Expert: Paul Soderman - 8/7/2009

Question
Hi Paul,

I am trying to design a wind tunnel capable of producing 125 Knts through a test section that is 200 mm square. I have all the dimensions for the contraction / test section / diffuser I need but am having trouble selecting a fan. I am looking to purchase a vane axial fan that can move about 5,500 CFM but am uncertain of the static pressure across the fan I  will need to be looking for. Is there any equation that can help me calculate this? The wind tunnel is going to be an open circuit design so my total pressure loss isn't of very much importance for me. I just need to make sure I can reach that 125 knts in the test section. Any suggestions would help a lot. Thanks so much.

Answer
Corinn
Fan performance is usually provided as volume or mass flow rate versus pressure head.  So to know where you are on the curve you need to estimate the head delivered by the fan so as to overcome the pressure drop of the circuit.  The dynamic pressure of the exhaust also enters. That is, some of the fan power goes into the energy loss of the circuit and some of the fan energy is thrown away in the exhaust. I suggest finding a report or book that helps estimate pressure drop of wind tunnels and computing how much head the fan needs at the mass flow rate needed to get 125 knots in the test section.  I recall providing that technique in the Appendix of this paper, but I can't be sure as I am out of the office today.


Soderman, Paul T. and Olson, Larry E.:  The Design of Test-Section Inserts for                      Higher Speed Aeroacoustic Testing in the Ames 80- by 120-Foot Wind Tunnel.                                                                                            
NASA TM 103915.  (Also - DGLR/AIAA Paper 92-02031, DGLR/AIAA 14th                                   Aeroacoustics Conf., Aachen, Germany, May 1992).