If a still or video camera and tripod are available then the camera should be mounted in front of the round hatch to give a good field of view around the cylinder. A plain background behind the wind tunnel is advisable (e.g. a plain sheet of white paper may be attached to the back of the tunnel, on the outside). Select camera settings that give the fastest possible shutter speed and then the best possible depth of field at that speed. The use of flash may cause inconvenient reflections on the working section sides, so where available use sufficient lighting to avoid the need for flash For best pictures the flow visualisation thread should be in sharp focus, which can be difficult to achieve especially if using autofocus. If the camera can be pre-focussed then it is possible to temporarily insert a focus guide such as the Pitot tube arm or a similar narrow object, through the roof tapping. Once the camera is focussed on the guide, the guide should be removed and the blanking plug replaced. If possible, take a test shot and display the results at a reasonable size to check that images will be acceptable.
If a tripod is not available then the camera can be hand held, but good results may be more difficult to obtain. An assistant to the camera operator is suggested who can make adjustments to the equipment as required, e.g. to position then remove any focussing guide.
Procedure
Check that the fan is set to 0%, then switch it out of standby mode by selecting the
‘Fan On’ button on the mimic diagram.
Check that the manometer readings are all the same at zero velocity.
Select the results sheet in the software and rename it to ‘40%’.
Measure the ambient temperature in Celsius and pressure of the laboratory in Pascals and enter the results in the appropriate boxes on the mimic diagram.
Gradually set the fan to 10% in 1% increments by using the up arrows. This allows the fan to start up gradually. Check that all fittings on the tunnel remain secure and that there is no safety hazard due to the inlet and outlet air streams. Gradually set the fan to 40% by typing in speed increments of 10% until 40% is reached. Be aware of the surroundings when operating the wind tunnel, keeping safety in mind at all times.
Allow time for the fan to stabilise at 40%.
Exercise D
Adjust the height of the flow visualisation tube and the length of the thread to give a good curve above the cylinder. Sketch or photograph the curve. Shorten the thread until the end trails immediately in the cylinder wake, investigating possible turbulence (it is difficult to visually record this unless a motion camera is available).
If using the C15-11 inclined manometer, take a reading for the water level in all the columns and enter the results on the mimic diagram. It is also possible to move the cursors along the tubes to match the readings, giving a clearer visual representation of the pressure variation around the cylinder.
Select ‘Cylinder’ in the ‘Model Used’ box.
Log the sensor readings by selecting the icon.
If the wake survey rake is available, create a new results sheet using the icon and rename it ‘Wake 40%’. Disconnect the pressure cylinder and connect the survey rake. Select ‘Rake’ in the ‘Model Used’ box. If using the C15-11, enter the new manometer readings on the mimic diagram. Log the sensor readings by selecting the icon. Disconnect the wake survey rake and reconnect the pressure cylinder.
Create a new results sheet using the icon and rename it ‘60%’. Select ‘Cylinder’
again.
Increase the fan setting to 60%.
Repeat the flow visualisation and pressure sensor logging as before. If using the wake survey rake, create a new sheet for ‘Wake 60%’, set the ‘Model Used’ to ‘Rake’
and connect the survey rake to take a set of readings.
Repeat at 80%. Remember to create a new results sheet and rename it each time, and to select the correct model for each set of readings.
Gradually shut down the fan: Type in a value of 50% for the fan setting. When the fan has slowed, type in a value of 20%. Once the fan has slowed again, reduce the fan speed to 0% by using the arrow keys.
Set the fan to Standby by selecting the ‘Fan On button in the software.
Save the software results by selecting ‘Save As…’ from the File menu. Give the results a suitable name for future reference, such as the equipment code, experiment letter and date.
Switch off the mains switch on the IFD7.
Results
For the Cylinder results, the software calculates the theoretical pressure at each tapping point around the cylinder, and the Reynolds number at that free stream velocity. For each fan speed setting, plot a graph of Theoretical Pressure and Surface Pressure against Tapping Position, and note the Reynolds number for each setting.
For the Rake results, the software calculates the pressure and flow velocity for each prong position, and the Reynolds number at that free stream velocity. For each fan speed setting, plot a graph of pressure against position. On the second y-axis, plot the flow velocities. Note the Reynolds number for each graph.
Conclusion
Describe the visual observations made of flow around the cylinder. What shape did the streamlines form around the cylinder? How did this vary with free stream velocity? Was turbulence a significant element of the flow pattern? Did this change with Reynolds number? Include sketches or photos to illustrate your observations.
How well does the theoretical prediction of surface pressure correspond to the measured pressure? Does the accuracy of the theoretical equation vary with Reynolds number? If so, how?
Describe the pressure variation across the wake. Relate this to the visual
observations of the streamline paths and the appearance of turbulence. How did the wake vary with free stream velocity/Reynolds number?