[ChazInMT]

A wind tunnel is like using a chain saw to slice onions, you get an idea about what might be happening, but the slices will look dramatically different.

The 3 primary differences between a wind tunnel and real life are,

1st) In real life, the car is moving through air that isn't. (Assuming a calm day) the drag created by the car is due to the energy required to move the air out of the way, and drag it along behind you. So, calm air just sittin there.....along comes a car, the air needs to move sideways/up/down to allow the car to pass. As the air is being shoved out of the way, some of it gets snagged on the car, or pushed along by it, making it move in the direction the car is going. In order for the air to move, you have to add energy to it, that is your drag. In the worst case, air moving at different speeds to get out of the way, and/or be entrained, creates differing air pressures due to the Bernoulli principle. When these differing pressures create a secondary air movement (Vortex) that secondary movement is also felt as drag, because, the air was just sitting motionless and the passing of your vehicle is what made it move, so the only place to get the energy to be moving comes from the car.

In a wind tunnel, the air is moving, and the car tries to slow it down.....can you see the significance? The wind tunnel car stops the air, the rolling car is starting the air.

2nd) The air affected by the car outside extends out many yards in each direction in front of, to the sides, and up. And by affected, I mean made to move....and we remember from before that making air move means energy expended. Behind the car, the air is in motion for many hundreds of yards before it settles back to being still and just sitting there again. Believe it or not, those air molecules hundreds of yards behind are still sort of attached to your car and anything that affects them, will be felt as a change in drag on your car.

In a wind tunnel, the sides and top of the tunnel don't allow the air to move like it would outside, and the air behind is seriously affected too. The rear wake has no opportunity to develop as it would outside, it is either ejected to still air, or turned around to be recycled way before it would have normalized. Either way, you lose how the air reacts outside, and thus have a different drag number.

3rd) The ground plane outside has still air sticking to it, as you drive over it, the air stuck to the bottom of the car is moving in the direction of the car. So outside, there is a shear effect happening, the air is essentially moving in 2 directions.

In a wind tunnel, the air on the ground and the bottom of the car are moving in the same direction....front to back. This is a dramatic difference, the air is moving very differently, thus, a different drag number.

There are other nuanced effects as well I'm sure, but these 3 are fairly glaring.

This is not to say that wind tunnels are complete crap, just that their results may not represent what a cars Cd will be in the wild. However, if you just want to see how one design element variation changes the aerodynamic characteristics of the car, it should work well. If nothing else, you learn a lot in how to conduct experiments, record data, and report the results.

I hope this helps you to understand things a little better. There is way more to aerodynamics than meets the eye. Also, what our intuition tells us is true, may need to be thought out much more deeply to understand the reality of it.

Aeroguy, I'm very heartened by your measured responses in this thread, and you seem to be a wicked smart person as well. With a keen intellect and good people skills, I'm sure you are going to be a massive success in what ever you choose to do in life. People like you make this forum very rewarding to participate in. Thanks for being here.