Ride Height/Diffuser Angle & Downforce

[trinidefender]

You're asking how opening DRS can cause a diffuser to stall.

You're saying that with a lower pressure gradient from diffuser to behind the car, this reduces the diffuser's need for pressure recovery. Somewhat true, but you have it the wrong way around. A low pressure region of the wing interacting with the car wake will reduce the adverse pressure gradient. The diffuser geometry is/was designed specifically for the car setup with maximum wing downforce and is usually counting on that low pressure region to drive the diffuser at its steep angle.

Without that there, the flow velocity within the diffuser is very low and because of the steep angle, there's still a large adverse pressure gradient, but with much less KE in the boundary layer. With the lower wing downforce and ride height/rake change, I think you do have chance for a "off design" separation occuring

I can guarantee you that no wing has ever stalled from having "too low a pressure gradient." If it did then we would have planes dropping out of the sky all over the place as what you are saying is akin to saying that a wing needs to be at a high angle of attack to force it to work otherwise the airflow over the top of it slows down and stalls. But does that ever happen? No it doesn't!

The greater the pressure gradient the greater the chance there is that the airflow will stall.

[trinidefender]

Here is an article from the MIT website. Pretty easy to understand to.

http://web.mit.edu/2.972/www/reports/ai ... rfoil.html

Scroll down to "limiting physics" and stalling and read that. Now imagine the bottom of the roof of a diffuser as the top of a wing.

"If the pressure gradient is too high, the pressure forces overcome the fluid's inertial forces, and the flow departs from the wing contour. Since the pressure gradient increases with an increasing angle of attack, the angle of attack should not exceed the maximum value to keep the flow following the contour."

Understand my point now?