Wheel well vents

[spacemanspif]

Originally Posted by aerohead

I'm not certain that the wheel well aft of the tire catches any air at all in the context of a parachute.A parachutes drag is created from the all-separation,enormous turbulent wake above it.
A pool of dead air may just travel along with the quarter panel,as with a rear bumper,with the active flow just ricocheting off the dead air.

If there is no air being caught in the well and the air is being pushed around the wheel/wheel well then what is the benefit of the radiused well behind the tire? I'm under the assumption that the radius or vents eliminate/aliviate pressure that causes drag from caught up in the wheel well.

[serialk11r]

Originally Posted by whatmaycome14

Interesting. Does this example works best with a rear engine car, or if like in the case of the Miata, the configuration of the engine doesn't make a difference and the is just the BEST way to get airflow out of the front wheel wells.

Some mid engined cars have the ducts at the front wheels too, so apparently ducting there is useful for cleaning up the aero around the wheel itself, but I think front engined (race) cars might have a bigger vent that they also exhaust cooling air to.

[renault_megane_dci]

Originally Posted by aerohead

I'm not certain that the wheel well aft of the tire catches any air at all in the context of a parachute.A parachutes drag is created from the all-separation,enormous turbulent wake above it.
A pool of dead air may just travel along with the quarter panel,as with a rear bumper,with the active flow just ricocheting off the dead air.

I have never quite understood this dead air theory.
What bothers me is air is highly compressible so the layer of air is more likely a turbulent area for me, with air coming in and out and maybe some air staying deep down.
I can only think about it like coffee cup you would try to fill with the tap wide open. The dead content of it would be quite shallow and you would end up with water all around ...

[aerohead]

Originally Posted by spacemanspif

If there is no air being caught in the well and the air is being pushed around the wheel/wheel well then what is the benefit of the radiused well behind the tire? I'm under the assumption that the radius or vents eliminate/aliviate pressure that causes drag from caught up in the wheel well.

If the wheel well communicates pressure to the forward stagnation point,via the engine bay,and high-pressure cooling system air is escaping from the bay through the well,it can create a 'jet' which upsets the airflow along the sides of the car.(You may have seen this during wet driving).
Professor Morelli spent a lot of time in the Pininfarina tunnel massaging the vents of the CNR 'banana' car such that the air emerged parallel to,and at exactly the same velocity as the surrounding air so as not to trip it into turbulence.
The trailing radii would be an attempt to turn the air parallel to the car sides before it contacts the side flow.
On a Ferrari,where there is a dedicated radiator extractor vent designed into the belly pan,no such air would be blasting out the wheel-wells,and nothing fancy would be required to enhance the flow there.
Remember though,these supercars,while very 'cool' have pickup truck-like drag coefficients,and are designed for high-speed stability more than any other parameter.They're not really a model of efficiency.

[aerohead]

Originally Posted by renault_megane_dci

I have never quite understood this dead air theory.
What bothers me is air is highly compressible so the layer of air is more likely a turbulent area for me, with air coming in and out and maybe some air staying deep down.
I can only think about it like coffee cup you would try to fill with the tap wide open. The dead content of it would be quite shallow and you would end up with water all around ...

Actually,air is considered an incompressible fluid until transonic velocities are achieved,typically 250-mph for cars.
Up to that speed,air is just like water.
If you take an open,full bottle of beer, and submerge it in a fast moving stream,with the neck of the bottle normal to the flow,the beer will just stay in the bottle,with the water skimming over the opening.You might get some resonance (Helmholtz),but that's it.
Depending on the well design,it's possible that air will pool inside portions of it and just travel along with it,with the outer active flow just skimming over it.
Just like a rear bumper cover which pools air (upside-down) under the car,as low as suspension components,with active flow skimming along the pool as flat stones skipping on a lake or pond.
It like shark skin in a macro environment.All the denticles capture dead water in between,and the 'water' touches 'water',not the shark's skin.

[serialk11r]

Originally Posted by aerohead

Professor Morelli spent a lot of time in the Pininfarina tunnel massaging the vents of the CNR 'banana' car such that the air emerged parallel to,and at exactly the same velocity as the surrounding air so as not to trip it into turbulence.
The trailing radii would be an attempt to turn the air parallel to the car sides before it contacts the side flow.
On a Ferrari,where there is a dedicated radiator extractor vent designed into the belly pan,no such air would be blasting out the wheel-wells,and nothing fancy would be required to enhance the flow there.
Remember though,these supercars,while very 'cool' have pickup truck-like drag coefficients,and are designed for high-speed stability more than any other parameter.They're not really a model of efficiency.

So the trailing radii are useless if you don't have air flowing through the wheel wells? Am I reading this correctly?

And I suppose cars that do have the trailing radii or other vents in the wheel well but also dedicated radiator extractor vents, it is there to handle whatever other flow that does make it to the wheel well?

[renault_megane_dci]

Originally Posted by aerohead

Actually,air is considered an incompressible fluid until transonic velocities are achieved,typically 250-mph for cars.
Up to that speed,air is just like water.
If you take an open,full bottle of beer, and submerge it in a fast moving stream,with the neck of the bottle normal to the flow,the beer will just stay in the bottle,with the water skimming over the opening.You might get some resonance (Helmholtz),but that's it.
.

You just made me waste a beer :-/








(Just kidding)

[Smokeduv]

The Toyota FT-1 concept has them also:

[chillsworld]

I like me some race car tech

This is an entire thesis written on the pressure zones and aero of wheel-wells http://www.tfd.chalmers.se/~sinisa/i...st_Jan2010.pdf, just in case anyone wants to read up on it (almost 50 pages) It's based on a rectangle body with smooth surfaces, it is of course theoretical and not real world. Each vehicle would be different.

"The findings indicate that addition of wheel and wheelhouse to basic model increases the drag force by 34% and 38.9% for different cases (the addition of them to a square body for theoretical aerodynamic study). The drag and lift forces are dependent on the wheelhouse depth. Increasing wheelhouse depth increase the total drag and the total downforce forces. The results also indicates that the wheel generates lift for both cases but the bigger wheelhouse volume generates higher lift forces. The results shows the importance of considering the effect of the rotating wheel. Reducing drag forces can be a challenging task to increase the efficiency of the vehicle."

Looking at the bountiful diagrams and graphs, it appears that the wheel well is a source of both high pressure and low pressure... Lift and downforce And the drag comes in the form of vortices off the tire itself not the spokes because the model used a solid wheel like our pizza pan covers, as well as the escaping air on the lower 1/3 of the rear of the opening (the part that you guys want to cut or vent), and the bottom edge of the wheel well below the car.

Here are a few of the diagrams I found of the most interest:






Based on these first two images, it wold seem that a vent on the upper portion of the wheel well might have a larger impact on the vortex coming off the tire... decrease in drag? Lower vent would be more in line with equalizing pressure zones (downforce/lift). Or I could have that backwards for all I know.






And a diagram I had on my computer from a long time ago:




Hope this helps someone

~C

[chillsworld]

Originally Posted by Smokeduv

The Toyota FT-1 concept has them also:

The external shape of the wheel well (sloping back to the body), upper vent, and body panel shape leading to the rear wheel and vents seem to be perfectly designed for the theoretical findings of the thesis I just posted! I love it when I can see how/why a car is designed the way it is.

~C