Drag reduction modifications in wind tunnel: Audi A2 from Cd 0.288 to 0.204

[California98Civic]

Originally Posted by NeilBlanchard

... I think the ideal thing would be to extend the moving belt out ahead of the car.

That's true. I wonder why they don't do that? Probably expense, I would bet. Maybe they can use the tunnel for more versatile types of testing if the Belt doesn't extend over the entire floor?

[Vman455]

Originally Posted by NeilBlanchard

Right, and vacuuming it off has the effect of lowering the air pressure under the front of the car. Combining the vacuum slot and the moving belt, this could artificially give downforce on the front of the car.

I was just thinking about this more--over the stationary floor, the velocity at the floor/boundary layer interface is 0, so all the kinetic energy of the airstream there is being converted to pressure. That could make the pressure under the front of the car overly positive if not corrected, and suctioning it off could address that.

[aerohead]

Originally Posted by Vman455

I was just thinking about this more--over the stationary floor, the velocity at the floor/boundary layer interface is 0, so all the kinetic energy of the airstream there is being converted to pressure. That could make the pressure under the front of the car overly positive if not corrected, and suctioning it off could address that.

The air immediately adjacent to the floor boundary is always at zero velocity,whether in a calm,or moving air mass.The difference would be whether or not a boundary layer was present,either laminar,or turbulent.For the size of a full-scale model,it would always be a turbulent boundary layer.
With a stationary-floor tunnel,without suction,you'd not only have a turbulent boundary layer forming and growing on the underside of the model,you'd also have one present and growing under the model on the floor.This creates 'Wall-Effect',choking off the inviscid free flow,accelerating it to an artificial high velocity and lower static pressure.This wouldn't happen in the real world.It is for this reason that you'd want to vacuum that layer away.Or provide a curved wall/ceiling to compensate for the blockage effects of the model in the test section.DARKO and A2 use this.

[TexasCotton]

Thanks for thread

[Vekke]

I am doing CFD simulations with the a2 3L basic body shape. Seems like 230mm diffusor extensions at the same angle is good for -0,013 drop in drag coefficient.
280mm long was good for -0,015

[Vekke]

Here comes few pictures on the topic



Best angle of attach was 0 degrees in cfd



3d printed air curtains in place in 3d model which is then rendered to look like car.



New rear spoiler



Rear fenderskirts+ rear air curtain.
, front tire deflectors, wheelcovers,front air curtains

More to come the diffusor and few other tweaks. The smooth covers are too dull looking so will put aluwheels with blocking plates to the model also.

[Phase]

wonder if those side skirts are hurting or helping the aero. they stick out a bit

[MeteorGray]

Protruding fenders usually indicate an effort to cover tires that are sticking out, which will prevent related traffic tickets. If so, the effect on fuel economy is probably a wash.

[Vekke]

Side skirts hurt, that are those narrow shapes at door lower edge. To protect the door from bumbs etc. They come out about 30mm which is too much. Will take them out.

Rear fenders come out about 40mm so thats why I think that rear air curtain will work. Shape still needs tweaking

[Phase]

Originally Posted by Vekke

Side skirts hurt, that are those narrow shapes at door lower edge. To protect the door from bumbs etc. They come out about 30mm which is too much. Will take them out.

Rear fenders come out about 40mm so thats why I think that rear air curtain will work. Shape still needs tweaking

The Prius and a few other cars use small side skirts I’ve noticed. Surprisingly the Ioniq doesn’t use any. I was thinking of adding some small side skirts to help guide the lower air over my custom rear fender/wheel spats/skirts.