Wind Tunnels - Rules of thumb?
Thread Starter
Wind Tunnels - Rules of thumb?
As part of a college aerodynamics project, we are supposed to make and test some aerodynamic shapes in the college's recently acquired wind tunnel.
At present we only have the tunnel, which is approx 18" x 18" (450 x 450mm) in cross-section and of plywood construction. The fan and motor, which have not yet been fitted, I am told should give a maximum airspeed through the octagonal working section of between 40mph and 80mph (roughly 18 to 36 m/s).
The test models are likely to be relatively simple shapes and aerofoils, and will be pitched at between 5° and 20°. Observation of tip vortices and boundary layer effects are likely to be the main objectives, although a balance giving lift and drag values is planned.
My question is this: What span of model would be the maximum that could be sensibly tested in an 18" cross-section, before wall effects seriously upset the results?
At present we only have the tunnel, which is approx 18" x 18" (450 x 450mm) in cross-section and of plywood construction. The fan and motor, which have not yet been fitted, I am told should give a maximum airspeed through the octagonal working section of between 40mph and 80mph (roughly 18 to 36 m/s).
The test models are likely to be relatively simple shapes and aerofoils, and will be pitched at between 5° and 20°. Observation of tip vortices and boundary layer effects are likely to be the main objectives, although a balance giving lift and drag values is planned.
My question is this: What span of model would be the maximum that could be sensibly tested in an 18" cross-section, before wall effects seriously upset the results?
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Before you dive in to the testing of models, I would suggest that you determine the working characteristics of your tunnel, if none are available. This looks like a project ripe for some sound tunnel calibration or behaviour characterisation, such as boundary layer thickness measurements, wall interference, blockage experiments, etc. Otherwise you'll never know if your models are too big, or capable of being made bigger!
Enjoy!
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Whilst I don't know the answer (being an avionics chap and not allowed to play with wind tunnels) I've just spent a year at Cranfield where they have a couple of them, so could certainly put you in touch with people who should be able to answer your question.
Best try and get it through the library given the price. I think it's been the bible for 40++ years in various versions.
As you are a student, get yourself a slot booked at the National Aerospace Library at Farnborough who should be able to find all the guides you want.
(N.B. Probably mount it on one wall and go straight across to the other wall for 2d shapes, for 3d shapes, I'd reckon on either mounting it on one wall and not going more than 2/3 across, or in the centre taking up no more than half the tunnel width as a crude rule of thumb. But I do agree with I-M about characterising the tunnel.)
G
Thread Starter
Thank you IM, Glum & Genghis.
I will certainly investigate the National Aerospace Library. I had not heard of it until last week. 'Low Speed Wind Tunnel Testing' is in our college library, but is reference only i.e. cannot be take out on loan. I will take a good look at it though.
We have a rack of 14 manometer tubes so some degree of tunnel characterisation should be possible.
I will certainly investigate the National Aerospace Library. I had not heard of it until last week. 'Low Speed Wind Tunnel Testing' is in our college library, but is reference only i.e. cannot be take out on loan. I will take a good look at it though.
We have a rack of 14 manometer tubes so some degree of tunnel characterisation should be possible.
