Expert: Paul Soderman - 12/18/2008

Question
QUESTION: I was trying to calculate wind tunnel blockage for H-rotor.
Can you please give me the expressions or atleast tell me as to how to calculate blockage correction for wake blockage, individual corrections for lift, drag and momments.
Can we algebraically add solid blockage with wake blockage to obtain effective wind tunnel blockage?

Please help!!



ANSWER: Wind tunnel corrections are too complicated for me to describe in this format.  I suggest you read "Low-Speed Wind Tunnel Testing" by Rae and Pope (or their new edition which has a new first author).  The procedures and equations are developed there in some detail.
Paul

---------- FOLLOW-UP ----------

QUESTION: Thank you very much Sir for all your help till now.

I have another four questions to ask you Sir, if you kindly help me-

1. Which of the following expressions for solid blockage correction factor is correct?
1/4 x (rotor frontal area/test section area) or only-

(rotor frontal area/test section area)


2. Can the solid blockage correction factor for wind tunnel testing, i.e. 1/4 x (rotor frontal area / test section area), be applicable for say 20%-50% rotor blockage? I have found in the literature (One of Sandia Lab reports) that they used this correction factor for less than  10% rotor blockage in case of Savonius rotor.

3. Is the rotor frontal area same as rotor swept area?

4. What will be the frontal area of three-bladed H-rotor (giromill rotor)for the given specifications-

i) Blade height - 20 cm
ii) Blade chord - 5 cm
iii) Central shaft length = 25 cm
iv) Central shaft diameter = 1.5 cm
v) Base size - 8 cm x 3 cm
vi) Supporting arms size (6 nos.) - 12 cm x 0.5 cm

To me it is around 198 sq cm.  Is this alright?

KINDLY HELP ME PLEASE..


Answer
It seems you are using the solid body blockage developed for wings and bodies in a wind tunnel.  I suggest you study Heyson's method for V/STOL aircraft in a wind tunnel because that is closer to your rotor case.  I have not done that analysis, but a quick read of the V/STOL section in Rae and Pope - Low-Speed Wind Tunnel Testing tells me that the Heyson method or one of the other V/STOL methods described in the book would be best for you.  You also need to consider flow breakdown induced by a rotor in a wind tunnel as discussed in the book.  Please don't ask me for details as I spent most of my career working with fixed-wing aircraft models in wind tunnels.
Paul