|
I've not done any real parametric tests with the number of strakes in a diffuser so i can't say what the optimum is likely to be. I know the primary function they serve is to reduce the amount of cross-flow in the diffuser and try to keep it more like the 2-dimensional idealised situation.
The more strakes you have the more '2D' the flow should be. I presume there would be a point though where the amount of effective diffuser area you're losing due to the unpleasant boundary layer goings-on in all the 90-degree corners betwwen strake & diffuser surface outweighs the other gain.
Also there are occasions where cross-flows can be beneficial, however, such as when you want to run a wing/diffuser steeper than you could normally get away with. By allowing some flow around the side edges (making it more '3D') you can re-energise the tired boundary layers and keep the flow attached for longer. There is a fairly hefty drag penalty for this but, if done cleverly, the overall balance between downforce/drag can still work in your favour.
If you're being really cunning you can deliberately manipulate the cross-flow to generate strong vortices whose extremely low pressure core acts on a conveniently located nearby surface and pulls the car down. A bit like how delta wings work on, though there are other things going on as well. It's also now a commonly used trick on the top flaps of F1 car rear wings.
|