W.J. Devenport and W. L. Hartwell
Last Modified December 20th, 2006

1. Introduction

This laboratory is designed to

  1. give you hands on experience of the smoke flow visualization technique, and
  2. provide you with an opportunity to use this technique to study several wind tunnel flows.
In general, flow visualization is an experimental means of examining the flow pattern around a body or over its surface. The flow is "visualized" by introducing dye, smoke or pigment to the flow in the area under investigation. The primary advantage of such a method is the ability to provide a description of a flow over a model without complicated data reduction and analysis. Smoke flow visualization (Barlow et al., 1999)  involves the injection of streams of vapor into the flow. The vapor follows filament lines (lines made up of all the fluid particles passing through the injection point). In steady flow the filament lines are identical to streamlines (lines everywhere tangent to the velocity vector). Smoke-flow visualization can thus reveal the entire flow pattern around a body.

You will be able to use this technique in a small smoke tunnel to study a number of flows, including particularly the flow past a stalled airfoil. Generating a flow in a wind tunnel that accurately models the flow over a real vehicle or vehicle component can be a lot harder than just having a model the right shape. During the course of this laboratory you will be able to investigate some of these problems. In particular you will be able to examine:

Reynolds number effects:  For an airfoil Reynolds number is usually defined as  where  is the free stream velocity, c the wing chord and the kinematic viscosity. It represents the typical ratio of the scale of inertial forces to that of viscous forces in the flow. Most engineering devices operate at large Reynolds numbers. A airplane wing with a chord length of 7ft, flying at 150 km/hr at sea level has a Reynolds number of 5,700,000. A hydroplane 1m in chord traveling at 35 knots (18 m/s) has a Reynolds number of 19,400,000. Reproducing such Reynolds numbers in a wind tunnel with a small model (say 0.25m chord) is usually not possible. Much wind tunnel work therefore relies on the assumption that flows remain largely unaltered by increase in Reynolds number. This is not always the case.

Blockage effects:  Blockage may occur as a result of the solid walls of a test section constraining the flow as it moves around a model. This constraint, which does not occur in free flight, increases the velocity of the flow as it passes the model, altering the flow pattern and characteristics. In the case of an airfoil, solid blockage tends to make the flow over an airfoil stall earlier than it otherwise would. This is because boundary layer separation occurs when in regions where the flow is decelerating, i.e. there is an adverse pressure gradient. The stronger the gradient the sooner separation will occur. Blockage increases the magnitude of the deceleration and thus the adverse pressure gradient produced by an airfoil. Interestingly, the reverse effect occurs in an open-jet wind tunnel. Here the wind-tunnel 'wall' is formed by the edge of the jet, and thus it can be deformed by the flow over the model. This deformation tends to reduce the magnitude of the accelerations and declarations experienced over the model.

2. Apparatus, instrumentation and methods

A. Instrumentation for measuring the properties of the air.
The wind tunnel you will be given to use in this experiment uses the laboratory atmosphere as the working fluid. The properties of the air in the lab vary depending on the weather so it is important that at some stage in your experiment that you measure them, so you know what fluid you are working with. From the point of view of the dynamics of the air, the important properties are its density and viscosity (think of Bernoulli's equation and the Reynolds number).

Rather than measuring density directly, it is best obtained by measuring pressure and temperature and then using the equation of state for a perfect gas. An aneroid barometer for measuring atmospheric pressure is provided on the side of the open-jet wind tunnel control panel which is in the same lab as your experiment (figure 1 ). A digital thermometer for measuring atmospheric temperature is located on the side of the open-jet tunnel next to the test section (also shown in figure 1 ). Pressure is read in milliBar (1 milliBar=100Pa). Temperature is read in degrees Celsius or Fahrenheit, depending on the thermometer setting. The gas constant R in the equation of state for a perfect gas (p =RT) is 287 J/kg/K.

The temperature can also be used to infer the dynamic viscosity of the air using Sutherland's relation. For SI units,

where T is temperature in Kelvin. Recall that kinematic viscosity  is dynamic viscosity divided by density. You can program Sutherland's relation in your Excel logbook, or use the calculator below:

  • Input the temperature in Kelvin  K
  • Press 
  • Read off the dynamic viscosity  kg m-1 s-1
Now might be a good time to start making entries into the logbook, noting the atmospheric conditions and properties.

B. Smoke flow visualization wind tunnel and equipment
A small vertical smoke tunnel (figure 2 ) will be at your disposal. The tunnel is an open-circuit design, powered by a small variable speed electric fan. The top mounted fan pulls air from the room through a turbulence screen into the lower entrance, which measures 35.6 cm x 17.8 cm. From the entrance the tunnel converges to the test section, which measures 10.2 cm x 17.8 cm in cross-section and is 25.4 cm long. Downstream from the test section, the tunnel converges to a 12.7 cm diameter circle at the fan. The speed of the flow in the test section is controlled using the dial located at the bottom right corner of the facility. The calibration that relates the flow speed in the test section to the dial setting is shown in figure 3 . The uncertainty of this calibration is not known, but it is probably no better than 10%.

The "smoke", which is vaporized kerosene, is produced using a Preston-Sweeting mist generator (figure 4 ). The resulting vapor is piped via a black rubber hose into a strut located directly upstream from the test section. A series of equally spaced holes in the trailing edge of the smoke strut introduce smoke filaments into the flow.  During operation of the smoke tunnel it may be necessary to give a sharp squeeze to the black rubber bulb attached to the end of the strut. This will clear anything that may be blocking the smoke holes. In addition, the black rubber hose connecting the smoke generator to the strut should be drained periodically or whenever the vapor flow is poor. To drain the hose, remove the end connected to the smoke generator, hold it towards the floor and give it a few shakes. Take precautions not to shake the kerosene on your pants leg, as this is easy to do.

 The tunnel comes with some additional supplies and equipment including,

  1. extra kerosene
  2. NACA 2412 airfoil model (section shown in figure 5 ), cambered flat plate wing model, sphere model and cylinder model, figure 6 , (your TA can show you how these are attached if you ask).
  3. A modified test section window, that reduces the effective width of the test section
  4. Also provided are calipers, a metal ruler and tape measure so you can measure the model and wind tunnel dimensions and a overhead projector pen that can be used to temporarily used to mark the wind tunnel window if you desire.
  5. Digital camera and tripod

3. Practical work

A. Getting familiar with the equipment
    The following procedures are designed to help you get a feel for the smoke tunnel, its models and peripheral equipment. It is important that everybody get a hands on feel of how to use the apparatus and what its capabilities and problems are. Feel free to play with the apparatus at this stage, but don't forget to record your impressions in the logbook.

B. Designing and implementing the smoke flow visualization experiment
Now that you are familiar with the capabilities and limitations of the smoke tunnel and smoke flow visualization system, you are ready to exploit them to achieve a scientific goal. Choose goals from the following list. (You may also modify these goals or choose a different goal of your own, but that goal must be scientific, and clearly stated in the logbook). Note that your grade does not depend upon how many goals you achieve, but on how complete, careful, scientific and documented your work is. For example, if you only complete one goal, but you document a systematic, detailed, and careful study, you have done well. In addition your grade does not depend upon how close your results agree with any other pre-conceived ideas of what the answers should be. Instead it depends upon how open mindedly and objectively you assess your results, their limitations, and what they appear to show.  (e.g. If the stall behavior of the airfoil disagrees with expectations, or is upset by tunnel imperfections, you are expected to report exactly that.) The group should leave few minutes at the end of the lab period for discussion and to check that everybody has everything they need. As a group go through the exit checklist.

4. Recommended Report Format

Before starting your report please read all of appendix 1 . Check out the report grade sheet for this experiment, available in appendix 1..

Title page
As detailed in appendix 1.

State logical objectives that best fit how your particular investigation turned out and what you actually discovered and learned in this experiment (no points for recycling the lab manual objectives). Then summarize what was done to achieve them. Follow this with a background to the technical area of the test and/or the techniques (e.g. the fluid dynamics and/or the experimental techniques you are using). This material can be drawn from the manual (no copying), classes, textbooks from prior courses, references cited in this chapter or even better, other sources you have tracked down yourself.  Finish with a summary of the layout of the rest of the report.

Apparatus and Instrumentation
 It is probably easiest to begin by describing the smoke tunnel itself. Include a diagram or labeled photo of the tunnel. Describe the figure in the text of the report including all the details that may be important to the flow it produces (e.g. open circuit, contraction ratio, dimensions and shape of the test section, type of vapor used, method of vapor production, method used to introduce vapor to the flow, location of smoke strut, flow speed uncertainty etc.). Next describe the model(s) you used. Diagrams or photos will be needed. Give the model dimensions (span, chord, diameter, airfoil shape designation) and location(s) that the models were placed in the test section (with the airfoil you should also include the chordwise location about which it was rotated to angle of attack). Don't forget to mention imperfections in the models and any uncertainties. Now mention the techniques used to make the measurements. Include in particular the digital camera, tripod and location. If you measured positions directly off the wind tunnel window, talk about how that was done (and the accuracy). Mention the thermometer and barometer, their accuracies, and what they were used for.

Results and Discussion
Before writing the results and discussion make sure all your results are analyzed and plotted, your photographs are properly annotated and labeled. Make sure your plots are formatted correctly - default Excel plotting format is not acceptable, see appendix 1 .

 A good way of writing this section may be to tie each set of tests and results to your objectives stated in the introduction. (If you find it hard to do this, try changing your objectives!) For example, you might begin with "Photographs of the smoke flow visualization were made with the airfoil model at between -15 and 15 degrees in order to define its stall behavior as a function of angle of attack. Figure ?? shows photographs of the flow at 3 degree increments in angle of attack. Table ?? and figure ?? show measurements of the chordwise location of stall at 1 degree increments in angle of attack. Included in figure ?? are error bars showing the uncertainty in the indicated stall locations...". Before you can really talk much in detail about variations seen with whatever parameter you are studying you will probably need to describe one case in detail, e.g. "Figure ??, which shows the flow pattern at 6 degrees angle of attack is typical. Flow approaching the airfoil stagnation point is deflected upwards and .... The streamlines passing over the top surface of the ..." Then talk about the variations, introduce your plots describe their axes, the symbols used and then discuss what they show". Don't forget the plots that show wind tunnel effects, e.g. "Figures ?? to ?? show flow through the test section for the same conditions as figure ?? but with the model removed. Comparing the figures measured at corresponding Reynolds number, some of the change in position of the streamlines with Reynolds number can be seen to be a inherent effect of the wind tunnel, and this would appear to bring into question...". Also remember to include any uncertainty estimates in derived results (such as for Reynolds number) here. You should reference a table (copied out of your Excel file) or appendix containing the uncertainty calculation.

Make sure your results and discussion include (and justify) the conclusions you want to make and that those conclusions connect with your objectives.

Begin with a brief description of what was done. Then a sequence of single sentence numbered conclusions that express what was learned. Your conclusions should mesh with the objectives stated in the introduction  (if not, change the objectives) and should be already stated (although perhaps not as succinctly) in the Results and Discussion.

5. References
  1. Barlow J. B., Rae W. H. and Pope A., 1999, Low-Speed Wind Tunnel Testing, John Wiley & Sons, chapter 5.

Figure 1. The Open Jet Tunnel.

Figure 2. Smoke Flow Visualization Tunnel

Figure 3. Smoke Tunnel Velocity Calibration

Figure 4. Preston-Sweeting Mist Generator

Figure 5. NACA 2412 Airfoil Section

Figure 6. Smoke tunnel models

Figure 7. Sample smoke flow visualization for the airfoil model.