Marine Technology II 249 - WIT Press · Marine Technology II 251 of about 1.31, corresponding to a...

Application of vortex generators in ship

propulsion system design

M. Oledal

TecAWogy, TV- 74 JOE-mail: [email protected]

Abstract

An experimental study of vortex generators for hydraulic applications has beenperformed. The mixing obtained by the presence of the vortex generators hasnot been addressed at this stage, but the main purpose of the study has been toget insight into the effects of captation occurring about the device. Threedifferent vortex generator principles has been used, two with sharp leadingedges and one smooth model with less added wetted area, in order to gainunderstanding of the geometries influences on the vortex generator design. Theexperiments showed that the geometry of the vortex generators was veryimportant, both concerning the mixing that could be visually observed, andconcerning the captation patterns. It is concluded that application of vortexgenerators in e.g. water jet inlets is both possible and advantageous, but that thedevices should be used with great care.

1 Introduction

The increased use of high speed crafts in sea transportation has raised theinterest for alternative propulsion systems such as water jets and contra-rotatingpropellers. As with most propulsion systems used in marine engineering, one ofthe main problems is that the propulsor partly operates in the boundary layer orthe wake of the hull. This results in complex inflow patterns to theimpeller/propeller, hence leading to vibrations, material fatigue, laboriouspropulsion system design and reduction of the overall performance of the craft.In particular this is true for water jet propelled crafts where the possibility ofseparation of the boundary layer in the inlet duct may decrease the efficiency of

Transactions on the Built Environment vol 24, © 1997 WIT Press, www.witpress.com, ISSN 1743-3509

248 Marine Technology II

the propulsor to a great extent. A general view of the flow conditions at theimpeller is given in figure 1 below.

Figure 1: Example of velocity distribution (m/s) at the impeller plane of atypical water jet flush inlet. From Oledal et al. [1]

The flow energy redistribution device supplied by nature, i.e. turbulence,feeds energy from the free stream towards the walls, but sometimes this energytransfer rate is not enough for a boundary layer flow to sustain an adversepressure gradient. It grows rapidly and may eventually separate.

To help nature, a number of devices has therefore been developed forimprovement of the flow field, such as the Grouthes spoilers and theSchneekluth wake-equalizing duct. Both have the same purpose: a redistributionof the flow field approaching propellers in order to minimize the effects of flowdistortion at the propeller plane.

The use of vortex generators (VGs for convenience) as a means of flowcontrol is another well known solution, widely used in the aircraft industry bothfor inlet flows and external flows. For marine applications however, a number ofother factors must also be considered, such as the possibility of erosivecavitation, fatigue because of the large masses involved in the flow compared toVGs used in aerodynamics, and finally impacts from water surface debris. Thismeans that the VG, when used in hydraulic environments such as hydropowerplants and marine vehicles, must be designed using a somewhat different andmore extensive approach. In the current report, the effects of cavitation becauseof the VG has been investigated without considering any other aspects at thisstage.

In the following chapter the equipment used for the experiments is presentedtogether with descriptions of the VGs used during the test series. This isfollowed by a chapter describing the results obtained during the current work


Marine Technology II 249

and a discussion of its influences on the design of the VG. Finally, someconclusions from the experiments are presented and some proposals for furtherwork concludes the report.

2 Experimental setup

2.1 Water tunnel

A vertical water tunnel with a free exit has been erected at the Hydropowerlaboratory at the at the Norwegian Institute of Science and Technology. Thelength of the vertical test section is 750 mm with a rectangular cross-section of130 by 190 mm. Three of the walls are fitted with transparent Lexan plateswithin the painted steel frame. A stilling tank of 1000 mm diameter upstream ofthe test section, contains two flow stabilizing screens of perforated aluminumplate with 40 % aperture. Two conical flow accelerating sections of 45° and 30°respectively and a circular-to-rectangular transmission section, each 200 mmlong, connect the stilling tank with the test section. Upstream of the stilling tankthere is one 45° horizontal bend and one vertical bend of 90°, fitted in order toutilize the permanent pipe installation in the Hydropower laboratory. A butterflyshut-off valve is mounted ahead of the bends, and 5 m upstream of the valve aflow sluice control valve is installed. The quality of the water, which isimportant when cavitation results are discussed, is relatively poor. On the otherhand, for practical applications on high speed craft, the water quality willseldom be better.

The vortex generator, was placed at the center of one of the walls, 530 mmabove the exit. The model and the area in its immediate vicinity were covered bya thin ink layer during some of the experiments. By this method, regionsexperiencing cavitation erosion could easily be traced without running the testfor very long times. The choice of ink was based on experiences from marineand Pelton turbine research, and was optimized for the actual cavitation numberexpected.

In order to capture the fast changes of the cavitation patterns, a KodakEktaPro HS4540 high speed video camera was used. The camera was able torecord 4000 full frames, 256x256 pixels, per second or by recording smallerregions, 64x64 pixels, of the frame, up to 40500 frames per second. Initialtesting showed that the most important and interesting information could beobtained at a recording speed of about 9000-18000 frames per second,depending on the angle of attack. A further increase of the recording speed didnot reveal any new information.

2.2 Vortex Generators

Most reports covering experimental investigations of vortex generators arebased on wind tunnel testing, e.g. Lin et al [2] or Brown et al [3]. This is alsothe case as far as simulations of the flow field is concerned, e.g. Anderson &



Gibb [4]. However, experiments on propeller tip vortices very much resemblethe flow field about vortex generators, and comparisons can be made with thevortex cavitation observations described by Brennen [5].

The models that have been tested all have simple shapes, such that theanalysis of the results could be used for actual design of VGs when informationon the mixing characteristics is available. Three shapes were chosen:

• Simple Delta Vortex Generator - A very simple vortex generator model,designed as a simple delta half-wing mounted on a rotary support was chosenfor the initial experiments. The model had a root chord length of 45 mm, amaximum height of 20 mm and a sweep angle of 24°. The angle of attackcould be varied from 0° to 45°.

• Wheeler Vortex Generator - The Wheeler VG is a so called low-profileVG, named after its inventor Gary Wheeler[6]. Because of the stagnationpressure at the trailing end of the VG, the fluid is forced upwards and a pairof counter-rotating vortices are formed with a height of up to five times thevortex generator height. The reason for choosing this VG is the attractivecombination of low drag because the size of the VG can be much smallerthan for normal VGs in combination with good mixing characteristics. Afurther reason for the choice of this VG design in the current report was anexpected lift of the cavitating vortex core from the surface, hence reducingerosive effects on the surface behind the VG

Based on the experiments with the simple delta VG, an angle of attack of20° was chosen for this model in order to see the interaction between thecavity and the vortex breakdown.* Dome Vortex Generator - A smooth dome vortex generator was used bySchubauer et al [7] because its presence in the non-uniform flow field in theboundary layer would create vortices without adding much extra wettedarea. They found that the separation of the boundary layer was delayed bymore than 20% in an adverse pressure gradient wind tunnel. This was farfrom the best device tested, but from a cavitation point of view, thegeometry of the dome VG is clearly attractive. An interesting idea wasproposed by Smith [8], that the separation itself was not a vital ingredient fora successful VG distribution. Instead the change of the vorticity vectordetermined the success of a VG, indicating that shear layer strength could bereduced without loosing the mixing properties by careful VG design. Duringthe current experiments, a hemisphere with a diameter of 30 mm was used.

3 Results

All experiments were performed at a free stream cavitation number, defined as



of about 1.31, corresponding to a velocity of 12.1 m/s. The visual observationswere made using the actual cavitation bubbles or, at non-cavitating cases, byusing small air bubbles ingested into the water. In general, large scalefluctuations of the flow direction could be observed in the test section, probablyowing to the circular-to-rectangular transition section. By fixing short cottonthreads just in front of the models, this problem was partly eliminated as it waspossible to see the main flow direction on the pictures .

Two different views were used during the recordings. A side view of thewater tunnel configuration proved to give much information about the flowfield, in particular about the characteristics of the cavitation bubble cloudbehaviour close to the surface. A view from above the vortex generator wasalso used but the results were more difficult to interpret.

The simple delta VG served as an introduction to the vortical flow fieldabove and behind the device. For lower angles of attack no cavitation at alloccurred about the VG. When the angle of attack was increased to abouta=13°, a cavitating vortex core was observed along the leading edge, extendingsome distance downstream the trailing edge before it was terminated by a spiralvortex breakdown region, figure 2. The appearance of the cavity above the VG

Figure 2: Cavitation pattern about a single delta vortex generator at a=13°.Flow is from the right to the left.

was relatively stable except for cavity surface waves. The main direction of theflow in the proximity of the VG was conical, but after the VG the flow quicklyadapted to the free stream flow, and the vortex core never reached the height ofthe VG In this case it seemed that erosive cavitation was almost negligible.

A further increase of the angle of attack resulted in more aggressivecavitation, where the onset of the cavity could be seen already at the leadingedge. It was also noted that the vortex breakdown, now occurring above the



VG, resulted in intensive bubble collapse on the surface of the VG and behind it,which resulted in ink erosion during the tests. High speed camera observationsrevealed that as the vortex bursted, a very complicated flow pattern wasformed, where a number of minor vortexes occurred, often with sometimescavitating cores.

At extremely high angles of attack, i.e. oc>40°, no cavitating vortex wasformed, but vortex breakdown occurred instantly. As such high angles of attackare of little interest for practical applications the discussion of this phenomenonis left out here and reference is made to the discussion by Oledal et al [1].

The Wheeler VG showed a somewhat different flow pattern, due both to thestagnation inside the VG and to the fact that the vortices interact after theyleave the VG.

The main impact upon the flow because of the upward flow caused by thestagnation point was the continued rise from the surface of the vortex core afterthe VG, see figure 3. It was observed that the vortex core was situated at about1.5 times the vortex generator height at only two VG lengths downstream.Clearly the mixing because of the low-profile VG is much more efficient thanthe single delta VG. In addition, the lift of the core means that cavitationbubbles formed at vortex breakdown collapses away from the surface anderosion is reduced. The interaction of the vortices after the trailing end seems tobe a very complicated phenomenon and requires much more investigationbefore any conclusions can be drawn.

Figure 3: Cavitation pattern about a Wheeler vortex generator at oc=13°. Flowis from the left to the right.

Finally, the cavitation pattern around the dome VG, fig. 4, didn't resembleany of the characteristics found on the other VGs, which should come as nosurprise. The shear layers produced at the sharp edges of the other VGs is not



present in this case, and no cavitation was observed behind the VG. This is ofcourse also an indication that mixing caused by the dome VG is much weaker.

Figure 4: Cavitation pattern about a dome vortex generator. The flow is fromthe left to the right, and the cavity can be seen close to the junction between theVG and the tunnel surface.

Instead, another region of cavitation was found in front of the VG. A horse-shoe vortex was formed at the upstream end of the dome, strong enough tocreate a cavitating core that spanned almost all the way to the trailing end. Thereason for this behaviour is probably that because of the boundary layer, astagnation point can be found some distance above the tunnel surface and avortex is formed. This cavity looked relatively harmless, but because of theshort distance between the core and the surface, erosive cavitation may occur.

4 Conclusions

As is often the case when cavitation is involved, conclusions tend to be of anempirical nature and the current report is no exception. As the author is notfamiliar with any other research on hydrodynamic VGs, one of the majorintentions has been to gain a basic understanding of the flow phenomenainvolved in vortex generation.

Nevertheless, the conclusions may be summarized as follows:

• The use of vortex generators on high speed craft can be a method forreducing losses because of distortion. However, the design of hydrodynamicVGs will be a compromise between drag increase, mixing and cavitationcaused by the device.



• The shear layers produced on the sharp edges of simple delta VGs areunfavorable from a cavitation point of view, but with careful design, thecavitation will take place away from the surface, and hence no erosion willoccur.

• Both the Wheeler type and the simple delta VG are sensitive tochanges in the flow direction. This could be the difference between erosiveand non-erosive cavitation.

• The Wheeler type VG has very attractive mixing features, but suffersfrom severe cavitation on the surface of the VG. Based on the experiments itcan be concluded that the more sophisticated "wishbone" shaped WheelerVG, may be a way of avoiding many of the cavitation problems experiencedduring the current work, without renouncing mixing properties. It is clearthat this VG concept is the most promising, with a combination of low dragand mixing properties. If the cavitation can be controlled, this device isprobably the best for reduction of inlet flow distortion.

It is finally suggested that some effort is done in testing modified versions ofthe Wheeler VG, i.e. studies of the cavitation pattern for different "wishbone"type VGs and VGs with smoother shapes. A question of later but veryimportant significance will be the distribution of the VGs in an inlet for optimummixing.

Nomenclature

a Angle of attack of vortex generatorGO Free stream cavitation numberpo Free stream static pressurepv Vapour pressurep DensityUo Free stream velocity

Acknowledgments

The author would like to thank Kvasmer AS for their support of the currentwork through their research program "Ship for the Future".



References

[1] Oledal M. & Ostman A. 3D-Simulation of the Flow in a Water JetMet, SIMS'96 proceedings, 1996, Trondheim, Norway

[2] Lin J.C & Howard F.G., Small Submerged Vortex Generators forTurbulent Flow Separation Control, Journal of Spacecraft 1990 5503-507 ' ' '

[3] Brown AC, Nawrocki H.F. & Paley P.N., Subsonic DiffusersDesigned Integrally with Vortex Generators, Journal of Aircraft 19683,221-229

[4] Anderson B.H. & Gibb I, Study on Vortex Generator Flow Controlfor the Management of Inlet Distortion, Journal of Propulsion andPower, 1993, 3, 422-430

[5] Brennen C.E., Cavitation and Bubble Dynamics, Oxford UniversityPress, New York, 1995

[6] Wheeler GO, Low Drag Vortex Generators, US Patent no5058837

[7] Schubauer, G.B. & Spangenberg, W.G. Forced Mixing in BoundaryLayers, Journal of Fluid Mechanics, 1960, 8, 10-32

[8] Smith, FT Theoretical Prediction and Design for VortexGenerators in Turbulent Boundary Layers, Journal of FluidMechanics, 1994, 270, 91-131

[9] Oledal M. & Kjeldsen M., Classification of Cavitation Patternsabove a Delta Shaped Vortex Generator, Internal report, HydropowerLaboratory, NTNU, Trondheim, Norway, 1996


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