FLOW VISUALISATION

AF17 Flow Visualisation Apparatus

Introduction

In experiments with fluid mechanics, it is often necessary to study the nature of the motion by direct observation of part or of the whole of the flow pattern. Important features may be observed, such as regions of steady or unsteady flow, thickening boundary layers and separation, secondary flows and so on. Flow visualisation can show characteristics of the motion, which might previously have been totally unexpected. It frequently explains phenomena which may otherwise defy explanation.

It sometimes provides the starting point for new theoretical or analytical studies. A good historical example is provided by Osborne Reynolds’ visualisation of laminar and turbulent flows in glass tubes, which was the starting point for a rational understanding of the resistance to flow experienced by a fluid as it moves along a pipe.

There are many techniques for visualisation. In water, dye filaments as used by Osborne Reynolds are still used. A more recent innovation is the use of tiny bubbles of hydrogen produced by an electrode which sheds a sheet of bubbles, or which may be arranged to shed discrete streams of bubbles which behave like dye filaments. In air, the most common practice is to use smoke injected through a tube or a row of tubes, usually called a rake. The smoke must be of nearly neutral density so that it does not rise or fall through the flow due to the effect of gravity and needs to be quite dense if the traces are to be observed for more than a short length downstream of the injector tube.

Description of Apparatus

Figure 10.1 shows the main parts of the flow visualisation module. Smoke from the generator passes through flexible tubing into a streamlined manifold that spans the duct at the inlet to the working section. This duct contracts from a settling chamber, which contains honeycomb and gauzes to reduce turbulence, to the working section.

Smoke emitted from the rake of tubes (smoke comb) accelerates with the airflow along the inlet duct into the working section and the filaments may be observed through the clear plastic front surface against a matt black background. A strong, diffused light from either side can help to give good contrast. The apparatus exhausts through the outlet in the bench top; a flexible air hose should be fitted to conduct the exhaust to a suitable ventilator, otherwise the smoke will easily fill a laboratory in minutes. The smoke is harmless, but you must ventilate it away for safety and common sense.

Figure 10.1 Flow Visualisation Apparatus

The individual filaments retain their separate laminar identities for the whole length of the working section for speeds up to approximately 1 m/s. It is important to adjust the injection rate so that the velocity of injection matches the air velocity past the injection tubes, otherwise premature turbulence in the smoke filaments is likely to occur. It is recommended that students gain a little experience in setting the air velocity and smoke injection velocity with the working section clear of any models, before proceeding to visualisation of flow around various bodies.

Operation

Important – make sure that you use this equipment in a well-ventilated area and the wind tunnel outlet is directed to a suitable air extractor.

Make sure the wind tunnel fan is off. Fit your chosen model into the working section.

Put the smoke generator onto the wind tunnel bench top as shown in the picture at the start of this section. Connect the outlet of the smoke generator to the smoke comb connector. Refer to Appendix B for full details on operation of the equipment.

Typical Results

Various objects may be placed in the working section and the flow pattern observed.

Three examples of the models supplied for use with the apparatus are described briefly here. Figure 10.2 shows the flow around a circular cylinder. The motion over the front part of the cylinder is steady as indicated by the almost unwavering smoke filaments. Separation occurs at around 80° from the front of the cylinder. A wake forms, which is shown to be unsteady by the mixing of the smoke. The unsteadiness is transmitted to the flow outside the wake. It is interesting to note that the pressure in the separated flow, as indicated by the surface pressure readings recorded in Figure 5.6, is almost constant.

Figure 10.2 Flow around a Circular Cylinder

Figure 10.3a Flow over an Aerofoil - Small Incidence

Figure 10.3b Flow over an Aerofoil - Large Incidence

Figure 10.3a shows flow over an aerofoil at a small incidence. The flow remains attached to the surface over almost the whole chord; this represents the normal or

‘unstalled’ condition, at which useful lift is generated, while the drag is comparatively small. The lift is due to the difference in pressures on the upper and lower surfaces of the aerofoil. Over the upper surface, convergence of the smoke traces indicate an acceleration of the flow, particularly over the first quarter of the chord, and this is accompanied by a fall of pressure which contributes much of the total lift on the aerofoil.

Figure 10.3b shows what happens if the angle of incidence is increased too much.

The flow no longer sticks to the upper surface but separates, causing stall. The lift is reduced considerably, and the drag increases as a consequence of the wider wake which results from the separation.

Figure 10.4 Flow through a Sharp-Edged Slit

Figure 10.4. shows an example of flow through a sharp-edged slit. The contraction of a vena contracta at approximately one half-slit width downstream of the edges can readily be seen.

Further Observations

1. Extend the observations to flow around a flat plate placed across the stream, flow along a long straight, flow through a round-edged slit, flow through a convergent-divergent pipe, and other cases which come to mind. Sketch the flow patterns and identify unsteady zones, separation points and so on.

2. The photographs of Figures 10.2 to 10.4 were taken with the smoke filaments running in the mid plane between the front and rear walls of the working section. Repeat these tests and observe what happens when the smoke is directed close to the front wall. If different patterns are now observed, what inferences may be drawn? Do you consider that secondary flows are present, and if so, how do they arise?