Flow Visualization

Module IV Compressible Fluid Flow Semester VIII

[Dept. of Mech. Engg.] [College of Engineering Adoor] [VENKITARAJ K P]

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FLOW VISUALIZATION

Flow visualization is the art of making flow patterns visible. Most fluids (air,

water, etc.) are transparent, thus their flow patterns are invisible to us

without some special methods to make them visible. Moving fluids often

form patterns so complicated that intuition fails when we try to imagine

them. Some flows are so complicated that we cannot analyse all their details

from the governing equations, even with the biggest computers now

available.

1. Tracers

Tracers are fluid additives that permit the observation of flow patterns. An

effective tracer does not alter the flow pattern but is transported with the

flow and is readily observable. It is important that tracers are not affected by

gravitational or centrifugal forces resulting from density differences.

2. Streamers

Flow visualization along a surface can be accomplished by attaching tufts of

wool, silk or cotton to the surface, or if flow away from the surface is to be

observed, they may be supported on wires. Surface tufts may be used to

observe the transition from laminar to turbulent motion. They may also be

used to study flow separation qualitatively, where the violent motion of the

tufts or their tendency to point in the upstream direction identifies that

separation is taking place.

3. Liquid films

This method makes use of contrast obtained on account of the unequal

rates of evaporation of a liquid film in the laminar and turbulent regions. A

film of some volatile oil is applied on the surface of the model prior to

starting the flow. When the air flow takes place over this surface the

evaporation of the oil film in the turbulent region is faster than in the

laminar region. A clearer contrast is obtained by using black paint on the

surface. This method can be used for aerofoil blade surfaces in wind

tunnels.

4. Smokes

Smoke has been used successfully to study the detailed structure of

complex flow phenomena. It is the most popular agent used for flow

visualization in wind tunnels. One injection technique is the so-called

smoke-wire method, where the smoke is generated by vaporizing oil from a

fine electrically heated wire. The method can be applied to flows where the

Reynolds number based on the wire diameter is less than 20. Smoke can



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also be released from a small-diameter tube or rake to create one or more streaklines.

5. Optical Flow Visualization Methods

Flow patterns in gas streams can be observed by means of optical

techniques which are sensitive to variation in gas density. At high velocity,

changes in density can be sufficiently large to cause comparable changes in

the refractive index of the gas. The velocity of light in a medium increases as

the density of the medium decreases. Also, the change in density of a gas

produce changes in the refractive index of the gas, which in turn changes

the direction of the light rays that passes through the gas. When these rays

are projected on a screen, the intensity of illumination becomes sensitive to

the direction of the light rays.

The optical index of refraction n of a medium is defined as ratio of the speed

of light in a vacuum (a0) to the speed of light in that medium (a).

The refractive index is related to the fluid density by Snells law as

where is a constant characteristic of the gas, is the local density and s is the standard density ( at 0C and atm pressure)

The refractive index n is related to the fluid density also through the

ClausiusMosotti equation, which for a gas reduces to the simpler form of the GladstoneDale equation

with being the gas density and K the GladstoneDale constant, which has the dimension of 1/, is specific for a gas, and depends weakly on the wavelength of light used.

In compressible flow of an ideal gas the density is a function of the Mach number, and, for these flows, the information obtainable with the methods

is therefore a measure of the Mach number or flow velocity.



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The optical methods in common use ( interferometer, schlieren and

shadowgraph) depends on one of the two physical phenomena: (i) the speed

of light depends on the index of refraction of the medium through which it

passes, and the index of refraction of a gas in turn depends upon its

density; and as a consequence of this first phenomenon, (ii) light passing

through a density gradient in a gas (and therefore through a gradient of

index of refraction) is deflected in the same manner as though it were

passing through a prism. In high speed gas flow the density changes are

sufficiently large to make these phenomena sizable enough for optical

observation.

The interferometer, based on phenomenon (i), measures directly changes of

density, and is primarily suited for quantitative determination of density

field.

The schlieren method based on phenomenon (ii), measures density

gradients. Although it is theoretically adaptable to quantitative use, it is

inferior to the interferometer in this respect, and its greatest utility is in

giving an easily interpretable picture of the flow field together with a rough

picture of the density variations in the flow.

The shadow graph method, also based on phenomenon (ii), measures the

second derivative of the density (i.e., first derivative of density gradient).

Therefore it makes visible only those parts of the flow where density

gradients changes rapidly, and it has found it greatest utility in the study of

shock waves.

Of the three methods mentioned, the interferometer yields the most

information and the shadowgraph the least. On the other hand the

interferometer is the most costly and the most difficult to operate, whereas

the shadowgraph is the least costly and the easiest to operate.

Interferometer

In this technique the variation of density in the flow field is directly

determined from the pattern obtained on the screen or a photographic plate.

The Mach-Zehnder interferometer shown in figure is extensively used in

wind tunnel experimentation. It consists of two fully reflecting mirrors M1 &

M2, and two half silvered mirrors (splitters) M1 & M2. Light from the source first passes througha collimating lense which renders

light parallel, and then passes through a monochromatic filter. It then

reaches the first splitter M1, which passes half the light and reflects the other half. The light which is reflected is changed back to its original

direction by the mirror M2 and then passes through the test section.



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The light which passes the first splitter M1 passes through the reference section (where the density is known) and reaches mirror M1. The mirror M1

reflects the light towards splitter M2 where the two beams now get combined into a single coherent beam. The combined beam is now focused by a lens

system on to a photographic plate.

When there is no flow through the test section (i.e, no density gradient), the

two beams joined at the splitter M2 will be in phase and the pattern of illumination obtained on the screen will be uniform. When flow is

established in the test section the beam of light passing through its varying

density field will be out of phase with the beam coming through the

reference section. This results in an interference pattern on the screen. the

pattern obtained don the screen consists a series of light and dark fringes.

Each fringe represents a region of consatnt density. The differences in

density between in gas in the test section and the gas in the reference

section are obtained by analysing the fringe pattern.

Schlieren Method

The schlieren system is used for the flow visualisation and is based on the

principle of refraction of light as being proportional to the density gradient.

It has a wide range of applications, including the visualisation of boundary

layers, combustion, shock waves, and convection currents within fluids

during heating or cooling, and air flow over models in wind tunnel testing.

A beam of light is sent through the test section from the light source by the

properly oriented concave mirror M1. The beam coming out from the test



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section is reflected on to a screen by the suitable located concave mirrors M2

& M3.

A sharp knife-edge is inserted at the focal point of the mirror M2 so that it

intercepts about half the light. When there is no flow through the test

section the screen is uniformly illuminated by the portion of the light that

escapes the knife edge. When the flow is established in the test section the

light rays passing through regions with density gradient will get deflected as

though it had passed through a prism. Depending on the orientation of the

knife edge with respect to the density gradients, more or less amount of light

escapes the knife edge and illuminates the screen. Thus the Schlieren

system makes density gradients visible in terms of illumination. A

photographic plate at the viewing screen records these density gradients as

different shades of gray.

Shadowgraph method

The shadow graph method is particularly suitable where there are large

density gradients, such as in the flow across a shock wave. This method is

simpler, less expensive and easy to operate compared to other two methods

explained above. But it does not provide any fine details of the density field,

and therefore is used for qualitative analysis.

A shadow system comprises a light source, a collimating lens, and a viewing

screen or photographic plate. If the source is far from the test section, then

the collimating lens is not required.



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When the gas is not flowing through the test section, there is no density

gradient and the screen is illuminated uniformly. When the flow is

established in the test section the light beam will be refracted wherever

there is a density gradient. However, if the density gradient were constant

each ray will be deflected by the same amount, and there would be no

change in the illumination on the screen. If the density gradient varies there

will be tendency for the light rays to get diverge or converge. Bright regions

appear where the light rays converge, dark regions where light rays diverge.

The resulting image on the screen is thus a series of light and dark regions.

Thus it is evident that the variations in the illumination of the screen are

proportional to the second derivative of density. The shadow graph is

particularly useful for viewing shock waves. In the region of shock wave, the

derivative of density gradient is positive on the upstream side of the shock

and negative on the downstream side. Hence the shock wave appears as a

dark region on the screen followed by a bright region. The upstream and

downstream of the

shock the screen is uniformly

illuminated.

Documents

Flow Visualization