How stalled diffuser/wing reduce drag at F1 car if in aerodynamics stall/separation is always related to increase in drag?
Any graph of pressure distribution when diffuser is stalled?
From "Fundamentals of Aerodynamics" by J.D.Anderson Jr -Fifth edition
It looks to me like the multi-element wing is key here.
When flow is attached, we get flow on this general order:
To me, it looks like it's key that most of the air flowing over the bottom side of the leading element also flows pass the bottom side of the second and third elements, so it's increasing down-force the whole way. We get down-force (and induced drag) somewhat like we would from a single, heavily cambered, airfoil--other than the minor detail that a single airfoil like that would most likely stall.
But then let's consider what happens in a stall. We now have flow separation and cavitation:
Because of the flow separation, the air that flowed past the leading element is no longer directed upwards, so it no longer flows past the second and third elements (and likewise, air that flowed over the second element no longer flows past the third.
So, instead of acting like a single airfoil, it now acts like three completely separate airfoils and (key element) most of the air only flows past a single element instead of all three.
Apparently the increased induced drag from air flowing over all the elements is (probably only slightly) more than the increase in drag from being stalled.
As an aside: although it doesn't talk specifically about what happens when the wings stall, you might find this paper interesting in any case (if you haven't already read it, of course).
My understanding for this has been that If you have attached flow you have downforce, opposite of what you have in the image, so lift force is downward in this case. If you have downforce you has a drag component due to turning the flow. If you stall the wing you have less downforce and less drag i.e. no turning. I'm not sure if F1 cars stall components anymore as the second image implies with angle of attack (flexible components). What does occur is DRS which is more like feathering the rear wing to stop it creating downforce.
