Term PaperFluid Mechanics

CIV 208Topic: - Flow visualization techniques and their use

Submitted To Submitted ByMr. Vikrant Sharma Atish Kumar

Reg. No.:- 4100070015Roll No.:- RH5001A53Class: - B-Tech-Civil (2nd Year)

AKNOWLEDGEMENT

I would like to express my gratitude to all those who gave me the possibility to complete this

term paper. I want to thank the Department of CIVIL ENGINEERING of LOVELY

PROFESSIONAL UNIVERSITY for giving me permission to commence this Term paper, to do

the necessary research work and to use departmental data. I would also like to thank to Mr.

VIKRANT SHARMA, Lecturer in constructional material, who gave and confirmed this

assignment and encouraged me to go ahead with my term paper.

I am finally thank of our friends whose help, valuable suggestions and encouragement helped

me in all the time of research for and writing of this term paper.

THANK YOU.

CONTENTSS.N. Description Page No.

1. Introduction 1

2. What we need to visualize 2

3. Visualization Techniques 3

4 Surface flow visualization

Surface Oil Film

Liquid crystals and temperature sensitive paints

Pressure sensitive paint (PSP)

Flow Visualization with Special techniques

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5 Particle Tracer Methods

Smoke Visualization of the Flow

Visualization using dye

Visualization by different small particles

The gas bubble visualization

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6 Optical Methods

Shadowgraph method

Schlieren method

Interferometry

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7 Uses of Flow Visualization 12

8 Reference 13

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IntroductionFlow visualization in fluid dynamics is used to make the flow patterns visible, in order to get aqualitative or quantitative information on them.

A model Cessna with helium-filled bubbles showing path lines of the wingtip vortices.

Flow visualization is the study of methods to display dynamic behaviour in liquids and gases.The field dates back at least to the mid-1400's, where Leonardo Da Vinci sketched images offine particles of sand and wood shavings which had been dropped into flowing liquids. Sincethen, laboratory flow visualization has become more and more exact, with careful control ofthe particulate size and distribution. Advances in photography has also helped extend ourunderstanding of how fluids flow under various circumstances.

Leonardo Da Vinci’s Hand drawings

Naturally occurring flow visualization

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What we need to visualize

Pathline:Pathlines are the trajectories that individual fluid particles follow. These can be thoughtof as a "recording" of the path a fluid element in the flow takes over a certain period.The direction the path takes will be determined by the streamlines of the fluid at eachmoment in time.

Streakline:Streaklines are the locus of points of all the fluid particles that have passedcontinuously through a particular spatial point in the past. Dye steadily injected into thefluid at a fixed point extends along a streakline.

Timeline:Timelines are the lines formed by a set of fluid particles that were marked at a previousinstant in time, creating a line or a curve that is displaced in time as the particles move.

Streamline:Streamlines are a family of curves that are instantaneously tangent to the velocityvector of the flow. These show the direction a fluid element will travel in at any point intime.

Steady Flow:a flow field which does not change with time. For steady flow, streaklines, pathlines,and streamlines coincide.

Particle Advection:Computing the motion of particles through a flow field

Vorticity:The curl of the velocity field, giving the magnitude and direction of angular velocity foreach particle in the velocity field

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Visualization Techniques

1. Surface flow visualization

This reveals the flow streamlines in the limit as a solid surface is approached. Colored oilapplied to the surface of a wind tunnel model provides one example (the oil responds to thesurface shear stress and forms a pattern)

Surface flow visualization over a 5 degree ramp. The 5 degree ramp with micro vortex generators.

Surface flow visualization over a 25 degree ramp withtest section unstart.

The 25 degree ramp with micro vortex generators.

Surface flow visualization past a cylinder. The cylinder with micro vortex generators.

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Surface Oil FilmOil film or dots on the model surface enable obtaining a picture of the flow pattern at thesurface of the model placed in the wind tunnel quickly and easily. The special mixture can beprepared from an appropriate oil and fine pigment (Al2O3; T iO2, powder, fluorescent dye,colouring pigments, graphite). The technique allows observation of the lines of separation andreattachment of the flow to the body.

Visualization with TiO2 + oil on the surface around two vertical cylinders fixed on the plate in the large windtunnel

Oil flow visualization, airflow on the end wall of a turbine blade cascade

Liquid crystals and temperature sensitive paintsA surface-temperature distribution can be gained by coating a test model with cholestric liquidcrystals. If they are illuminated with white light under a certain angle of incidence, liquidcrystals reflect only one light wavelength at each viewing angle, depending on smalltemperature changes in the crystal sheet. Liquid crystals are able to respond to finer changes oftemperature in the boundary layer, due to laminar-to-turbulent transitions or indicate theplace of shock waves. The colours of liquid crystals are reverse if the temperature changes inthe opposite direction. Therefore, liquid crystals are very attractive for boundary-layer studies.Model to be tested should be made of a material with low heat conductivity and coated withblack paint as base. Fig. demonstrates the application of liquid crystals for hot streamsvisualization in a little smoke wind tunnel.

Flow visualization in the small wind tunnel with liquid crystals

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Pressure sensitive paint (PSP)

The spatially continuous pressure and temperature distribution on aerodynamic test surfacesis important for understanding complex flow mechanisms and comparison with predictions ofcomputational-fluid-dynamics models. Conventional pressure measurements are based onpressure taps and electronically scanned transducers. Pressure taps provide pressureinformation only at discrete points.

A comparison of pressure results between PSP (right side of model) and Computational Fluid Dynamics (left side)

Flow Visualization with Special techniques

Third group of visualization methods is based on two principles: introducing a foreign invisiblesubstance into the incompressible flow and visualizing the density variations in the flow by opticalmethods. The foreign substance in this case is energy transferred to certain portions of the flow toincrease the energy level (spark, electron beam and glow discharge methods) and make artificialdensity variations. Such portions of the flow have an altered density and can be visualized by theoptical methods.

They are applied to visualize the rarefied gases that are for several reasons distinguished from theordinary compressible flows. The gas flow with extremely high level of kinetic energy becomesluminous in a stagnation point where the kinetic energy is transferred into heat. That heat exitselectronic transition in the gas and the flow itself is visible.

Flow visualization by electronic beam in hypersonic wind tunnel for M =10

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2. Particle Tracer methods

The visualization technique of streamlines, filament lines or particle paths, which injects someforeign material into a flow as a tracer is the most popular one and has been widely used overa long period, up to now. These three curves coincide if the flow field is stationary. But in theflow that depends on space and time as well, the three types of curves are different from oneanother. Which curves will be visualized depends on: where the particles are introduced, thelength of the exposure time and the reference system from which the flow is observed orphotographed.

There is no difference between liquid and gaseous flows. The tracer may be smoke, dye,pigment, milk, air or hydrogen bubbles, ozone, fluorescent dye, powder, sawdust, aluminiumparticle, bakelite etc.

Smoke Visualization of the Flow:-Recent developments indicate that smoke visualization in wind tunnel, one of the oldest flowvisualization techniques, will continue as an important experimental tool in the study ofcomplex flow dynamic phenomena. Improvements in generation and injection of smoke aswell as in lighting (laser as a light source), in techniques of acquisition and computation havecontinued to increase the scientific value of this method. Similar results are obtained by flowvisualizations with fog and vapour.The smoke can be very useful in a wind tunnel with low turbulence. There exists no upperlimit of speed for smoke line visualization (it was possible to extend the range of smoke linevisualization even to supersonic flow velocities).

Flow visualization in the VTI smoke wind tunnel (a and b) and in Onera smoke tunnel (c)

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Flow visualization with smoke from ship chimney in small subsonic wind tunnel

Visualization using dye

The visualization of the liquid flow patterns by ejection of dye is an analogy of the smoke visualizationtechnique. The mixing of smoke and air is more intense than that of dye and water. A dye for the flowvisualization of filament line has to fulfil several requirements: stability with respect to diffusion, thesame specific weight as the working fluid and high contrast. Dye can be injected in a tested flow eitherfrom a small ejector tube placed at a desired position or from small orifices, that are provided in thewall of a model (Fig. 6a), without the component perpendicular to the model surface. Dye can also begenerated in the flow, without disturbing the flow.

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Visualization by different small particles

Adding small particles in the flow (water or air) can enable visualization and measuring of the flowvelocity. The fundamental assumption is that the velocity of the particles and fluid is identical. Theparticle tracer can be either solid, liquid or gaseous and the fluid liquid or gaseous, for e.g.: dust,magnesium (Mg), Al2O3, TiO2, aluminium (Fig.8) and polystyrene or cosmetic powder, licopodium,hostaflon, cigarette smoke, metaldehyde, atomized DOP, glass sphere, marble dust, oil drops, waterdrops, hydrogen, gas, helium bubbles,... The diameter of the particle is between 0.1 to 20 microns.

(a) (b)Flow visualization around hydrofoil (a) violet aniline dye, (b) experimental and numerical path linevisualization (layers opacity 50%)

The gas bubble visualization

Gas bubbles visualization is a tracer method where tracer particles have lower (in the water) or densitysimilar (in the air) to the flow. The observation of such gaseous tracers in a gaseous flow requires theuse of optical visualization methods. The gas bubbles change their shape during the motion and inconsequence, the drag coefficient of these gaseous tracer particles is not only a function of the velocitydifference between the fluid and particle, but also a function of the deforming forces acting on theparticle. The gas bubbles can be injected in the flow or generated by electrolysis.

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3. Optical MethodsSome flows reveal their patterns by way of changes in their optical refractive index. These arevisualized by optical methods known as the shadowgraph, schlieren photography, and interferometry.More directly, dyes can be added to (usually liquid) flows to measure concentrations; typicallyemploying the light attenuation or laser-induced fluorescence techniques.

Shadowgraph method

The oldest and the simplest of all optical methods for flow visualization is the shadowgraph .Fig. 1shows a typical setup for shadow methods. A light beam passing through the wind tunnel test sectionis parallel. A spherical mirror or lens makes the light parallel. The light source should be small toensure good sharpness of the obtained image. Observation and recording the deflected beam parts arein the perpendicular plane screen at a distance of l from the test section.

Figure 1. Schematic arrangement of the shadowgraph system, deflection of light rays in a field of thevariable ∂2n/∂y2

Shadowgraph visualization around a sphere Typical shadowgraph images showing the sphericaltipped cylinder mounted on the flat plate

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Schlieren method

As mentioned before, the Schlieren method is sensitive to the changes of the first derivative of density(or refractive index) and it can record the angular deflection of the disturbed ray with respect to theundisturbed in a transparent medium with local non homogeneities.

Today the Schlieren method is the most frequently used in aerodynamic laboratories, since it isrelatively simple and very useful.

If a parallel beam of light passes trough the air where there is a density gradient normal to thebeam direction, the light travels more slowly where the density is greater and the beam is refractedtowards the region of greater density.

Schlieren effects around a cone and a slanted Rainbow Schlierenslot in the bottom wall for M∞= 0.8 in thesupersonic wind tunnel

Schieren system with laser as a light and the schlieren effect around a cone for M∞ = 1.1

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InterferometryIn most gas dynamics applications, it is useful to know flow density changes in wind tunnels, shocktubes osupersonic jets. The phase alteration beam passing through a disturbed section of a tested fieldcan be compared with an undisturbed beam. The effects of interference make the basis ofinterferometry. The application of this principle in visualizing compressible flow fields is as old as theschlieren method.

a. Classical interferometry

The most used type of interferometers tests is the Mach-Zehnder interferometer (MZI). Two lightbeams (test and reference ones) in the MZI are separated by its four plates. This instrument is suitablefor quantitative density measurements in large wind tunnels. It requires an extremely high degree ofmechanical precision and complexity of construction. Mechanical and optical tolerances are in order ofa wavelength or below. This makes the instrument expensive and its cost grows rapidly withincreasing the diameter of the desired size of the field of view.

Mach Zehnder interferometer

b. Holographic Interferometry

Holographic interferometry is an optical method that enables complete flow field testing. The methodis non-contact (it does not disturb the flow field) and is used for testing different objects andphenomena.

The flow density can be measured directly using interferometry. The greatest advantage ofholographic interferometry, in relation to the schlieren method, is the fact that it provides completeinformation stored in a single plate, allowing a postponement selection of specific types of flowvisualization.

Arrangement for holograms recording a) and reconstruction b)

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Uses of Flow Visualization

Understanding flow phenomenon

Verifying model or theory results

Easier measurements for designing

To get a priori knowledge of solution

In computational fluid dynamics the numerical solution of the governing equations can yieldall the fluid properties in space and time. This overwhelming amount of information must bedisplayed in a meaningful form. Thus flow visualization is equally important in computationalas in experimental fluid dynamics.

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Reference www.vti.mod.gov.rs/ntp/rad2007/2-07/rist/rist.pdf

http://caos.iisc.ernet.in/hpg/students/francis/Students_talk/kiran_fv.ppt

http://web.cs.wpi.edu/~matt/courses/cs563/talks/flowvis/flowvis.html

http://en.wikipedia.org/wiki/Flow_visualization