Well being as I have experience in fluid measurement here is an idea. May be invovled but it is used in high precision applications.

Air is a fluid. As the circumference of a tunnel decreases, the speed of the fluid increases until it reaches a supersonic velocity then the inverse applies. So construct your wind tunnel so that the diameter decreases after the wing that is being tested. Take a pressure measurement before the decrease in size and after the decrease in size. The change in pressure (should go down as velocity goes up) is directly proportional to the velocity. Pressure can be taken with pressure transducers. You would then multiply the change in pressure by a constant and that will give you the velocity. I will calculate the constant if you should choose to use this as it can be complicated and is based on the change in size. Hope this helps.

Travin