Diffuser functions

[marekk]

But the diffuser doesn't really create base suction. The car does. The rear tyres, the sidepods and the rear wings do.

The diffuser helps to slow the air back down to ambient in order to reduce drag and aid downforce production. It doesn't suck the air through itself.

Bear in mind that the air is just minding its business until the car comes along. Then the localised pressure difference across the car (front to rear) makes the air move.

And why do tyres, beam wing and rear wing create localized low pressure area behind the car, and diffuser don't ?

My understanding of fluids is, when you move any object (except symmetrical airfoils in perfectly laminar flow conditions) through it, you will have low pressure area behind. And bodies ending abruptly even more so.

You can create downforce just by blowing at diffuser (or by moving diffuser through the flow, doesn't matter).

Seeing is believing, so anyone can make simple card board model of diff and blow it with hair dryer to see it working
I did.

[PlatinumZealot]

Diffuser and downforce are not mutually inclusive.

You can have a diffuser without downforce (Ferrari F150 front wheel diffuser rings) and you can have downforce without a diffuser.

On the Formula1 car the rear diffuser is reducing the energy losses of the air as it passes under the floor from the front splitter.

The kink before the diffuser is interesting. The air seems to increase in speed at the kink on a CFD plot. What Is happening at the kink is the body of air taking a curved path around a corner into a "empty" space (as the car moves forward). The faster part of the air stream (the middle part) will come closer to apex of the bend. (Think of a water fall). So for a certain range of distance from the surface of the bend the velocity will increase. Too much of a sharp bend this can destroy the flow in the other parts of the diffuser.


It depends on the radius and angle of the diffuser.. but all in all that's just a transition point. You notice this in elbows of pipes too. There is a low pressure on the inside radius of the elbow. This causes a loss of energy.


Similar concept shown below. You see it's because the faster fluid in the middle (Red) has to take the bend and it as it bends around it comes closer to the apex of the bend. In a CFD plot you will see it as an increase in velocity closer to the surface (boundary layer gets encroached on).

[shelly]

Agree with n smikle.
Suction peaks on the diffuser are due to the pressure gradientthat builds up radially when you force the flow through a curved path (you can sy it is the pressure gradient taht provides centripetal force to the flow).

The gradient is radial and pointing toward the center of the curve: lower pressure on the inside.

[marekk]

The kink before the diffuser is interesting. The air seems to increase in speed at the kink on a CFD plot. What Is happening at the kink is the body of air taking a curved path around a corner into a "empty" space (as the car moves forward). The faster part of the air stream (the middle part) will come closer to apex of the bend. (Think of a water fall). So for a certain range of distance from the surface of the bend the velocity will increase. Too much of a sharp bend this can destroy the flow in the other parts of the diffuser.


It depends on the radius and angle of the diffuser.. but all in all that's just a transition point. You notice this in elbows of pipes too. There is a low pressure on the inside radius of the elbow. This causes a loss of energy.

You almost got it - air seems to increase in speed at the kink.
But using bended pipes, saxopohone horns and other analogies is not always useful.

Except for gravitation, the only reason for a fluid to change speed and direction is pressure gradient.

As the car move forwards, area behind diffuser (in aerodynamic shadow) has lower pressure then ambient.

Pressure gradient (low behind diffuser, higher under the car) accelerates flow, which reach it's maximum speed (maximal downforce) at diffuser kink.

Downstream from this point flow decelerates (still generating some downforce).

Flow's speed profile inside diffuser:



To stop any further speculations:

The diffuser increases in volume along its length, creating a void that has to be filled by the air passing under the body. This venturi effect means that the flow is accelerated through the throat of the diffuser, creating the desired low pressure, then gradually returned to the same velocity at which it joined the wake (See Fig 1).The angle or slope of the diffuser is also important, the diffuser must have a gradual change of angle to prevent flow separation from its roof and sides. (McBeath, 1998, Competition Car Downforce)

[Dragonfly]

Thanks marekk,very useful for a non-specialist like me. A picture speaks a lot more.
Would someone please explain what exactly you call a "kink" in English? Intuitively I think I know, but don't have a proper technical dictionary from English to Bulgarian. As far as my understanding goes, it's the flow entry side of the diffuser, but is there something special there, some special shaping for example, to call it "kink"?

[volarchico]

Kink: the pink lines in the darkest blue region. Where the slope of the diffuser connects with the floor.

[marekk]

Thanks marekk,very useful for a non-specialist like me. A picture speaks a lot more.
Would someone please explain what exactly you call a "kink" in English? Intuitively I think I know, but don't have a proper technical dictionary from English to Bulgarian. As far as my understanding goes, it's the flow entry side of the diffuser, but is there something special there, some special shaping for example, to call it "kink"?

AFAIK "kink" is just rapid change shape's angle (but not 90 degree - that's called "step").

[horse]

Yeah, I guess a kink is a rapid change in direction usually having a sharp edge.

[shelly]

marekk, I think what you posted is useful but needs some corrections.
This time comparison with flow in pipes has some relevance.[quote="marekk]
Except for gravitation, the only reason for a fluid to change speed and direction is pressure gradient.[/quote]
Agree on that: in this case the pressure gradient we are talking about is the radial pressure gradient that makes the flow turn upwards in the diffuser.
The more abrupt the turning (smaller radius) the more centripetal force is needed, the strongest the gradient, the lower the pressure on the kink line (until stall).

That's why on the kink line you get lower pressure, as is shown here:

The kink line, as volarchico said, is the small radius edge where flat floor finishes and diffuser begins.
In the two other cfd images you posted there is a big transition radius, so you do not see any suction peak; it is not representative of the detail we are talking about atm.

About this image:


I think it is not representative of how a diffuser works, because if it is a diffuser, it is stalled (see blue negative velocity).


About McBeath quote: it is just a basic explaination of what we agree already, i.e. diffuser smoothening transition of the flow to the wake.
It is very general - we are looking with greater detail than that. Agree that posting it fixes a common starting point of discussion.

[horse]

About this image:


I think it is not representative of how a diffuser works, because if it is a diffuser, it is stalled (see blue negative velocity).

You can get a separation region after the kink on a diffuser where the flow will reattach later on (spatially). Don't know if that's going on in this particular simulation, however.

[marekk]

This time comparison with flow in pipes has some relevance.
Agree on that: in this case the pressure gradient we are talking about is the radial pressure gradient that makes the flow turn upwards in the diffuser.
The more abrupt the turning (smaller radius) the more centripetal force is needed, the strongest the gradient, the lower the pressure on the kink line (until stall).

Respectfully disagree. Just can't see this vertical pressure gradient turning flow upwards. That's main advantage of diff vs open wing - small flow momentum change = small induced drag.

Failed to find pics of current F1 diffusers, but i believe there is no real kink, just gradually increased curvature. Like this one:



unlike this one, used in old good days, with very distinct kink:

[Raptor22]

Jeepers all this discussion on a topic thats covered under theormodynamics 101.

Diffusors work better without sudden changes in direction. Its function is too raise pressure and reduce velocity without shock.
Without Shock is important since shock waves ina diffusor increase the onset of choking.
Again 'm with Marekk on this one.
Diffusor kink lines?!! Who makes these things up? Diffusors have to be joined to the floor somehow and typically its at a dfined edge where floor and diffusor meet. the larger the radius the less the opportunity for shock waves to develop, the less the chance of diffusor choking the less senistive the car is to underbody airflow changes.

Its just a diffusor, its not a trilithim warp generator. there no science fiction involved.

[horse]

without shock.

Pardon my ignorance, but aren't shock waves a supersonic phenomena?

[Raptor22]

in a subsonic diffusor its the boundary layer separation we want to avoid. We still term the sudden reduction in velocity "Shock". We are not refering to shockwaves such as in a supersonic nozzle/diffusor.

So in any diffusor we want a gradual pressure rise without boundarylayer separation. If we do encounter boundary layer separation we run the risk of choking the diffusor.
IS there an enginering need or a sudden and rapid rise in pressure over a small radius? I don;t know of aneed but F1 engineers seems to have differeing needs to the rest of us so I'm open to an F1 engineer explaining it.

[horse]

in a subsonic diffusor its the boundary layer separation we want to avoid. We still term the sudden reduction in velocity "Shock". We are not refering to shockwaves such as in a supersonic nozzle/diffusor.

Must be a semantic nightmare for those working on transonic flows.