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rego April 29, 2012 16:26

Drag and Lift coefficient for diffusor
Hallo everybody,
I have a problem. Im a member of a formula student team and we want to use a diffusor for our car this year. We designed 3 different diffusors and now I would like to know which one has the biggest down force.
In the first place I just want to simulate only the diffusor without the body and tires. After I know which one is the best, I will make the complete simulation with the tires and the body. The most important value for me is the down force.
I run the simulation but I have to big values for drag and lift coefficient.
Drag coefficient 1,72
Lift coefficient -9,95
Drag force 51,4 N
Lift force -296 N

Info about my model:
Wind tunnel: 35mx14mx4m LxBxH
Physics: three D, steady, turbulent, SST K-omega,
segregated flow, gas, constant density, all y+ wall

Mesh: prism layer mesher, trimmer,
I used 4 prism layers on the ground and 4

volumetric controls around the diffusor to refine

the mesh. My mesh has 863000 cells

Can anybody help me to understand why I have these values. I read that a plane in a stream has a Cd value of 1.1. I assumed much smaller value of Cl for the diffusor.

With kind regards


abdul099 April 30, 2012 02:56

In my opinion, it doesn't make much sense to run the diffuser without the rest of the car. The flow through the diffuser will be greatly influenced by the flow around the car body.

Anyway, I suspect it's the mesh resolution. You've got a big domain but a very small mesh. (Don't misunderstand that, the domain size is fine for your simulation.) And when you have 4 prism layers, the cells must be huge to keep the cell count that low. So I would refine the mesh and have a look what happens.

rego May 7, 2012 16:21

Hi Abdul099,
thank you for the answer. I know that I need to simulate the whole car, if I want to know the real Cd & Cw. I just wanted to know what kind of design is the best.
I changed the resolution and the result is the same. I just have a beter convergance of my residual.
But I read in a Forum that the Ca value in books is that small because they use the front area of the whole car.
Next week I will start to simulate the whole car and I will see what will happen.

PS: And thank you again for you help (it is the second time you helped me, the last time it was about Cd for ahmed body. The post was from my friend)

abdul099 May 7, 2012 18:05

Hm, different reference areas are nasty to go wrong, since you often don't think it could be that simple :D

But I still think, it doesn't make much sense to compare different designs without taking the whole car into account. Even when you just want to evaluate the best design, it will be influenced from the car. And therefore you might get a different result.
Looking just at the diffusor is nice to get a feeling for the mesh sizes etc, but you shouldn't judge without looking at the whole car.
My (short) F1 experience told me, it's even not easy to have a look on the FRONT wing without taking the car into account. And the front wing is the most forward part of the car. But its strongly influenced by the following body, so we needed to include the body to judge the front wing. And now you want to judge diffusor designs without the rest of the car? You know, the diffusor is at the rear of the car? Hm...

Of course you can do however you like. It's just my very own opinion that it doesn't make too much sense.

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