Expert: Paul Soderman - 4/6/2006

Question
I have been researching into blockage corrections for wind tunnel testing; namely the "area ratio method" suggested by Barlow, Rae and Pope (referenced from Rae, W.H., and Pope, A., Low Speed Wind Tunnel Testing, Second Edition, John Wiley and Sons, Inc., 1984).

When applying a blockage correction to a scale model (Frontal Area approximately 0.013m²) placed in a closed return wind-tunnel wind-tunnel, is it necessary to apply a correction to lift, drag and moment coefficients, as I was only going to apply the correction to the measured drag coefficient? Also can this same blockage correction be applied to the forces mentioned, or are different blockage corrections required?

The blockage correction being used is CD (Corrected) = CD (Measured) / [1 + 1/4*(A/S)]²

Thank for our help.


Answer
Carlo

Wind tunnel walls produce numerous effects on wind tunnel models that are not found in flight.  Rae and Pope list nine effects in the beginning of chapter 6 in my book.  The so-called solid blockage causes an increase in dynamic pressure, which affects all the aerodynamic forces.  In addition, wake blockage primarily affects drag and requires a special correction to drag.  Other factors alter the model angle of attack, which primarily affects lift.  So, to answer your question, you will calculate different corrections for lift, drag, and moments except for the dynamic pressure correction which affects all forces equally.

Paul