Stalling Diffuser

[sasquatch]

the flow in a diffuser is very three dimensional with large streamwise vortices on the edges. These structures help pump the flow but cause lots of the drag.

for a stalled diffuser, the strong streamwise vortices would not attach and then the wake would be fully separated. this would result in less drag on the diffuser surfaces.

we are talking about a very unsteady and complex flow strucuture. the flow is already largly disturbed.

[RH1300S]

Thanks for that sasquatch - I've been waiting for this answer. I don't have the technical knowledge to express it. Is this basically losing drag referred to as "induced drag" - which, AFIK, is drag created by the work done by the surfaces (even if the flow is still attached) as against friction drag and turbulent drag?

[Guest]

Sick.

[theSuit]

Form drag is the net force that arises due to pressure differences around an object.

Skin Drag is the result of friction between the surface and the fluid.

Consider a trivial example, a flat plate in a uniform flow.
Perpendicular to flow: >>> |
High pressure on left, low on right. Net force acts to right. The drag here is almost all form drag.
Parallel to flow: >>> –
Negligible pressure variation along the plate, so all drag is skin drag.

Induced drag is when the direction of the lift is not perpendicular to that of travel. So in our case the downforce acts down and a little back - the little back slows the car.

Note - induced drag is a handy term but what's really going on is that the pressure distribution all around the wing produces a net force pointing in some direction. The vertical component we tend to call lift, the horizontal component drag. It's a useful distinction, but artificial - they're components of the same force.

You'll also get a force from skin friction, but it's often negligible compared to that from the pressure distribution.

As to 'by the work done by the surfaces' be careful, remember that work done is the dot product of force and displacement, so it's only the component of force in the direction of travel (or vice versa) that counts. Hence true downforce does almost no work as the car doesn't move vertically much (the suspension is there for a reason!).

[Mikey_s]

Now I'm not an aero guy, but I think some issues are being confused here.

The main objective of the diffuser is to provide a low pressure area at the back of the car in a controlled fashion. Technically speaking a stall is when the flow ceases to become laminar and the lift:drag ratio tends to 1.

The main purpose of the diffuser is to slow the air flow down by increasing the volume, in turn this provides a low pressure area at the back of the car, pulling the air under the sloping undertray and thereby speeding it up, and thereby reducing the pressure under the car and increasing d/f.

What I have not understood is the comment from Sasquach about air being incompressible... it is highly compressible, as evidenced by all of the aero bits and bobs squeezing and stretching the stuff all over the car.

MC is correct that a stalled diffuser would be advantageous insofar as it would reduce the d/f, raise ride height (reducing d/f even more) and thereby making the car go faster as energy is no longer required to push the car down onto the track and can be used to push it forward instead.

However, stalling a diffuser must be rather tricky technically... if anyone has done it I say good for them, to do so in a controllable fashion is a daunting technical challenge, particularly as the rear wing also produces a low pressure zone behind the car further energising the airstream exiting the diffuser...

It's fascinating technology if it works, but I can see that there could be some issues in ensuring that it all starts working again in time for the next corner!!

[mep]

I wonder why some of you still think about it,
because a stalling diffuser is rubbish.

[mini696]

I wonder why some of you still think about it,
because a stalling diffuser is rubbish.

Care to provide reasons?

[Mikey_s]

mep, technical discussion would be good... this is a technical forum after all.

[kilcoo316]

Stalling the diffuser would affect the whole flow under the car, from front wing back [especially from splitter onwards].

Stalling the diffuser would also increase the rake angle, increasing drag from other areas [like the rear wing for instance].

Also, do you really want a diffuser to be run that close to the limit that it stalls going through 130R or pouhon [for instance].



There is no way a team would want a diffuser to stall, they want a car thats as stable as possible, not one that has a knife edge at X m/s.

[mep]

Exactly kilcoo316.

The reason why I don't want to writte here much is
that the idear of stalling the diffuser is so absurd.

[tf1]

actually, the ideal aerodynamics is a car that has a full diffuser stall in the straights but with one that magically comes back to full strength as soon as the driver stomps the brake pedal or turns the wheel. Of course, this would require movable aerodynamic surfaces. But this is definitely the ideal case. It isn't stupid or crazy at all.

[kilcoo316]

actually, the ideal aerodynamics is a car that has a full diffuser stall in the straights but with one that magically comes back to full strength as soon as the driver stomps the brake pedal or turns the wheel. Of course, this would require movable aerodynamic surfaces. But this is definitely the ideal case. It isn't stupid or crazy at all.

No the ideal aerodynamics would be the rear wing dropping to 0 lift angle of attack - thus reducing drag, while still retaining an aero-balance of sorts as both front wing and diffuser approach the ground plane.

Of course, the front wing could reduce AoA too... But we'd be talking a whole new F1.

[Mikey_s]

I'm on shaky ground here as i am not an aero guy, but Kilcoo, the AoA does reduce at high speeds on the front wing as can be seen on the nose camera shots; the trailing edge (of all the teams I have seen) definitely bends as the speed increases, thereby reducing drag (and d/f).

I recognise that the challenges of stalling the diffuser at high speeds and asking it to work perfectly at low sppeds is technologicall challenging. However, some (presumably knowledgeable people) in F1 are suggesting that at least one team is doing this, so irrespective of how difficult it may be I would suggest that it is not impossible, just rather difficult to do.

I certainly am not qualified to say whether it is being done, or whether it works in practice. Furthermore, I think you make a good point kilcoo that you would not want to lose d/f in a fast corner, but conceptually it is rather intriguing - I'd like to see the evidence for it working (e.g. suddenly see the rear suspension lifting along the back straight on some of the rearward facing cameras).

carry on techies... !

[mep]

One think you should not forget is that a F1 kind car
has a lot of air resistance even withoud wings and
is very lightweight.

So even with very gripy tires you will
need some downforce to get the force
over the tires to puss the car against the air.

So no downforce at straight line does not work.
And you get the downforce from the underside
of the car with the highest efficiency.

[tahadar]

Exactly kilcoo316.

The reason why I don't want to writte here much is
that the idear of stalling the diffuser is so absurd.

the idea of stalling diffusers isn't absurd at all. If you recall the 'hot' topics from a few years ago, you will remember that the whole concept of a flexible rear wing was to flex at high speed, resulting in the closing of the gap between elements. This effectively made the rear wing a highly cambered, single-element device as opposed to a 2- or 3- element one, resulting in stall. This reduced drag as a result and gave cars a higher top speed.