Stalling Diffuser

[PNSD]

Ok, MartinB said this could be the reason for Ferraris straight line speed.

Explain...

I thought of electronically controlling flaps in the undertray.. but nah

[wowf1]

You say that, but I think I remember a few years ago hearing Ron Dennis claim that Ferrari had a flexible undertray. I think the story was that it flexed at high speed much like the front/rear wings do now, but probably in a more complex manner. I wouldn't be surprised if flexy diffusers are used today in F1!

[manchild]

viewtopic.php?p=32011#32011

[zac510]

I don't see how a stalled diffuser would decrease drag.

[manchild]

Two ways to stall it - increase attack angle of some inner element too much and get stall and drag or change attack angle of that element to horisontal and get stall with no drag.

[kilcoo316]

Ron Dennis claim that Ferrari

At that point you should be very dubious of what follows.


Ron Dennis has an ingrained dislike for Ferrari - and every comment he makes on them has a large inherent bias. I wouldn't believe anything he says with regard Ferrari.





Anyway, I don't have that much experience in under-car aerodynamics, so cannot really comment on the stall margins of diffusers [if its run quite critical, I would expect them not to play with flexi versions].

[zac510]

Two ways to stall it - increase attack angle of some inner element too much and get stall and drag or change attack angle of that element to horisontal and get stall with no drag.

But how would that change the overall drag of the car?

[manchild]

Since diffuser is there to enable downforce generating of car's bodywork and having in mind that downforce isn't needed on straight than non-functioning diffuser (stalled) would increase aero efficiency of the car. Also, when diffuser would stall car would increase ride height (like with active suspension on straights) and gain even more aero efficiency because air would flow below car's bottom more easily.

[miqi23]

The diffuser is there to expand the air coming from underneath the car in a controlled manner or I should say to mantain the pressure difference across the ends. Hence, the expanded air would have more pressure which be equal to the free stream pressure and as a result the lower pressure underneath the car produces more downforce and the controlled pressure difference produces less form drag.

Now, if you stall the diffuser by say removing it or causing the flow to separate, it will increase the drag and affect the downforce being produced at the rear end of the car reducing its effeciency also called our good old lift to drag ratio. The higher the ratio the most effecient it is.

[manchild]

But diffuser sticks car to the ground (speeds up flow below car leading to decreased pressure). It is good for downforce but bad for aero efficiency. When it would be stalled by flexing that would mean that flow on top of the car and flow below the car would become much similar than with functioning diffuser which would improve efficiency/reduce drag.

Car with perfect aero efficiency and zero drag would have identical air flow values (amount, speed, pressure) on both top and bottom. Stalled diffuser would decrease bodyworks possibility to generate dowforce, previously mentioned air flow values would become more similar than with non-stalled diffuser and drag would be reduced. At least that's how I see it.

[miqi23]

Manchild, its the smooth underbody that sticks the car the the ground, not the diffuser.

Yeah, its a car not an aerofoil where you are trying to look into it as the pressure difference between top and bottom. You should appreciate that the the car's upper body also produces lift due to its curvature and its often reduced by using trapped vortices, see those gills! it produces vortices and are trapped where the airflow over them and keeps the drag to minimum by not separating and the loss of surface shear stress reduces lift!

[flynfrog]

Manchild, its the smooth underbody that sticks the car the the ground, not the diffuser.

Yeah, its a car not an aerofoil where you are trying to look into it as the pressure difference between top and bottom. You should appreciate that the the car's upper body also produces lift due to its curvature and its often reduced by using trapped vortices, see those gills! it produces vortices and are trapped where the airflow over them and keeps the drag to minimum by not separating and the loss of surface shear stress reduces lift!

the diffuser accelrates the the air under then car thus sticking it to the ground a smooth underbody is realy just a basic diffuser

[mini696]

Manchild, its the smooth underbody that sticks the car the the ground, not the diffuser.

No. That is the flat bottom enforced by the rules, the diffuser is the "box" under the rear of the car.

...and its often reduced by using trapped vortices, see those gills!

The gills arent really used for this purpose, otherwise you would se them on more cars more often.

[sasquatch]

The diffuser works by increasing the velocity of the air under the car. The higher the velocity of the air, the lower pressure it has. The downforce is produced by low pressure air acting on the large surface area on the underside of the car. The diffuser helps expanding the air out of the rear efficiently. Since the flow is incompressible, the mass flow rate under the car should be conserved (ignoring all the stuff comming in from the sides). so the faster the air can move through the diffuser, the quicker it has to go under the car, creating a lower pressure.

For a stalled diffuser, the expansion doesn't occue since the flow has separated. As a result the mass flow rate through the diffuser section reduced and this decreases the speed of the airflow under the car. So there is less lift.

In terms of reducing the drag on a car, it is difficult to create lift without creating drag at the same time. Because of the large upwash created by a f1 car, you will get vortex induduced drag. So if you had a stalled diffuser, I would say the drag is decreased since the downforce is also reduced.

The gills is one method to combine the free stream air with the air from the radiators. I doubt they produce vorticies. If they did, it would be bad since a vortex has a low pressure region in the centre and you don't want low pressure on the top side of the sidepod.

[Tp]

So if you had a stalled diffuser, I would say the drag is decreased since the downforce is also reduced

Well if you stall the diffuser, the flow will become seperated like you said but surely this would cause a large increase in drag due to disruption of airflow.