is distortionfrom the hull or fuselage of the ship or aircraft.Movement of air around these bodies can impactnot only the flow speed and direction, but the con-centrations of various constituents or particulatematter being measured. In fact, airflow distortioncan generate errors that are larger than those inher-ent in the measurement sensor. To make the mostaccurate atmospheric measurements, these errorsmust be understood and minimized by a suitableselection of sampling location.

To better understand the influence of objects onmeasurement error, wind tunnel studies have tradi-tionally been done, even though such tests are cost-ly and time-consuming to perform. They are limitedby the wind tunnel speed and the physical size ofthe model. Numerical modeling has also beenemployed to simulate the airflow disturbance overships and around aircraft. From 1988 to 1994,potential flow codes were used to understand the flow characteristics in critical areas where sen-sors are placed. These codes provided reasonably accurate estimates of flow and particle behavior atlocations outside the aircraft boundary layer.

In recent years, advances in CFD have permittedfaster, more accurate representations of airflowaround bodies. Airflow characteristics (speed anddirection, for example) and particle trajectories inmeasurement regions can be predicted using CFD,allowing better placement of research instrumenta-tion and measurements of greater accuracy. Since1995, Mto-France has used FLUENT for this purpose, tasking some individuals with full-timeeffort. CFD is currently being used to optimizemeasurement equipment and its positioning on anumber of aircraft and ships.

ENVIRONMENTAL


The Aircraft Fleet at Mto -FranceIn cooperation with other French governmental organizations, Mto-France hasoperated several research aircraft over the years, such as a Piper-Aztec, a Merlin-IV, and a Fokker-27 [1]. For these aircraft, CFD has been used to correct in-flightmeasurements made by sensors and instruments designed and mounted on thefuselages prior to the introduction of CFD. In 2005, Mto-France, throughSAFIRE [2], began to operate two new instrumented aircraft, an ATR-42 (a bi-turboprop) and a Falcon-20 (a bi-turbojet). For these aircraft, CFD was used tostudy the best position of the instruments, and to make sure that they, in turn,have no harmful influence on the aircraft for all possible flight attitudes.

The Design of New Airborne InstrumentsMto-France has also used CFD to develop new aircraft instruments, includingspecial sensors, and to optimize their shape. This effort has proved to be veryimportant for the design of sensor inlets that measure airspeed or aerosol concentrations. For manufactured instruments, CFD is used to illustrate the airflow disturbance in the volume of measurement. The aerodynamic forces andmoments on the outside body of these instruments are also calculated by FLUENT for certification by the French Aviation Administration.

Airflow Studies Around AircraftCFD simulations for specific aircraft have proved to be valuable to the researchcommunity, both for optimal sensor placement and for interpretation of meas-ured data. In most uncertainty analyses, it is assumed that the air reaching thesampling inlet or sensor is representative of the free stream atmospheric flow.However, the movement of air around an aircraft fuselage can impact not onlythe flow speed and direction, but the concentrations of various atmospheric con-stituents as well. The additional uncertainty caused by the fuselage is a functionof the location and type of measurement being made. For studies of aerosol particles in clouds and in the atmosphere, for example, a wide range of particlesizes (0.001 to 1000m in diameter) must be measured accurately. Their distribution at various locations around the aircraft varies significantly from freestream conditions, and CFD is useful for quantifying this discrepancy.

Mto-France

The research aircraft ATR-42, a bi-turboprop operatedas part of the SAFIRE project

The airflow disturbance in the volume ofmeasurement for a manufactured sensor

Pressure is used tocompute theaerodynamic forcesand moments onthe pylons mountedunder each wing ofthe ATR-42; thepylons are used tocarry additionalinstruments

For studies of aerosol particles in clouds, the trajectories of particles ranging from 0.1m (left) to 100m (right) in diameter must be measured accurately

ENVIRONMENTAL


Airflow Studies over Research ShipsMeasurements made from ship borne instruments are biased due to the effect ofthe ship on the flow of air to the instruments as well as turbulence from theair/water interface. The presence of the ship causes the air flow to a particularinstrument site to be either accelerated or decelerated, displaced vertically or, toa lesser degree, in the horizontal direction. Although recognized for some time,it is only recently that the problem has been addressed using 3D CFD models.These simulate the flow over particular ships, quantify the effects of flow distor-tion, and hence correct the ship-based measurements.

Since 1998, Mto-France has used FLUENT to model the flow around one of the research ships operated by a French Institute [3] for various relative winddirections and wind speeds. It has used the resulting estimates of error in themeasured wind speeds to correct measurements of fluxes between the air andthe sea. Comparison of the data recorded by the ship with the CFD results hassuggested that the flow distortion on the measured wind speed is dependent onthe incident wind speed. Pathline traces of the predicted flow field, beginning farupwind of the ship and passing through the instrument inlet, allows the verticaldisplacement of the flow reaching the site to be estimated. These and other CFD results have been useful to researchers who have obtained meteorologicalmeasurements from aircraft or ships in the past or to those making comparisonsbetween ground, aircraft, ships, buoys and satellite data systems.

References:1 Fokker-27 was co-funded by Mto-France, the National Center for Scientific Research

(CNRS), the French Space Center (CNES) and the National Geographical Institute (IGN).

2 Service des Avions Franais Instruments pour la Recherche en Environnement (SAFIRE,Facility for the French Aircraft Instrumented for Environmental Research), created in2005 with Mto-France, CNRS and CNES.

3 Institut Franais de Recherche pour lExploitation de la Mer (IFREMER, French ResearchInstitute for Exploitation of the Sea).

The presence of a research ship can distortthe flow around it; contours of pressurecoefficient are shown

An illustration of how CFD has beenused to correct ship-based measurements

Gauging By Andrew J. Newman and Paul A. Kucera, Department of Atmospheric Sciences,

ACCURATE PRECIPITATION MEASUREMENTSare required for many different applications such asriver/flash flood forecasting, water resource man-agement, and agriculture. Precipitation measure-ments are retrieved through the use of a variety ofinstruments located on the surface (e.g. raingauges) or from remote observations such as satel-lites or ground-based radars. Despite advances intechnology, rain gauges are still considered thestandard for surface rainfall measurements. Often,these data are used for verification of remotelysensed rainfall estimates. Therefore, it is importantto understand and quantify the errors associatedwith the in situ precipitation estimates.

When using a rain gauge, a variety of errors canoccur due to calibration and sighting problems andinstrument failure. While these errors are mostlypreventable, environmental conditions can lead towind induced catchment errors that are not gener-ally preventable. Catchment errors occur because arain gauge modifies the environment in which dataare collected. Field measurements show that theseerrors can range from 1-2% to over 10%, depend-ing on the rainfall intensity and free stream windvelocity [1]. A few prior studies have used CFD tosimulate precipitation gauges with results that havefairly good agreement with observations [2, 3].Using FLUENT, similar instruments are now beingmodeled for comparison with previous studies, andfor the development of a tool for evaluating the flowcharacteristics around new precipitation monitoringinstrumentation.

As an example, the flow around a Qualimetrics tipping-bucket rain gauge mounted on a 1m polehas been studied. The computational domain is 1.8m x 1.2m x 2m and comprised of roughly200,000 tetrahedral cells, with local refinementaround the rain gauge to better resolve the detailed

ENVIRONMENTAL


RainfallUniversity of North Dakota, Grand Forks, North Dakota, USA

flow there. A steady wind velocity with a height profile was specified along with the standard k-turbulence model. The results were compared withone of the earlier studies [2], even though a slightlylarger wind speed (4m/s versus 3m/s) was used. Theresults illustrate the acceleration of the flow over thetop of the gauge along with the formation of a vortex in the gauge catchment area. A flow profilesuch as this can cause raindrops, especially smallones, to miss the rain gauge, giving rise to anunderestimate of the actual rainfall.

The flow around an instrument that is used toimage hydrometeors (raindrops or snowflakes, forexample) has also been studied. The instrument,called a disdrometer or Rain Imaging System (RIS)and developed at NASA/Wallops Flight Facility byDr. Larry Bliven, photographs hydrometeors that fallthrough a sample volume located between thecamera and light source. The RIS is unique becauseit can provide measurements with less wind interfer-ence than other instruments of similar capability [3].In a FLUENT simulation of this device, roughly

450,000 tetrahedral cells were used, again with arefined mesh in the vicinity of the instrumentation.The worst-case scenario has the wind coming frombehind the camera housing and flowing toward thesample volume. The CFD results demonstrate thatthis condition creates a shadow effect in the samplevolume that may cause under catchment of thehydrometeors. Wind flow conditions that are notalong the camera line-of-sight do not affect thehydrometeor sampling as much.

References:1 Sevruk, B.: Wind Induced Measurement Error for High-

Intensity Rains. Proc. International Workshop onPrecipitation Measurement, WMO Tech. Document328, 199-204, 1989. [Available online athttp://www.wmo.ch.]

2 Nespor, V.; Sevruk, B.: Estimation of Wind-InducedError of Rainfall Gauge Measurements Using aNumerical Simulation. J. Atmos. Oceanic Tech., 16,450-464, 1999.

3 Nespor, V.; Krajewski, W.F.; Kruger, A.: Wind-InducedError of Raindrop Size Distribution Measurement Usinga Two-Dimensional Video Disdrometer. J. Atmos.Oceanic Tech., 17, 1483-1492, 2000.

Velocity magnitude for the rain gauge simulation

Streamlines colored by velocity magnitude with staticpressure contours on the rain gauge

Contours of velocity for the Rain Imaging System with the worst case wind scenario, in which the wind blowsfrom behind the camera (left) towards the sample volume (right)

Pressure contours on the camera and mount andpathlines colored by velocity

WATER TREATMENT


WasteWater TreatmentBy Alban Poirier, Vincent Perrin, and Jrme Cluzeau, AIR LIQUIDE, Gaz Industriels Services, DAP, Les Loges en Josas, France

OXYGEN PLAYS AN IMPORTANT ROLE inwaste water treatment. Most of the bacteria that areresponsible for the decay of organic material areaerobic, so the dissolved oxygen in the waste watermust be replenished by an outside source. The aeration process allows bacteria and sludge to beput into contact. Efficient oxygenation is, in fact,essential to the success of aerobic biological treat-ment. In the most difficult cases, particularly withindustrial effluents, pure oxygen boosting (ratherthan air boosting) is a very efficient solution; it canbe applied to most basins, even those that were notoriginally designed for oxygen. CFD can be used to validate the technical choices for oxygenationwithout industrial risks and to predict the perform-ance of existing and future basins.

In a recent project, FLUENT was used to character-ize the hydrodynamic behavior of an industrial rec-tangular waste water tank in the presence of fourfloating turbine aerators and two types of oxygentransfer device: a TURBOXAL (floating at the basinfree surface) and a VENTOXAL (immersed in thewaste water basin). The goal of the project was tosimulate the initial performance of the basin, andthen to improve its performance before the installa-tion of the equipment by optimizing two parame-ters: the location of the new equipment and theflow rate distribution. Using a Lagrangian (DPM)calculation, a discrete phase of oxygen bubbles wascoupled to the continuous phase of water, takinginto account the hydrodynamic effect of the oxygenplume. Of particular interest were the oxygenationhomogeneity, the mixing efficiency, and the interac-tion of the plumes with the different equipment.The presence of low velocity zones, which representa significant risk for sludge deposits (and the development of filamentous bacteria) and short-circuiting (hydrodynamic flows with low residencetime) were sought as well.

Pathlines illustrate the flow field for high (top), nominal (middle), and low (bottom)oxygen flow rates for VENTOXAL

Waste water treatment basin

RecirculationEffluent

VENTOXAL

TURBOXALTurbine aerator

Water outlets

VENTOXAL

WATER TREATMENT


Validation of these two AIR LIQUIDE oxygen boost-ing devices was difficult because of the high recip-rocal impact of the gaseous phase on the flow. Inparticular, one of the fitting parameters is the lengthof the bubble streams, measured experimentallyand compared with the modeling results using a typical bubble size (measured and correlated). The VENTOXAL device was first simulated on aninstrumented biologic water treatment plant, andprovided computed velocity and concentrationfields for comparison with data. The validationallowed simplifying modeling assumptions to beidentified. The TURBOXAL was also the object of apreliminary 3D CFD simulation, used for the devel-opment of this new device, that took into accountthe impeller rotation. These results also allowed asimplified 3D model to be developed for the wastewater basin simulation that used values of axial,radial and tangential velocities. A simplified modelof the turbine aerator was developed as well, basedon technical data supplied by the manufacturer.

The CFD simulation allowed the influence of various parameters to be studied. For example,varying the location of the oxygen boosting equipment or modifying the flow rate distributionswere considered in order to improve the mixingperformance of the basin. The oxygen residencetime could also be optimized by displaying theupward velocity of the plumes.

Once the basin began operating, the experimentaldata showed very good agreement with the CFDmodeling results. The plumes were located exactlywhere the CFD model predicted they would be, anda very good quality of floc was observed at the exitstation as a result of a good velocity distribution.Overall, the CFD modeling effort saved time in determining the best implementation of the equipment, and it will contribute to new oxygenboosting projects in the future.

Gets an Oxygen Boost

Cross-section of the velocity field 0.57 m above thebottom of the basin, showing the low velocity zonealong the right edge

Iso-surface of 0.1 m/s velocity, showing the low flow zones in the basin

Validation of a simplified VENTOXAL CFD modelof an industrial basin

SPORTS


THE INSTITUTE FOR RESEARCH AND DEVELOPMENT of Sporting Equipment(FES) in Berlin has been described as a gold mine, as the equipment that hasbeen developed there has often proved to make a difference during competi-tions. Since 1962, engineers, mathematicians, physicists, and craftsmen at theFES have been involved in the development of customized sports equipment foruse by athletes in training and competition. Along with sports such as rowing,canoeing, sailing, and cycling, the recent emphasis has been to focus on theupcoming 2006 Winter Olympics in Turin, Italy. Of particular interest are thesleds used for the skeleton and bobsledding events, where it is as important to minimize friction as it is to keep the air resistance of the athletes and theirequipment at a minimum.

To achieve enhanced results in this area, the FES engineers have been performingflow simulations using FLUENT. In their view, CFD has become indispensable forequipment development in competitive sports. Over the years, hardware gainshave enabled numerical simulation of three-dimensional flow patterns withample precision and effective comparison between variants. At FES, the goal isnot to achieve a comparable result in relation to the wind tunnel, but to createa foundation that makes systematic comparison between variants more reliable.

In the bobsled research, a Linux cluster fitted with AMD Opteron 64-bit CPUs was used for the CFD simulations. Using the cluster, the calculation of acomplete model with six million cells could be completed in about an hour. Adual-processor Intel Xeon 3GHz computer with 4 GB of RAM also accomplishedcomparable tasks in an acceptable time.

The realizable k- model with standard wall functions was relied upon for all ofthe computations. Initial trials with the shear-stress transport (SST) k- modelwere also successful. The realizable k- model had the advantage of being well-adapted for simulating regions of stalled flow. In addition, the flow underneaththe sled is known to generate increased resistance due to the shear between thefloor (base layer) and the upper layers of the fluid that are moving faster. Toinvestigate the effect more precisely, a moving base layer was used in the simu-lations in conjunction with the oncoming air flow boundary condition. Suchdetailed scenarios could not be properly reproduced in a wind tunnel.

The simulations of different sled variants at FES led to the development of designmodifications. One change gained prominence because it caused a significant reduction in resistance. The modification involved the shape of the hood, and itresulted in better formation of the wake area and a subsequent decrease in thestrongly turbulent area behind the sled.

As is done for Formula One (F1) race cars, the bobsled prototypes were tested inwind tunnels at TU Dresden and the BMW plant in Munich. The results in thosecases were found to be in good agreement with the CFD simulations. After having complied with all of the technical prerequisites for a good performancein the Olympics, Germanys bobsleds should again be in peak position to returnfrom Turin with even more medals.

The 4-man bobsled in the wind tunnel

Dynamic pressure contours on the sled, ice, and athletes

The German bobsled rounds the bend

BerlinsOlympicGoldsmiths

By Ralf Gollmick, FES, Berlin, Germany and Mathias Jirka, Fluent Germany

SPORTS


THE UNITED KINGDOM IS NOT RENOWNED forits climate being conducive to winter sports.However, with the Winter Olympics in Turin, Italycoming up in February 2006, a new breed of athletehas come along. Kristan Bromley, the top-rankedskeleton bobsled competitor in the UK, and currentEuropean Champion, is a leading British medalprospect. He was also the first British male to win awinter sports world cup series when he dominatedthe 2003/2004 season. Bromley is perhaps uniquein the sport because he has a Ph.D. in ice sportstechnology, and saw the value of CFD in his previ-ous job with the defense company BAE. His passionfor skeleton grew while working on a BAE-sponsored skeleton sled project, and it has led himto dedicate himself to becoming the next Olympicchampion. Indeed his gold medal quest is tightlycoupled to his passion for engineering. He and hisbrother Richard, who is also his coach, have set upa pioneering technology company to support his icesports dream and coordinate his UK Sport-fundedperformance program for the Olympics. Membersof the press in the UK have been fascinated by this unique athlete, and have taken to calling him Dr. Ice because of his sporting, technological, andacademic achievements.

In 2003, Bromley approached the Elite Sports CFDUnit a part of the Sports Engineering ResearchGroup (SERG) in Sheffield, UK and asked them toprovide CFD flow simulation support to increase hischances of success. SERG has had considerable suc-cess in the past with the British Olympic Cyclingteam in Athens during 2004. Bromley maintains aphilosophy of using advanced technology toenhance on-ice performance, We look to ourstrengths in R&D to help bridge the gap we havewith the stronger winter sports nations, so that I cancompete on equal terms. His belief that elite-level

athletes also require elite-level support led him tocollaborate with SERG and make use of Fluents CFDsoftware, since both have a proven record of successin elite sport.

Dr. John Hart and his colleagues in the Elite SportsCFD Unit began by laser scanning a full-size flexiblemannequin on a skeleton sled, They then used spe-cialist surfacing software to produce a high-qualityCAD surface on top of the scanned data. This pre-pared the model for import into GAMBIT to createthe required volume mesh. A computational meshsuitable for CFD analysis was then constructed usinga combination of GAMBIT and TGrid. A typicalskeleton sled and mannequin mesh consisted ofapproximately seven million tetrahedral and prismaticelements. The prisms were required over the entiresurface of the modeled geometry to accurately capture the surface boundary layers and flow sepa-rations. FLUENTs CFD solver captured turbulentflow effects by using the realizable k- turbulencemodel in conjunction with non-equilibrium wallfunctions to accurately resolve the boundary layerflows.

The initial area of CFD interest for Bromley has beenskin friction characteristics associated with the cus-tomized skin-suit in which he competes. He wantedto assess small changes in surface texture in terms oftheir impact on minimizing his overall aerodynamicdrag. The ultimate proof of the CFD work will comein Turin when all of Bromleys hard work, his mentaland physical conditioning, and the technologiesbehind his sled will be put to the test over the fewshort minutes of the Olympic competition.

[email protected]

Dr. Ice and his Skeleton Crew

By David Curtis, Sports Engineering Research Group (SERG), Sheffield University, Sheffield, UK

Pressure contours on a simulated skeleton slider, with pathlines colored by velocity magnitudePostprocessed by Ensight

OIL & GAS


Distilling Exergy SavingsBy Ricardo Pulido, Leodegario Monroy, and Ricardo Rivero, Instituto Mexicano Del Petroleo-Exergy Group, Atepehuacan, Mexicoand Yi Dai, Fluent Inc.

IT IS NO SECRET that oil makes the worlds economies go around. Becauseoil processing can be expensive, energy intensive, and detrimental to our envi-ronment, efforts to make the oil refining process cleaner and more efficient areongoing at a number of research laboratories. At the Instituto Mexicano delPetroleo-Exergy Group, engineers have been focusing on distillation. Atrefineries, distillation is used to separate crude oil a mixture of hydrocarboncompounds into a number of constituents. Of particular interest to thisgroup is diabatic distillation technology, whose benefit is measured in terms ofexergy savings. Exergy, simply stated, is usable energy that can do work, andis a quantity that tends to decrease over time. It is emerging as an increasinglyuseful measure of efficiency as process improvements are considered through-out the refining industry [1].

In a classical adiabatic distillation tower, heat is supplied externally by meansof a heater. As the exhaust vapors rise, the temperature drops and condensa-tion occurs on plates positioned in stages at different heights. The condensedliquids are extracted separately, since condensation occurs at different temperatures for the exhaust constituents. Any remaining exhaust vapors exitafter the uppermost condenser, carrying with them excess heat that is extract-ed externally by means of another condenser. In a diabatic distillation tower,heat is supplied or extracted not only by an external source or sink, but fromprocess fluids inside the distillation process itself, which is why diabatic distil-lation is considered exergy efficient. There are a number of studies that sup-port this technology for its future potential in the refinery industry worldwide[2, 3, 4].

A numerical and experimental study is currently underway to gain a betterunderstanding of a diabatic distillation tower. In the first phase of the study,the isothermal flow of liquid and gaseous naptha is considered in a typicalstage. The process simulation package ASPEN is used to generate gas and liquid mole fractions and flow rates at various zones within the stage. Theseare used as boundary conditions in a FLUENT calculation that makes use of thevolume of fluid (VOF) model to track the motion of the two fluids on a meshof 425,000 cells.

The early results have provided a clear view of where the liquid tends to collect on the plates within the stage, and they are serving as a basis for thefirst round of physical testing. Using the numerical results, proposals for modifications to the tower will be made and tested in a quicker, less expensiveway than would be required if only experiments had been run. In the future,heat and mass transfer (condensation) will be included, and different diabaticdesigns will be compared numerically prior to fabrication, to save time and money.

References1 United Nations Environment Programme. DTIE.

hwww.uneptie.org/energy/act/wssd/index.htm

2 Kenney, W.F.: Energy Conservation in the Process Industries, ISBN 0-12-404220-1,Academic Press, Inc. New Jersey 1984.

3 Linnhoff, B., Polley, G.T. and Sahdev, V.: General Process Improvements ThroughPinch Technology, Chemical Engineering Progress, June 1988.

4 Rivero, R.: LAnalyse dExergie: Application la Distillation Diabatique et auxPompes Chaleur Absorption, Thse de Doctorat, Institut National Polytechniquede Lorraine, Nancy 1993.

The diabatic distillation pilot plant

The geometry of one stage of thediabatic distillation tower

Pathlines, colored by liquid napthavolume fraction, illustrate the flow of

the liquid and gaseous components: gasenters through the inlet at lower left,passes around the plate at right, and

enters the upper region; liquid napthaenters through a slot inlet on the top

left and drips down the left-hand plateonto the horizontal condenser plate

Contours of liquid naptha volumefraction show the liquid running down

both vertical plates and collecting onthe horizontal plate

Focus on CFDFor the Ariane 5 Launcher

N E W S L E T T E R S U P P L E M E N T

s2 INTRODUCTIONAriane 5 Reaches for the Skies

s4 CRYOGENICSSpace Engineering Activitiesat CRYOSPACE and AIR LIQUIDE

s8 CRYOGENICSCryogenic Flows in Rocket Engines

s10 CRYOGENICSThe Path to Passivation

s12 HEAT TRANSFERAriane 5 Internal CavitiesBeat the Heat

s14 SOLID ROCKETSPressure Oscillations in Solid Rocket Motors

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S2 Fluent News Fall 2005

Ariane 5Reaches for the Skies

By Vincent Canu and Gilles Lebiez, Fluent France and Keith Hanna, Fluent News

EUROPES ACCESS TO SPACE todaydepends on the Ariane family of heavy-duty(610 ton) launchers and the future Soyouzand Vega launchers for medium and smallpayloads (13 tons). Since their inception,Ariane rockets have been the most successfulcommercial satellite launcher systems ever,with Ariane 4 alone being responsible for thelaunch of 182 satellites of all kinds in 14 yearsbefore its retirement in 2003 over 450 tonsin all.

The space industry in Europe today employsthousands of men and women in hundredsof companies across the continent. Space-related systems contribute a tremendousamount to our scientific knowledge andtechnological development in the modernworld. They are responsible for the hundredsof satellites launched since the 1970s thathave paved the way for global positioningsystems in our cars, improved weather fore-casting and global climate monitoring,detailed mapping of our land masses andocean depths, modern high speed telecom-munications, and increasingly sophisticateddefense and security aids.

EUROPES ACCESS TO SPACE todaydepends on the Ariane family of heavy-duty(610 ton) launchers and the future Soyouzand Vega launchers for medium and smallpayloads (13 tons). Since their inception,Ariane rockets have been the most successfulcommercial satellite launcher systems ever,with Ariane 4 alone being responsible for thelaunch of 182 satellites of all kinds in 14 yearsbefore its retirement in 2003 over 450 tonsin all.

The space industry in Europe today employsthousands of men and women in hundredsof companies across the continent. Space-related systems contribute a tremendousamount to our scientific knowledge andtechnological development in the modernworld. They are responsible for the hundredsof satellites launched since the 1970s thathave paved the way for global positioningsystems in our cars, improved weather fore-casting and global climate monitoring,detailed mapping of our land masses andocean depths, modern high speed telecom-munications, and increasingly sophisticateddefense and security aids.

Ariane 5 (last heavy version) qualificationflight in February 2005. The launch wasperformed during the day (it usually happensduring the night) in order to allow a moredetailed post flight analysisPhoto copyright ESA/CNES/ARIANESPACE Service Optique CSG

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INTRODUCTION

Fluent News Fall 2005 S3

The Ariane 5 rocket was first put on the drawing board in 1987 and subsequently had its first suc-cessful launch in 1997 from the Guiana Space Center near Kourou, French Guiana. Its success isthe result of a unique collaboration between many European companies who all make differentparts of the launcher system. Over the years, engineers have faced immense technological chal-lenges, including cryogenic propulsion systems, aerothermodynamics, fluid-structure interaction,thermal protection, and aeroacoustics to name a few. A variety of in-house and commercial CFDcodes, including FLUENT, have been used to successfully model these diverse phenomena. In fact,CFD has been used to model nearly every part of the launcher and launch process [1], such as:

By comparison with the Ariane 4 rocket, which relied little on CFD, Ariane 5 has made widespreaduse of this technology for understanding, refining, and ultimately accelerating the space transportation design process, within the constraints of safety and quality assurance demanded of such a unique system. Of the many companies involved with Ariane 5, CNES, the French governmental space agency, led its technological development in association with commercial andacademic partners such as EADS Space Transportation, Air Liquide, Snecma, Sener, Fiat Avio Spa,and Cryospace, among others. CNES was formed in 1962 to assist the French government in shap-ing Frances space policy. It leads the programs funded by the French government and representsFrance at the European Space Agency (ESA) and in international space activities and partnerships.It has about 2,500 personnel at four sites across the world, and in addition, it promotes andencourages space applications and industrial space related R&D. For 40 years it has been involvedin driving the design and development of European Launchers with respect to safety, production,quality assurance and launch ground facilities.

Real tests to validate or compare CFD results for Ariane 5 are expensive, hardto do (especially for cryogenic liquids), and limited in generating useful data.Actual Ariane flight data is best for CFD validation, but it, too, is very expensive to generate. Thats why we need reliable CFD codes.

Currently, CNES uses some 90 different pieces of simulation software, most of which need to be ISO 9001 certified. For future space systems, CNES isinterested in using validated software (including CFD codes) to evaluatereusable launcher components; electrical, nuclear and solar propulsion sys-tems in space; as well as miniaturization and nanotechnology, using lightermaterials with different compositions. Ultimately, performing virtual launcheswith simulation software will be a challenge.

- Isabelle Rongier, CNES

The civil space industry has become highly competitive in the post cold war world and the Arianespace program wants to keep its dominance in the satellite launching arena by constantly updatingits technology. However, as with NASAs recent space shuttle troubles, the modern Ariane 5-ECA rock-et also had a failed mission. On December 11, 2002, the cooling tubes in the central Vulcain 2 liq-uid fuel rocket failed, ultimately leading to the rockets mission being aborted. This caused a majorreevaluation of the Ariane 5 launcher and CFD was used extensively for the next two years to identify the source of the failure and fix the cooling problem. The changes, many of which were simulated and later tested, led to a successful launch of a new rocket with enhancements to its thirdstage in February, 2005. In honor of this achievement, all of the stories in this supplement focus onAriane 5 applications that have made use of FLUENT software. Cryogenics applications are featuredin stories by Air Liquide and Cryospace (page s4), Snecma (s8) and CNES (s10), and heat transfer inpressure-controlled cavities is described in an article by EADS (s12). The complex flow characteristicsof gaseous combustion products in solid rocket motors are shown on the supplement cover, anddescribed in the article by Fiat Avio Spa on page s14.

Reference1 Launch-Vehicle Modeling, European Space Agency (ESA) Bulletin 120, November, 2004.

EADS carries out most of the construc-tion of the Ariane 5 Launcher, and usesFLUENT for aerothermal analysis andaerodynamics. Each Ariane launcher is unique because each payload is different. CFD is used to give technicalconfidence and reliability to the designteam. In their planning it helps to cutcosts and it also yields technical insightsthat would not otherwise be available.However, senior engineering experiencealso plays a major role in the designprocess in terms of checking the CFDpredictions rigorously.

- Loic Cheriaux and Jean-Marc Carrat, EADS

Time (sec) Process

Prior to launch Launcher roll-out and local climate modeling

0 to 7 Launcher main engine ignition

7 Solid booster ignition and take-off

30 to 50 Transonic flight

70 Flight at maximum dynamic pressure

540 to 550 Stage separation; ignition of upper stage engine and upper stage engine plume impact on the main stage

Classical launch in geostationary transfer orbit (GTO)(with reentry of the main stage into the sea), followed bysatellite maneuvers to reach geostationary orbit (GEO);solar arrays are then unfolded to get the operativeconfiguration of the spacecraft Courtesy of CNES

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Space Engineering at CRYOSPACE

By Jerome Lacapere, Air Liquide, Sassenage, France, and Mathieu Gardette, Cryospace, Les Mureaux, France

ENGINEERING ACTIVITIES relative to the cryogenic propellant tanks ofthe European space launch vehicles were first developed at Air Liquide DTA(Advanced Technology Division) for the Ariane 4 upper stage tank (alsoreferred to as H10). Subsequent development occurred at CRYOSPACE, ajoint venture between Air Liquide (55%) and EADS ST (45%), for the Ariane5 main stage (EPC) and upper stage (ESC) liquid hydrogen (LH2) tanks.Development of the Ariane 5 upper stage liquid oxygen (LOX) tank (ESCLOX) and the helium sub-system (SSHEL) was later performed at Air Liquide.

Engineers at Air Liquide developed in-house software in the 60s and 70s tocompute and predict the thermo-hydraulic behavior of the propellants (liquid hydrogen as fuel and liquid oxygen as oxidizer) in their respectivetanks. The software models have been periodically improved, and theyremain dedicated engineering tools. During the past five years, CRYOSPACEand Air Liquide DTA have also been using and benchmarking FLUENT fortheir specific cryogenic applications.

The main function of a stage tank is to thermally condition the propellants inorder to feed them to the engine in well-controlled temperature and pressureranges. Over the years, a specific expertise has been developed and main-tained for modeling the thermodynamics inside these tanks. In particular, thesimulations must:

compute the heat fluxes entering the tank volume when it is subjected to a severe external environment (ground conditions onthe launch pad or flight conditions during the ascent phase, forexample)

accurately predict the heating rate of the liquid propellant andespecially the stratified temperature profile that develops at thetop of the tank due to natural convection along the tank walls

compute the gaseous mass flow rate needed to maintain theullage pressure (or that in the space above the liquid) in a specified range throughout the flight (before and while the liquidis draining)

All of these computations lead to a calculation of the thermal residuals, which correspond to the mass of cryogenic liquid that is not compliant withthe engine specifications in terms of temperature and pressure. Other calculations are performed to compute the diphasic residuals. These are thesmall amount of cryogenic liquid remaining in the nearly empty tank at theend of the engine feeding process. When this point is reached, external perturbations and the very high flow rate of ingestion could cause bubbleingestion, which must be avoided. Both types of residuals can be predictedusing complex 3D simulations.

He sphere

ESC(upper stage)

EPC(main stage)

Diagram of the Ariane 5 rocket, showing the cryogenic storage tanksCourtesy EADS SPACE Transportation / SERGE WITTEMANN

Boosterrocket

Payload

Liquidoxygen(LOX) tanks

Liquidhydrogen(LH2) tanks

Vulcain/Vulcain2 engine

Activities and AIR LIQUIDE AR

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LH2 temperature (K)

3.0

2.5

2.0

1.5

1.020.4 20.8 21.2 22.0 22.822.4]

LH2

tank

pre

ssur

e (b

ar)

Phase Description:A: initial saturated stateA to B: ground pressurizationB to C: waiting phase w/o drainingC to D: draining phase (engine on)D: engine cut-off)

23.2 2423.621.6

max temperature = 23.5K

minimum NPSP

A

B C D

LH2 saturation curveThermodynamic evolution at tank outlet

LH2 saturation curve over a range of temperatures, and the pressure evolutionprior to and during launch and ascent

Thermal stratification predicted by a 2Daxisymmetric model of the ESC LH2 tank duringdraining; warmed propellant starts to stratify at thetop of the liquid volume, and a hot spot appears atthe tank bottom in a zero velocity area

Thermal Residuals Thermal stratification Before launch, but following the filling of the tanks, the cryogenic propellantsare thermally stabilized into a saturated state. Since the tank pressure valuesare typically close to 1.1bar, the stabilized temperature values are 20K (-253C) for LH2 and 90K (-183C) for LOX. A few minutes before launch,the tanks are pressurized with helium and brought to the flight pressure values. The pressurization process is aimed at stabilizing the tank structures,which will have to withstand significant mechanical loads during the ascentphase, and at providing a sufficient net positive suction pressure (NPSP, ordifference between the static pressure in the tank and the saturation pressurein the pump) to prevent any cavitation in the turbo pumps during the drain-ing (engine boost) phase.

If the tank pressure is regulated at a constant value during the flight, theNPSP depends mainly on the pressure loss in the engine feed lines betweenthe tank and the turbo-pump, and the temperature of the propellant that isfed to the engine. The minimum acceptable NPSP value corresponds to amaximum acceptable draining temperature. The mass of propellant with atemperature exceeding this upper limit is unburnable. Since a launch vehiclealways needs to optimize the ratio of its used propellant mass to its loadedmass, the residual mass should be minimized. It is therefore important tolimit the heating rate of the propellant by an adequate insulation design and,if thermal stratification occurs, to be able to predict the draining temperatureand its evolution over the full propulsive phase.

One place where thermal stratification occurs is in the upper stage liquidhydrogen (ESC LH2) tank. FLUENT was chosen to simulate this problembecause of its volume of fluid (VOF) model with heat transfer, and its unstruc-tured mesh capability. The goals of the simulations included predictions of thermal stratification and feed temperatures at the tank outlet with anaccuracy better than 0.1K. When deemed acceptable, the computationswere done with 2D axisymmetric geometries. However, the particular shapeand location of the feed line (tank outlet) sometimes required the use of a180 3D problem domain, which made use of a symmetry plane. One of themain concerns was the optimization of the mesh. In addition to capturingthe turbulent convective boundary layers and moving free surface, therewere local areas where the fluid velocity was known to be relatively high (inthe boundary layer and close to the tank outlet), and other areas that couldbe considered dead zones with near zero velocity. Additional complexity wasdue to the fact that almost all of the parameters were time-dependant and many, such as heat fluxes on the walls, longitudinal accelerations, anddraining flow rates, were highly variable.

Liquid sloshing in the tanks The behavior of the cryogenic fluids inside the tanks is complexfor a number of reasons. First, external perturbations during thelaunch cause sloshing and rolling of the free surface of the liquid.As learned from many years of development and flight measure-ment processing, the sloshing amplitudes experienced by the pro-pellant can become high enough to modify the heat and masstransfer at the liquid/gas interface and to perturb the thermo-dynamic equilibrium in both phases. Typically these conditionscause the tank pressure to drop and the liquid temperature to rise.Second, in the gaseous dome above the liquids in the tanks, thereis some amount of non-condensable helium present (which isused as a pressurizer). While it is not enough to violate theassumption of pure vapor in the LH2 tanks, it is significant in theLOX tanks, so must be taken into account in numerical modeling.Third, in the near future, the upper stages of Ariane will berequired to operate in ballistic mode, and to have engine reignition following this phase of operation. During ballistic flight,the vehicle is in a micro-gravity environment. The walls of thetanks are wetted because of the predominance of capillary forces(with Bond number about 1) and because of the specific wettingproperty of cryogenic fluids (with contact angles less than 5).Because of the increased surface area, the cryogenic liquid under-goes increased heating by external heat fluxes and by heat andmass transfer with the gas phase.

To address these needs, the VOF calculations need to take intoaccount heat and mass transfer at the liquid/gas interface. A special model has been developed at Air Liquide for this purpose,and is now undergoing a complete validation. As part of the validation, characteristic tests have been performed with cryo-genic liquids, including sloshing tests in a small cryostat (diameter~ 20cm) with liquid nitrogen and liquid oxygen (performed in theframework of the French-German program COMPERE). These

tests have shown the characteristic evolution of gaseous pressureand propellant temperature that are directly linked to the heatand mass transfer at the interface. In some cases, the pressure evolution can be large, and this was shown to result in significantcondensation at the free surface. The numerical analysis of theseexperiments was carried out using FLUENT and the special heatand mass transfer model was incorporated through a user-definedfunction (UDF). The simulation results were found to be in goodagreement with the experimental data.

After a complete validation of the model, it will be used to simu-late thermal stratification and pressure evolution in a cryogenictank in a micro-gravity environment, so that temperature andpressure can be computed precisely for the ballistic phase.

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Liquid sub-cooling Although the more frequent concern is the heat-ing rate of propellant inside the tanks, there arealso reverse situations in which problems resultfrom excessive sub-cooling. For the EPC tanks inthe main stage, the LOX and LH2 tanks share acommon bulkhead, and a strong thermal cou-pling occurs at the wall interface. At the bottomof the LOX tank, heat is conducted toward theLH2 side, which serves as heat sink. Locally, theliquid oxygen is driven below its saturation tem-perature into a sub-cooled state. The main stageengine must operate within a given temperaturerange at the pump inlet, however, and this leads

to a requirement for a minimum sub-cooled tem-perature. Part of the functional studies performedon the propellant tanks included modeling thethermodynamic evolution inside the tanks andverifying that the feed temperatures do not dropbelow the allowable range.

A 2D axisymmetric simulation was performed forthe bottom of the LOX tank in the region of thecommon bulkhead. In addition to the liquid vol-ume, the surrounding metallic tank structure andtank insulation were meshed as well. This allowedthe external thermal environment specificationsto be imposed directly as boundary conditions.

Natural convection inside the liquid propellantwas simulated along with conductive heat trans-fer in the structure walls and insulation during thetransient simulation. The results were used to predict the temperatures in the sub-cooledregion and to make sure that the temperatures atthe pump inlet were within the necessary range.

2D axisymmetric model of the bottom of the EPCLOX tank, showing a sub-cooled (below 91K) layerof LOX growing in the tank bottom

2D axisymmetricmodel of theinsulated lowerskirt and ring ofthe ESC LH2 tank

Temperature stratificationin the cryostat duringsloshing, with the rangelimited to 85K

Volume fraction of helium in thegaseous phase during sloshing,showing the condensation ofnitrogen close to the free surface

Temperature gradients in the tank structure The tank structure is made primarily of aluminum alloys, and another set of simulations was performed for analyzing temperature gradi-ents. Using fine meshes for the solid wall material, one objective of the simulations was to accurately compute the conductive heat flux-es resulting from thermal gradients, which can represent a fair amount of the heat budget entering the tank. The simulations were alsoused to export temperature profiles for input to mechanical analysis models. For these calculations, the temperature boundary conditionwas typically imposed on the cryogenic side of the structure at the wall/propellant interface.

As an example, an axisymmetric simulation was performed for the insulated lower skirt and ring of the upper stage LH2 tank. The tankwas completely full, and still on the ground. The steady-state simulation showed a 265K temperature gradient along the 700 mm longskirt that generates a steady heat flux of 9 kW toward the tank inside.

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Diphasic Residuals As the tank draining nears the end, some free surface disturbances mayoccur, particularly for the LOX in the upper stage tank, leading to sudden gas inclusion in the outflowing liquid. Because feed pump operation is notrecommended in such conditions, the engine must be stopped before anygas ingestion occurs, and a residual mass of liquid will remain in the tank. Thequantity of diphasic residual mass depends on the dynamic conditions during the draining process.

In quiet conditions, perturbations in the liquid surface are minimal, so gasingestion is small. However, rolling can be imposed to the stage and trans-mitted to the drained propellants by the launcher attitude control system,and sloshing can occur. A vortex can form with an associated residual massthat increases with the intensity of these disturbances. To compensate for this,anti-vortex devices have been added to LOX tanks since the development of the Ariane 4 upper stage in order to delay the ingestion of bubbles in the collector and reduce the mass of the diphasic residuals. The impact ofthese devices is now being computed for a number of different flight config-urations with different degrees of rolling associated with different lateral perturbations. These computations are being performed with the complete3D geometry, starting from the very beginning of the launch phase and finishing at the end of the upper stage thrust phase, when the tank is draining.

The computations focus on two periods of time. The first is dedicated to thethrust of the first stage, and lasts about 10 minutes. The tank is not draining,but the fluids are subjected to external perturbations, particularly spinning.The second is dedicated to the thrust of the second stage, when the tank isdraining and subjected to external perturbations. Strong vortex formationappears at the end of this phase with bubble ingestion. This phase lasts about15 minutes.

Preliminary studies have included a comparison of numerical and experimentalresults, with tests performed on sub-scale tanks filled with water and at conditions with a similar Froude number. Very good agreement wasachieved, from quantitative and qualitative points of view. For example,when the first bubble was ingested, the predicted residual mass of water waswithin 10% of the measured value. In addition, the qualitative behavior ofthe free surface in the numerical computations matched the observed behav-ior, both in terms of the time when dips in the surface were seen to occur andthe location and size of the subsequent vortices. Following the validations, further computations were carried out using an actual flight configuration.

SummaryFLUENT is now used intensively to compute the pressure and temperatureevolution in cryogenic tanks during all flight phases from pressurization onthe launch pad to the last droplet ingestion in the turbo-pump. In the future,the tool will be adopted for use in a microgravity environment. For this purpose, the development of new local models is needed, and validation of these models will be a difficult task. Non-dimensional numbers will beheavily used to recreate experiments correctly on earth, since cryogenicexperiments in true microgravity conditions are difficult and expensive toperform. Despite the fact that transient 3D computations are CPU intensive,the final goal is to perform them with complex internal geometry coupledwith thermodynamic analysis and heat and mass transfer at the wall and atthe liquid/gas interface. After complete validation of the relevant models,computations such as these could be performed within a decade.

All these activities have been performed with support from CNES.

Geometry of the LOX tank bottom with internal anti-vortex equipment

Anti-vortex devices

The last phenomenon to be observed is the massive gas ingestion in thecollector, which always occurs at the same angular location, due to the curvatureof the collector

Massive gas ingestion

Three seconds after the onset of a smooth dip in the surface, a stable vortex iscreated as the free surface approaches the anti-vortex device. Some bubblesescape from the free surface, and this instant corresponds to the end of theengine feeding process; the remaining liquid in the tank corresponds to thediphasic residual

Stable vortexand bubbleseparation

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Cryogenic Flows in By S. Zurbach and L. Ballester, Snecma, SAFRAN Group, Vernon, France

THE DEVELOPMENT OF THE EUROPEAN LAUNCHER ARIANE was grantedto CNES by delegation of the European Space Agency, and Snecma has beenresponsible for all of the Ariane cryogenic rocket engines. The Ariane 5launcher and its powerful, expanded capacity version Ariane 5 ECA arethrusted by the cryogenic rocket engines Vulcain or Vulcain 2 and HM7.Vulcain 2 is an extension of the Vulcain cryogenic engine, operating in a gasgenerator cycle with two separate turbopumps. The combustion chamber is cooled by cold hydrogen flowing through regenerative circuits and thenozzle extension is cooled by cold hydrogen that flows through helicoidaltubes (a dump cooling system).

Rocket engines are exposed to severe mechanical and thermal loads, such ashigh vibration, a wide range of temperatures (from 20K to 3600K), and awide range of pressures (from vacuum to 200 bar). Due to this extreme environment and the performance quality and reliability required, an understanding of the physical processes and complex technologies relevantto these operating regimes is needed. As the prime contractor for the Arianecryogenic engines, Snecma has been modeling cryogenic flows for the sub-systems of the rocket engines for several years. The specifics of cryogenicflows depend largely on the thermodynamic behavior of the propellants. Foroxygen, hydrogen, and methane, a real gas approach is mandatory, since theideal gas equation of state is no longer valid. For both injection and cooling,the propellants operate in subcritical and supercritical regimes.

Besides hot tests, validated simulation tools are being used more and moreto ensure the reliability of space propulsion equipment. CFD allows new tech-nology to be evaluated without performing high cost tests. It is also a tool forassessing development risks and production non-conformance. One impor-tant example of the potential of CFD simulation was illustrated during theFlight Recovery Program (FRP) [1]. The FRP was set up after the maiden flightof the Ariane 5 ECA in December 2002, during which the nozzle extensionlost its mechanical integrity. During the FRP, a reinforced concept for the Vulcain 2 at the test bench

Thrust chamber with the helicoidal cooling system and a thermal map of thehelicoidal tube (supercritical hydrogen + solid)

Oxygen density as a function of temperature and pressure

Rocket Engines

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nozzle extension was defined, and the new design was produced and qualified within a very tight time schedule.

As the thermal load of the nozzle extension is the major contributor to itsmechanical integrity, the efficiency of the dump cooling system has beenextensively studied using FLUENT. It consists of welded rectangular tubes carrying supercritical hydrogen as a coolant. In the simulations, heat exchangein the regenerative circuit is modeled with a coupled approach involving thehot gas side and the coolant side, since very high heat fluxes are generated (10- 100MW/m2). On the coolant side, real gas effects are accounted for and amodified real gas equation of state is used. A 3D approach is necessary to fullycatch the three-dimensional nature of the flow inside the tubes, generated bythe helicoidal shape of the tube walls. The thermal methodology has been validated on a subscale nozzle experiment, and on full scale nozzles, with ded-icated thin thermocouples implemented on the tube walls. Post test expertiseof the tube and nozzle hardware has been systematically used to anchor theCFD predictions to a metallographic analysis. The test results are in good agreement with the CFD predictions. The 3D effects predicted in the flow ofhydrogen in the helicoidal tubes was confirmed by the test measurements. Thepredicted wall temperature and the thermal gradients within the solid partwere in line with the measured thermal map as well.

In addition to the nozzle cooling analysis, Snecma has developed specific com-bustion models to predict the combustion efficiency in a rocket chamber [2]. Forcryogenic engines, the atomization of the reacting fluids is often performed bycoaxial injectors. Liquid oxygen at 90K flows at low speed through a tube, whichis surrounded by an annular high speed flow of gaseous or liquid hydrogen. Inorder to guarantee sufficient atomization of the liquid oxygen and efficient mixing of the combustion products, optimization of the injection plate and injec-tors is necessary. For this purpose, a balanced analysis of both experimental testsand FLUENT predictions has been performed. One challenging component ofthese calculations is the prediction of the turbulent combustion at very high

pressures, in excess of 100 bar. Because the combustion chamber pressure is higher than the critical pressure of the injected propellants, the turbulent combustion regime is called transcritical. Despite these complexities, an accurateprediction of the reacting cryogenic flows has been obtained by the develop-ment and validation of specific models created within the framework of R&D programs at Snecma. These models, implemented in FLUENT through user-defined functions (UDFs), have been extensively compared to sub-scale measure-ments and applied to characterize full scale rocket gas generators and chambers.

For the forthcoming years, cryogenic flows will be computed for real gas mixtures. In the combustion chamber, oxygen and hydrogen are injected atvery low temperatures so that the classical ideal gas equation of state is nolonger valid to describe properly all of the thermodynamics, such as densityand enthalpy. It is therefore mandatory to have a CFD solver with a real gas formulation for a single fluid (for the cooling tubes) but also for mixtures topredict the flow field in the main combustion chamber, where supercriticalturbulent combustion of hydrogen and dense oxygen occurs. Importantcomponents for the success of this work include numerical solver robustnessand stability, thermodynamic integration of the necessary models, and a timereduction for large eddy simulation (LES) calculations.

For the evolution of the Vulcain Engine, a new nozzle extension was developedby Volvo Aero in Sweden, under a contract from EADS-ST GmbH in Germany.EADS-ST is responsible for the Vulcain 2 Thrust Chamber, under a contract withSnecma in France, who is responsible for the Vulcain 2 Engine.

References1 Ferrandon, O.; James, P.; Girard, P.; Terhardt, M.; Blasi, R.; Johnsson, R.; Damgaard,

T.: Vulcain 2 Nozzle Extension: Integrated European Team and AdvancedComputational Model to the Service of Nozzle Design; AIAA-2005-4535, July 10-13,2005.

2 Vingert, L.; Zurbach, S.: LOX / Methane Studies for Fuel Rich Preburner; AIAA 2003-5063, July 20-23, 2003.

Mean temperature field for a LOX/H2 rocket engine injector Characterization of a LOX / CH4 coaxial injector for two different operatingpoints; the predicted (left) and visualized (right) mean flame length are compared

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The Path to PassivationBy S. Petitot, S. Casalino and B. Vieille, French National Center for Space Studies (CNES), Evry, France, and B. Lazaro and E. Gonzalez, SENER, Universidad Politecnica Madrid, Spain

THE ARIANE 5 ECA IS AN EXPENDABLE LAUNCH SYSTEM. This means thatafter it delivers its payload, which is typically satellites into geostationarytransfer orbit, none of its parts will be reused. For these missions, the cryogenic upper stage (ESC) is designed to achieve a stable status so that itcan remain in orbit, structurally intact, for 25 years. A sequence of operationshas been developed to make this outcome problem-free. An important stepin the sequence involves the removal of any residual liquid propellant in thetanks. During this so-called passivation phase, the tanks are depressurized bysafety valve openings and the liquid is allowed to evaporate. The passivationphase is very important to study, because the evaporation occurs at a timewhen the ESC is subjected to linear and angular accelerations, and theinduced liquid motion can give rise to forces and moments that can strong-ly impact the dynamic behavior of the ESC module.

A study has been conducted using FLUENT to simulate the liquid motion inthe ESC in the period leading up to and during the passivation phase. Whenthe simulation begins, the ESC has forward axial motion, and a constanttorque is applied to make it start spinning about this axis. After 150 seconds,it is spinning with a rotation speed of 45 degrees/s and the torque is stopped.The axial speed decreases throughout this period, and after a total of 400seconds, the valves open and the passivation process begins. After 300 moreseconds, the ESC is accelerated abruptly and torques about the other axes areapplied. In the final stage, which lasts for another 50 minutes or so, the ESCundergoes a tumbling motion as it gradually decelerates.

The ESC has two cryogenic tanks, which contain liquid oxygen (LOX) and liquid hydrogen (LH2). Both tanks are included in the FLUENT simulation,using a total of 513,000 hexahedral cells. The fluid properties for the isother-mal calculation are defined using CNES software. The coupled motion of theESC and contained liquids is captured using a six degrees-of-freedom (6DOF)model, implemented through user-defined functions (UDFs). Viscous stressesare included through the use of the RNG k- turbulence model. The volumeof fluid (VOF) model with the geo-reconstruct scheme is used to capture theevolution of the free surface. Three fluids are defined: helium gas (the primary phase), liquid hydrogen, and liquid oxygen, and surface tension istaken into account. The external forces and torques described above are prescribed in a UDF, and are modified by the forces and torques that resultfrom the fluid sloshing.

Diagram of theAriane 5 ECAcryogenic upperstage (ESC)

Surface grid for the LOXtank (center, bottom) andthe LH2 tank (outer, above)

LOX tank

LH2 tank

Time (s)

150,000

100,000

50,000

0

-50,000

-100,000

-150,0000 2000 4000

Torq

ues

(Nm

)

3000

Mx

My

Mz

1000 350025001500500

Time (s)

1009080706050403020100

0 2000 4000

Torq

ues

(Nm

)

3000

Fx

Fy

Fz

1000 350025001500500

X-axis forceX-axis momentum

X-axis forceY & Z-axis momentum

X-axis force

0 200 400 3400Time (s)

Start ofsimulation

Spin of45/s

Beginning ofpassivation

phase

End ofpassivation

phase

The chronology of forces and moments applied tothe ESC before and during the passivation phase

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At CNES, a turbulent approach was used, and the results were compared tolaminar calculations done by SENER, a Spanish consulting firm with scientif-ic advisers from the University of Madrid. To achieve a comparable angularspeed after 150 seconds, the torque in the laminar case had to be stopped10 seconds early, at 140 seconds. Turbulent effects make a full 150 secondspin-up period necessary, but the axial angular acceleration remains non-zerofollowing this time as it takes time to decay. The difference in behavior is dueto the increased viscous dissipation in the turbulent case. When a turbulentapproach is used, the liquid phase forms a nearly uniform layer that rotatesalong the outer wall. The laminar calculation, on the other hand, predictsthat the fluid forms a rotating localized mass instead. The movement of thismass back and forth inside the vessel gives rise to angular accelerations in thenon-axial directions, and these grow in amplitude, causing an instabilityinside the vessel. For the turbulent case, small off-axis oscillations developonly after 700 seconds, when additional thrusters of the attitude control system are activated. This initiation time of oscillation development is veryclose to the time seen during the firs

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