Expert: Paul Soderman - 11/23/2010

Question
Dear Paul,

I am trying to design a open section closed circuit wind tunnel with circular test section of 1m diameter. And I need a flow velocity of 15m/s in the test section.  I am having trouble of designing. Should i start with the geometry of the wind tunnel first or start doing the required power calculation of wind tunnels to find the fan size and power needed to get the flow velocity and work towards the geometry of the wind tunnel. I would be great help if you could help me to point out to the correct step of designing wind tunnel.

Thanks
Dunil

Answer
Dunil
You need to make a list of requirements.  For example, a wind tunnel should be as efficient as possible with a high energy ratio, which is the ratio of energy in the air at the test section to the input energy at the fan. To do that you need a high contraction ratio so the air moving in the return legs is at low speed and low loss compared to the test section.  But you will also have real estate and construction cost requirements.  Corner vanes, for example, must be aerodynamically efficient, but can be expensive.  You also must have requirements on the kind of testing you want to do and what flow quality that requires.  High flow quality leads to special screens and honeycomb sections.  One way to start might be to choose a good existing wind tunnel and scale to your test section.  Examples are give in W. Rae and A. Pope:  Low-Speed Wind Tunnel Testing, John Wiley & Sons.  Rae and Pope also give wind tunnel and fan design guidelines.  Then do your power loss analysis.  In any system design, you will want to iterate the design until as many things are optimum as possible.  A spreadsheet that allowed you to change the variables and see the result would be helpful.  The paper by Eckert et al that I mentioned before could help with that.

Incidentally, open-jet wind tunnels have poor flow quality near the shear layer, so the usable test area will be less than 1 m diameter.  See my NASA TP 3020 for data on the DNW open jet wind tunnel (fig. 6).
Paul