Ok before going to the fluidic switch let me take a whack at how the drd reduces drag in the first place.
From the F duct days, we have the wake control theory. I don't know where i can dig up that thread or the you tube video i made but any how. Wake control is the name of the game when unloading the wings to reducing drag.
The f1 wing is almost vertical, and the up-wash from the air flow around the surface is almost vertical as well. You notice this rooster tail when the car is driving in the wet. The size of the wake behind the wing has a relation to the drag of the car.
Naturally more downforce means a larger wake and also more drag.
The drd is used to unload the weakest flow on the wing; the centre flow. This is probably how this flow looks when stalled:
Air is blown through a pipe perpendicular to the flow on the wing. Blown just where the flow begins that upward climb round the back.There is roughly a 5-10mm gap between the wing and the tube. This air "collides" with the underflow, stripping it from the wings surface, detaching flow; if you will.
And what you see is the above results. Of course this air needs to have a certain amount of momentum to do this.
Now how does creating this kind of separated flow reduce drag?
We know it unloads the wing, but drag reduction how?
Well, if you notice that v in the middle. It's basically a hole where there is no upwash. Effectively the upwash volume is reduced by a sizable percentage. The "weir notch" is where flow from to top of the wing will deviate to, instead of taking that upward stream which would normally be the case if there was the typical upward momentum from the wing underflow in the center.
Looking at the above, you can see the normal flows on the outside near the endplates. The arrows indicate flow direction.
You have your rear wing and the general flow and wake in blue. The red is the flow on the top side of the wing.
Notice how the V notch in the middle allows the flow to pass through, no longer taking the upward pathway like the flow on the outdide. It is this reduced wake size that gives that drag reduction. This is what happens when a wing unloads, the momentum of the airflow is reduced which is related to wake size. So they key is wake control.
Soo... from the back, here is a representation of what is happening in a nutshell. The light blow is how much your wake size will be reduced. And we all know the relationship between area and drag. Smaller area, smaller drag.
Basically this is how i reason the drag reduction device. A stalled wing is a very draggy and inefficient wing, but and F1 wing, due to it's vertical momentum nature, will be have a drag increase in the region of the wing, but due to a smaller wake the drag reduction from the wake trumps the increase on the wing, resulting in an overall drag reduction.
And keep in mind this is what i think, it is not verified.
To add to this, if you can allow that V to broaden the drd will be more effective. The wing shape from sauber is suggesting that the curvature underneath is there to make the effect spread out a little more.
On the topic of increasing drd effect, you can also say that having two pipes or "elephant trunks", would create 2 super imposed Vs further reducing wake size wouldn't you think?
