SiLo wrote:Surely that could be created by having the latter part of the tube opening up to create a venturi? Therefore sucking the air through the hole. Wouldn't that be very beneficial if is was sucking air from infront of the rear wheels, creating a loss of pressure and a loss of drag.
Please keep in mind how the underbody/floor of the car creates downforce - in a nutshell it´s the same principle then a wing.
You need a pressure difference above (higher) and below (lower) the floor, then you have a higher pressure left on top of the floor/car. Now you multiply the pressure with the area of the floor and you have the downforce the floor/underbody generates.
p=F/A --> F=p*A
F= (down)Force
p= Pressure
A= Area
So the downforce you can create is direct poportional to the pressue difference and the floor area - more = better.
The diffusor is "only" used to evacuate air from under the floor/car, as more air it can evacuate as lower the pressure will be under the floor -> higher pressure difference = more downforce. So bleeding air from above the floor is not a good idea in general, as it will reduced said pressure difference.
Now having said all that, we need to keep another aspect in mind.
There is a limit as how steep you can make your diffusor, it´s very similar to the angle of atack for a wing. At one point, the air will simply not being able to follow the contour of the diffusor, and slow down, seperate and killing of the low pressure. The diffusor will stall, in the same way a wing will stall at too high angles of attack.
To avoid this flow separation you need to re-energize the flow (speed it up) at one point.
This is where the exhaust comes into play, or you can bleed air with a higher speed from
above. It´s similar to what they do on the rear wings (blown rear wing/F-duct).
If you bleed air, you lose a bit of downforce in theory, but if your diffusor works better now, it will compensate for the loss by higher overall downforce. It´s a trade off.
Remember the discussion about the rake of the cars.
If some/one team is not able to keep the flow attached in their diffusor, they can´t run higher rake, as it will make their diffusor stall --> losing downforce.
Diffusor stall is critical under braking, if your diffusor is allready on the limit, the additional pitch/rake in braking can make it stall --> loosing (rear)downforce when you need it the most.
I´m sure some teams, can´t get the optimum out of flexi wings (wing in gound effect WiG), because their diffusor is not working at high pitch angles(rake).
But that´s just my opinion, I could be wrong
