Expert: Paul Soderman - 7/6/2008

Question
Hello,

I am currently doing a project about Vertical Axis Wind Turbines and i am planning to build a straight blade version to test in a wind tunnel. The dimenions are approximately 7 m wide by 6.5 m high. My model is planned to be about 2.5 m wide by approximately 2 m high. Do you think there will be significant problems with blockage or will blockage correction suffice for my case. I am ready to scale down my model if i have to.  

Answer
Eshwar
I don't know enough about your model to answer definitively.  If your model was a solid 2 m by 2.5 m, the blockage would be 11 percent, which is high.  Rae and Pope (Low-Speed Wind Tunnel Testing) suggest that a maximum ratio of model frontal area to test section cross-sectional area should be 7.5 unless errors of several percent can be accepted.  Your model, however, is not solid - I assume it has blades which spin around a vertical axis and produce lift and drag.  So your situation may not be as bad as a bluff body.  On the other hand, high lift can cause high wall effects.  I recommend reading Rae and Pope and summing the effects of all blades as if you had one larger wing.  That should give you a feeling for the magnitude of the blockage correction.  Other factors such as test objectives, error tolerance, etc. need to be considered.  Rae and Pope list several references that may help.  Perhaps other wind machines of your type have been studied and reported in the literature.  Some research is needed that is beyond my experience.  Good luck.
Paul