Wind tunnel

[Nacho-RT74]

Even these videos shows, winged cars, the reason why not post them on aero section is because the question is about the tunnel itself. I allways imagined the tunnel with blowers in front, blowing out, and not sucking in from rear... why?

http://wowmusclecars.com/1971-superbird-daytona-wind-tunnel-test/amp/

[JB400]

Air being sucked into a fan would be straight, whereas the air being blown in would be turbulent and need to be run through ducting to diffuse it.

[Nacho-RT74]

But outside the winds "blows", so is the read deal

[ws23rt]

The wind outside sometimes blows and in all different directions and speeds.
A car running at speed on a track is normally doing so on a calm day through still air. A wind tunnel is the only way to test airflow as it would be at speed on a calm day.
Either move the car (at speed) for testing or move the air instead.
Laminar (straight line flow of air) airflow is ideal. Apparently sucking the air is better than blowing the air to replicate still air for testing.

[Nacho-RT74]

I understand everything. Many variables outside, but still running a car is going ( average ) against the wind, is the same than the air blowing over the car and not sucking from behind...

[ws23rt]

I understand everything. Many variables outside, but still running a car is going ( average ) against the wind, is the same than the air blowing over the car and not sucking from behind...

As Jb400 pointed out. The fan creating the air flow will create huge turbulence by blowing on the test object. That inherently upsets the whole idea of the test which is to see how CLEAN (laminar) air flows over the object in test.
A car running at speed on a calm day will feel calm air flow. Not turbulent air flow.

[odcics2]

Another variable are turning wheels. They present their own set of turbulence, which can't be duplicated in a wind tunnel.
One reason that testing reverse facing "scoops" over the front tires in a tunnel made no sense, so they were never tested.

Some things just have to be done on a full scale car, out in real time track testing.       

[XH29N0G]

My son is learning aero in school, works in a wind tunnel and has been modelling the airflow on the front element of their Formula car.  He said technically the wind tunnel is a loop so it pulls and pushes air, but he said that it pulls near the thing being tested because turbulence will kill everything.  He also said that the walls mess things up and that even their small formula car (which you can walk around in the wind tunne) is too close to the walls to be effective.  I asked him how they did the GT40 tests (apparently his was used in the 60's fo rthat) and he said they were 1/5 to 1/2 scale models.  It is fun to talk with him. He is finding (says there are) all sorts of strange things happen because of nearby parts (or the ground).  He is building the front wing on their formula car and he said he got a 30% reduction in drag by considering the other parts, and has a wing with less downforce, but it works better with the body so there is a net downforce gain, and that it all has to be integrated.  So I can believe the wheels spinning could have an effect, but don't have a clue (myself) about how big it would be.

Mind you, this is the same kid who seven years ago came into the house with a broken RC car and told me that he learned that cars don't steer when they are in the air.  His car hit a wall after going airborn.

Me, I'm still at the cars don't steer when they are in the air.  I can't do the math he does and just am happy he can and has found something he likes