Matching Reynolds Number in Scale Model Wind Tunnel Testing

[Andres]

For my senior design project, I have to reduce the aerodynamic drag of a truck and trailer. I am doing CFD simulations and wind tunnel testing. The dimensions of the wind tunnel (test section) are 94 in long, 13.5 in tall and 20 in wide and a cross-sectional area of 270 in^2 and it could go up to 110 mph. The truck and trailer has a total length of 712 in and a frontal area of 10560.2175 in^2. I'm trying to figure out a way to match the Reynolds Number of the full truck and trailer using a scale down model but have a blockage ratio of less than 5%. The Reynolds Number of the full scale is 34,695,627.05 at a velocity of 65mph and 712 inches for characteristic length. A scale of 1:17 gives me a blockage ratio of about 5% but a Reynolds number of 3,137,150 at 100mph and 41.88 in long, which is nowhere close to the actual Reynolds Number. What do you guys recommend I do about the wind tunnel testing if I can't match the Reynolds Number to the real one? P.S, I need the velocity to be 65mph because at that speed, most of the fuel is used to overcome drag.

Sorry for the lengthy paragraph but I'm stuck.

 

[jack action]

The drag coefficient is the most important dimensionless parameter for your study. The drag coefficient is a function of the Reynolds number. Here are some typical relationships:

For high Re (>3500), you can usually assume the Cd is constant. The exception is for an object with not well-defined fixed separation points (like as sphere) where a dip is seen before stabilizing (usually somewhere around Re = 10⁶).

What you can do is plot the Cd vs Re for your scale model. If you see a dip and it is below your testing Re, then you can assume that the Cd will be similar for your full scale truck (at a greater Re).

You also mentioned CFD simulations, so you can plot the Cd vs Re that way as well to further prove your point (since you won't be able to measure high Re with your scale model).