Aerodynamic flow in wheel arches and behind wheel

[MadMatt]

I am searching for a simulation of the air flow inside the wheel arch of a sportscar, and also behind the wheel itself. Is the air quantity high behind the wheel ? By high I mean how is it compared to direct air-flow ?

Thanks in advance !

[E86]

MadMatt,
there was an article in the Feb. issue of RaceCar Engineering. That was of a ALMS Porsche GT2 that they ran at the MIRA wind tunnel. They did some different modifications to see how they could balance the car better. They made some openings behind the front wheel and noticed a significant change(7%) increase in front downforce.I hope this helps some.

[Onch]

MadMatt, sorry I can't help with your request, but I hope someone will as I am also very much interested.

E86, the big problem at MIRA is that the wheels are not turning. So testing rear wings is fine, but I would not really trust results regarding something like a hole in the bodywork next to a wheel...
That said in theory the hole should indeed reduce pressure in te wheel well and therefore increase downforce. As for the magnitude it could be as well be 0.7% or 14% once the wheel is bactually turning.

[747heavy]

maybe not 100% what you are looking for, but nevertheless, it may helps with some informations regarding the generell topic of flow inside wheelarches.

http://www.tfd.chalmers.se/~sinisa/imag ... an2010.pdf
http://www.hindawi.com/journals/ame/2010/763718.html
http://ltces.dem.ist.utl.pt/lxlaser/lxl ... s/15_1.pdf
http://www.ara.bme.hu/~regert/publicati ... rected.pdf
http://www.scribd.com/doc/48793244/bassim
http://www.ara.bme.hu/~regert/publicati ... ff2003.pdf

you can find another article on the topic here, but you will need an ATZ discription or buy it

http://www.atzonline.com/index.php;do=s ... =3/id=8005

[marcush.]

MadMatt, sorry I can't help with your request, but I hope someone will as I am also very much interested.

E86, the big problem at MIRA is that the wheels are not turning. So testing rear wings is fine, but I would not really trust results regarding something like a hole in the bodywork next to a wheel...
That said in theory the hole should indeed reduce pressure in te wheel well and therefore increase downforce. As for the magnitude it could be as well be 0.7% or 14% once the wheel is bactually turning.

I did some crude tracktesting with holes in that area (VauhallSpeedster) with phenomenal results.Admitedly the channels you could open were enourmous.(around 220x200 Channelcrossection if I remember correctly.)

The sideskirts and front splitter and flipups were also working very nicely.

[747heavy]

some discussion on the topic going on here



it´s still quite common in GT/LMP race cars, and used to balance front downforce and drag at the front.

LE MANS: The Aerodynamic Influence of Louvers on Prototypes
http://auto-racing.speedtv.com/article/ ... rototypes/

opening on the top in the inner wheelarches which will join into louvres in the front bonnet (Maserati MC12).

[marcush.]

to open the wheelarches to the top front is good ....but don´t try this for the road...the stones fly really high and wide..

[MadMatt]

Thank you guys for the answers ! A lot (too much !) informations there ! It will take some time to read, but worth it !

[Ciro Pabón]

The answer is in one of the links given by 747 Heavy, that shows the pressure coefficient in the arch of an sport car wheel:



You know that

Cp=1-(V/Vinf)^2

where Cp is pressure coefficient, V is the volume of air at the point measured, and Vinf is the volume in free flow. So,

V/Vinf = (1-Cp)^0.5

In the first graph you can see that behind the wheel, at the 180 degrees mark, Cp is -0.19.

This means that V is square root of 1-(-0.19), which is square root of 1.2, which is 1.09 so, I say the air quantity behind the wheel in this arch is 109% of the air quantity around the car (air quantity in free flow).

In english, the volume of air flowing behind the wheel is around 10% larger than the one flowing around the car.

You can use the numbers given in this graph to find estimated values for the air quantity in different points around the arch, using the same formula.

[ringo]

you mean velocity.
A cp of 0 is atmospheric conditions at Vinfinit and Pinfiniti
More negative means low pressure and positive means high pressure.

You want negative Cp bellow surfaces and positive on top.

[MadMatt]

Sorry to bump this thread, but I have a question regarding the pressure distribution inside the wheel well.

As we can see on that last picture, there is a low pressure area behind and over the wheel, causing lift. When we see the hand drawing with the 3 situations described (with colors), they say there is a high pressure on top of the wheel well.

In Katz' book, the situation is very very similar to the black and white illustration above so I would tend to believe it, but that still raises questions.

If we have low pressure there and we want to get rid of it, that means we have to either get some positive pressure air to flow through it or to suck the low pressure by an even lower pressure. Is that right ? How comes then that they open the back of the wheel well for example.

I am confused.

[gixxer_drew]

How comes then that they open the back of the wheel well for example.

I am confused.

Its not just whats happening behind the tire but what effect that has on the whole wheel house and underbody flows.

[MadMatt]

Yes, well I can imagine this, but can you be more explicit andrew ?

For example :



Does the air flowing on the side of the car suck the air from the wheel well ? From few sources that I have read, there is a low pressure area on the whole (or so) side of the car. That low pressure would be sufficiently low enough to create a positive pressure gradient to drive the air out of the wheel well ?