# Does Cd number change at different speeds during wind tunnel testing?

When meassure Cd or Cl of airplane/car in wind tunnel, does number change at different speeds, for example at 100km/h compare to 250km/h? If yes,how much?

I know from theory that coefficents change with Re, and Re is change with speed, but I know what real pracital data show when you change airflow speed in wind tunnel?

Because if difference in results is big then we have to agree at what speed test the object..

Aerodynamic coefficients like $$C_d$$ and $$C_l$$ depend at least on Reynolds number, as visible for example in these plots from this answer:  So yes, they depends, among other factors, on speed. Anyway, in order to compare results from different tests, the Reynolds number has to be the same, not just the speed.

And if you are testing some specific phenomenon then also other "numbers" have to be the same: for example if compressibility effect are important, then the Mach number has to be the same. If unsteady characteristics are important (like in a flutter test), then the so called "reduced frequency" (which is also a dimensionless number) has to be the same. If the inertia of the fluid is not negligible (like in hydrodynamics), then the Froude number has to be the same. And so on.

As long as you stay subsonic these values are (near) constants. But the forces acting on the objects do have the speed squared... That's where speed is coming into our equation.

Things do change when nearing the "sound barrier".

See the graph on this question: Acceleration of a supersonic aircraft after breaking the sound barrier

Deviations of that sort can be caused by imperfections in the uniformity of the airflow in the tunnel throat and by the model under test being too big for the throat dimensions, leading to interference between the walls and the model. This sort of thing has to be carefully checked for anytime you get an unexpected result out of a wind tunnel test.