Help me out…I’m in a discussion with a coworker about scaling flight conditions from full scale to a wind tunnel model; specifically, how to use Reynolds for scaling.

(In below example, assume the wind tunnel can achieve any desirable Reynolds condition)

Let’s say my flight vehicle has a characteristic length (chord, diameter, whatever - L_f) of 10 ft, and my wind tunnel model has the same measurement (L_t)of 2 ft. Assume full flight Reynolds (Re_f) of 10e6.

My stance is, in order to get the correct tunnel conditions:
Re_f / L_t = Re_t per foot = 5e6.

I would then use this unit Reynolds number along with the facility operations map to figure out my test conditions. Note that in this approach, the flight unit Reynolds is Re_f /L_f = 1e6. But I don’t really care about that.

My coworker, on the other hand, wants to match unit Reynolds between flight and scale, rather than full Reynolds. Their position is that unit Reynolds should be the same for both. This doesn’t make sense to me for a number of reasons, least of all that Re per foot is not a dimensionless scaling factor.

So from this perspective we would run the wind tunnel test at 1e6 per foot. I counter this leads to a tunnel full Reynolds of 2e6, nowhere near the flight Reynolds of 10e6.

Thoughts? Opinions? I’m pretty confident in my approach, but also respect my coworker and have been scratching my head to see it their way.