Expert: Paul Soderman - 2/9/2006

Question
Paul,

Thanks for the input.  As you can tell this project was somewhat mismatched (and to think I was originally going to major in Aeronautical Engineering...) but we are trying to overcome....

Without boring you with all the details, the tunnel is being designed to allow a MAV to achieve free flight inside the tunnel.  Not really novel but a continuation on an idea that was designed in the 60's.  Anyways, what we are interested in is flow quality in the test section.  We want to see how good our design is in terms of flow inside the wind tunnel.  Not interested in flow separation over an object or anything like that, just the quality of the undisturbed flow and maybe extent of the boundary layers.  It is pretty much impossible for us to achieve the same reynolds number because we are using a 1:12 scale (meaning we would have to use 12 X Velocity or 12 X Pressure -- impossible with our budget constraints).  You mentioned little tricks that aerdynamicists use to achieve this... is this too complex or are there resources that would be helpful to a senior student like myself?

Thanks again!

-Alex

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Followup To
Question -
Hi Paul,

I stumbled across your question section at the website here and was pleasantly surprised to find out that you had significant experience in wind tunnels.  
I am a senior at Florida State University majoring in Mechanical Engineering.  For our senior design project (year long "real life" project) we are building a scaled model of a wind tunnel we have designed.  (yes not really Mechanical but more aeronautical... long story <Sigh>)  
We are putting it together out of plexiglas but have run into a slight problem with the contraction cone.  We cannot shape this cone out of acrylic sheets (mainly expense and availablity of parts) so we are thinking of using either a fiberglass mold or cutting it from wood (which would then be polished).  I realize you probably weren't very involved in the manufacturing process but do you know of any industry/trade secrets for shaping the contraction cone? (We are using a square cross section)  

Along with this goes the question:  How accurate will this model be of the full scale version?  Not an actual question but if you have any resources/books on scale design/theory I would appreciate it.  (I have been looking up Buckingham's pi theorem but think there is more to it)

Again, thanks for your help!

Alex
Answer -
Hi Alex

Sounds like an interesting project.  I can't imagine a big difference between plexiglass and smooth wood for a wind tunnel surface.  The larger factor for wind tunnel performance is geometry.  The contraction cone shape is critical and is not a trade secret.  A reference that comes to mind is Aeroacoustic Measurements, Thomas J. Mueller, ed., Springer-Verlag, Berlin,  2002.  Mueller describes a wind tunnel design at Notre Dame U. that works well and involves square-to-round transition as I recall.  There are many other examples in the literature.

Your question about scale effects is hard to answer because you didn't give me a scale or an objective.  The Wright brothers did very well with a small wind tunnel.  But the reality is that the aerodynamic effects of model testing depend on Reynolds number.  The closer the model Reynolds number is to flight, the better will be the results.  However, aerodynamicists have many tricks for correcting the data for Reynolds number, so the simulation depends on the accuracy of the modeling and the experience of the engineer.  Same old story.

Good luck.

Paul

Answer
Hi Alex

I am not sure what a MAV is (micro air vehicle ?), but I think a scale of 1/12 should work well for you.  Many wind tunnels are at similar scales.  I shouldn't have used the word 'tricks' relative to Reynolds number effects because the extrapolations depend more on understanding than on easy fixes.  For example, if you measure drag on a sphere at one Reynolds number, you can extrapolate to other Reynolds numbers using the experimental data compiled by many researchers throughout history.  I recommend Schlichting's classic book: Boundary-Layer Theory for much information on these effects.  Likewise, you can search any of the NASA library records for studies of aircraft stall, which depend on Reynolds number.  

For your purposes, however, it seems that your main concern is how flow quality changes with Reynolds number.  Again, I would refer you to Schlichting to see how turbulence and laminar flow change with speed.  I assume you are going to measure velocity and turbulence distributions.  You should be able to generate a good realistic flow for a small vehicle like a MAV.  Free flight is going to be the challenge.

Paul