Ride Height/Diffuser Angle & Downforce

[flyboy2160]

Td: how does an increase in speed cause a "stall?" - at least when you use the term in it's correct sense: flow separation due to an excessive AOA. Speed bases stall (at these speeds) isn't backed up by any science or engineering I know.

Henra: Assuming the flow in the diffuser throat is attached, what gives the increased AOA to cause the separation? Pitching of the car during braking without ride leveling?

I'm skeptical of using the term "stall" when I suspect the real culprit for the loss of downforce is losing the floor side seals.

[chuckdanny]

Td: how does an increase in speed cause a "stall?" - at least when you use the term in it's correct sense: flow separation due to an excessive AOA. Speed bases stall (at these speeds) isn't backed up by any science or engineering I know.

Henra: Assuming the flow in the diffuser throat is attached, what gives the increased AOA to cause the separation? Pitching of the car during braking without ride leveling?

I'm skeptical of using the term "stall" when I suspect the real culprit for the loss of downforce is losing the floor side seals.

I think an increase in speed prevent the air from following the bend like when we are to fast entering a corner, reynolds number is the ratio between inertial forces over the viscous shear forces, it should increase locally up to the transition between laminar flow to turbulent. Then what happend is because the initial incident flow can't "fill the gap" to respect the pretty accurate constant density approximation there must be reverse flow hence separation.

As for the sealing of the diffuser, i tend to think that it is not that much coming from the side vortices which i think are used more to energize low pressure area behind the rear tires which create massive drag. So my view is that the vortices by diminishing drag allow more aggressive more efficient wing, you are exchanging bad lift/drag ratio (in this case there is very small lift created from the rear wheels except that they are forming a convergent section at the intake of the diffuser) areas for high lift/drag other areas so the overall is an increased Lift/drag.

My opinion on this is that this is mainly the rake of the car hence the very low front of the floor under the tea-tray that create a convergent flow pattern toward the rear diffuser like when you put your finger on a thin layer of water flowing over a flat surface or like a flow over a perfectly symetric wing profile. In fact without all the profile going from leading edge to trailing edge, just an obstacle create a convergent pattern downstream but with instabilities like the famous von karman vortex street. Maybe those streamwise vortices stabilizes this process of cross flow vortex shedding.

[trinidefender]

Td: how does an increase in speed cause a "stall?" - at least when you use the term in it's correct sense: flow separation due to an excessive AOA. Speed bases stall (at these speeds) isn't backed up by any science or engineering I know.

Henra: Assuming the flow in the diffuser throat is attached, what gives the increased AOA to cause the separation? Pitching of the car during braking without ride leveling?

I'm skeptical of using the term "stall" when I suspect the real culprit for the loss of downforce is losing the floor side seals.

In all honesty I'm not exactly sure hence that is what I stated before. However this is my current project concerning F1, learning about diffusers. I'll be combing through various aerodynamics books over then next 2 months or so and hopefully I'll be able to answer that question if the diffusers really do stall as a result of speed.

From my current understanding if it does happen it might have something to do with the convex shape at the very front of the diffuser and with the car running closer to the ground at high speed chocking the flow somewhat relative to low speed/static ride height.

[gixxer_drew]

Td: how does an increase in speed cause a "stall?" - at least when you use the term in it's correct sense: flow separation due to an excessive AOA. Speed bases stall (at these speeds) isn't backed up by any science or engineering I know.

Henra: Assuming the flow in the diffuser throat is attached, what gives the increased AOA to cause the separation? Pitching of the car during braking without ride leveling?

I'm skeptical of using the term "stall" when I suspect the real culprit for the loss of downforce is losing the floor side seals.

In all honesty I'm not exactly sure hence that is what I stated before. However this is my current project concerning F1, learning about diffusers. I'll be combing through various aerodynamics books over then next 2 months or so and hopefully I'll be able to answer that question if the diffusers really do stall as a result of speed.

From my current understanding if it does happen it might have something to do with the convex shape at the very front of the diffuser and with the car running closer to the ground at high speed chocking the flow somewhat relative to low speed/static ride height.

This may help: http://en.wikipedia.org/wiki/Adverse_pressure_gradient

[marcush.]

you cannot look at the difusser alone .The whole venturi formed by the throat nozzle and difusser all play into the stalling chcharacteristics...as ride height comes big time into this and the rest of the car as well ...thereis no such thing as components showing the exact same behaviour isolated or when run in the full system.Teams struggle to even produce correlation between cfd ,tunnel and real world figures ...
sure the middle section running a lot closer to the ground will starve flow a lot earlier so what does this do to the flow in 3d ? -must be the side channels trying to fill the middle diffuser section ....the transition phase will surely show some hysteresis (the characteristic may not be the same with the floor moving upwards avs downwards...),same goes for yaw conditions ,roll etc etc ..a myriad of variables here and its up to the development team to identify and concentrate on those driving the performance.

[chuckdanny]

maybe that's what vortices are also used for, to prevent reverse flow hence stall with the dissipating vortex more able to fill the low pressure area when there is to much curve given the speed of the flow, that's how leading edge vortices are used on the F18 at high angle of attack.

Because the rotation momentum of a vortex is up/down down/up orthogonal to the overall flow and naturally dissipate by enlarging itself via centrifugal force.

Look


It's almost* perfectly the same kind of geometry compare to the underside of the modern f1 cars with the planck/step plane as the central body of the plane and the diffuser being the wing close to the body. Vortices are preventing chaotique flow separation, it's no more a laminar pattern but it's stable, consistent.

* cut the canopy, nose, part of the wings and scale down the vertical fin so that they are the size of the diffuser fins...

[Pierce89]

Goodness people the reason f1 diffusers start to "stall" at high speed, is because the pressure gradient becomes to extreme for flow to remain attached.

[Pierce89]

Td: how does an increase in speed cause a "stall?" - at least when you use the term in it's correct sense: flow separation due to an excessive AOA. Speed bases stall (at these speeds) isn't backed up by any science or engineering I know.

Henra: Assuming the flow in the diffuser throat is attached, what gives the increased AOA to cause the separation? Pitching of the car during braking without ride leveling?

I'm skeptical of using the term "stall" when I suspect the real culprit for the loss of downforce is losing the floor side seals.

In all honesty I'm not exactly sure hence that is what I stated before. However this is my current project concerning F1, learning about diffusers. I'll be combing through various aerodynamics books over then next 2 months or so and hopefully I'll be able to answer that question if the diffusers really do stall as a result of speed.

From my current understanding if it does happen it might have something to do with the convex shape at the very front of the diffuser and with the car running closer to the ground at high speed chocking the flow somewhat relative to low speed/static ride height.

This may help: http://en.wikipedia.org/wiki/Adverse_pressure_gradient

Thanks Drew, I was ripping my hair out with these people that are supposed to know this stuff.

[chuckdanny]

It's hard to figure out what a turbulent boundary layer really mean.
A laminar one is clear, it's the thickness of air next to the body that is subjected to shear stress because of differences in speed between layers but even in this case i've read in reality it's not really like that, it's a model.

In fact, as flow reverses this so called laminar layer (a thin layer here) detach and move in an unsteady manner shedding vortices à la von karmann.

But in a controled turbulent boundary layer, the layer actually is quiet big and a laminar one could exist that divide two different part of the flow, one still laminar far from the surface and one very agitated but with less statistical variations hence the "controlled" term when it is fed with streamwise vortices.

Flow patterns around an f1 car create different 3d boundary layer in free air in area not in contact with the solid body just because of different "rivers" flowing next to each others at different speed. It stores energy in shear that can be released everywhere exacly like the von karmann vortices that occure past a cylinder which if occuring in front a leading edge of a wing change the incident flow conditions hence the efficiency of the profile.

[trinidefender]

Goodness people the reason f1 diffusers start to "stall" at high speed, is because the pressure gradient becomes to extreme for flow to remain attached.

Yes the large pressure gradient is why it stalls. That part isn't in contention. What exactly I am trying to figure out is why would the diffusers stall on the 2014 cars while they ran fine on earlier year cars which have much larger diffusers and in some cases ran closer to the ground restricting airflow even more. The modern diffusers that cars use aren't nearly as steep as before or even as steep as some road cars use. Hence an overall lower pressure gradient.

Which is why currently my idea rests on the convex vs concave diffuser design. As far as I'm aware, the older generation cars (pre-2009) had concave diffusers while the new generation has a hybrid design with the lip, the throat of the diffuser, being a convex design.

[Pierce89]

Goodness people the reason f1 diffusers start to "stall" at high speed, is because the pressure gradient becomes to extreme for flow to remain attached.

Yes the large pressure gradient is why it stalls. That part isn't in contention. What exactly I am trying to figure out is why would the diffusers stall on the 2014 cars while they ran fine on earlier year cars which have much larger diffusers and in some cases ran closer to the ground restricting airflow even more. The modern diffusers that cars use aren't nearly as steep as before or even as steep as some road cars use. Hence an overall lower pressure gradient.

Which is why currently my idea rests on the convex vs concave diffuser design. As far as I'm aware, the older generation cars (pre-2009) had concave diffusers while the new generation has a hybrid design with the lip, the throat of the diffuser, being a convex design.

I believe the bell shaped diffusers were around before '09. Also, I recall talk in '12 & '13 about diffusers stalling once drs opened. That leads me to believe the df producing elements are just very critical in general from being pushed to the limit.

[trinidefender]

Goodness people the reason f1 diffusers start to "stall" at high speed, is because the pressure gradient becomes to extreme for flow to remain attached.

Yes the large pressure gradient is why it stalls. That part isn't in contention. What exactly I am trying to figure out is why would the diffusers stall on the 2014 cars while they ran fine on earlier year cars which have much larger diffusers and in some cases ran closer to the ground restricting airflow even more. The modern diffusers that cars use aren't nearly as steep as before or even as steep as some road cars use. Hence an overall lower pressure gradient.

Which is why currently my idea rests on the convex vs concave diffuser design. As far as I'm aware, the older generation cars (pre-2009) had concave diffusers while the new generation has a hybrid design with the lip, the throat of the diffuser, being a convex design.

I believe the bell shaped diffusers were around before '09. Also, I recall talk in '12 & '13 about diffusers stalling once drs opened. That leads me to believe the df producing elements are just very critical in general from being pushed to the limit.

When the DRS opens it actually reduces the low pressure region behind the car and to some extent would reduce the amount of air that the diffuser is trying to draw through it by the reduction of the low pressure region. So how is it that opening the DRS can cause it to stall?

[NoDivergence]

Wait wut...

You need the low pressure region behind the car to drive the diffuser aka maintain proper flow velocity so the diffuser does not stall. You also lose rear downforce, causing a small change in ride height/rake. If your diffuser was already at optimum position, the increased rake can put you into stall angles.

[trinidefender]

Wait wut...

You need the low pressure region behind the car to drive the diffuser aka maintain proper flow velocity so the diffuser does not stall. You also lose rear downforce, causing a small change in ride height/rake. If your diffuser was already at optimum position, the increased rake can put you into stall angles.

"Maintain proper flow velocity" used in this contest is a fairly horrible term that doesn't really apply to anything.

The faster the air flows though the diffuser the larger the pressure gradient becomes. When the pressure gradient becomes to large then you get airflow separation on the underside of the diffuser. This also what causes aerodynamic stall as well as pure diffuser angle. (However the two are a violist linked to each other, increase one and the other generally has to decrease.)

When the DRS flap opens the low pressure region behind the car gets smaller because the rear wing isn't forcing the airflow upwards as much anymore. This also means that the low pressure region behind and above the diffuser is smaller and therefore has less of an effect on "pulling" air through the diffuser. Airflow will still be going through it but at a slower rate, I.e. The diffuser creates less downforce.

A theory I have is that when DRS opens it, in addition to the rear wing, cuts drag at the diffuser by making it not "work" as hard. As in it isn't that the diffuser stalled but that the diffuser was simply producing both less downforce and drag.

Pierce89, thoughts?

[NoDivergence]

You're asking how opening DRS can cause a diffuser to stall.

You're saying that with a lower pressure gradient from diffuser to behind the car, this reduces the diffuser's need for pressure recovery. Somewhat true, but you have it the wrong way around. A low pressure region of the wing interacting with the car wake will reduce the adverse pressure gradient. The diffuser geometry is/was designed specifically for the car setup with maximum wing downforce and is usually counting on that low pressure region to drive the diffuser at its steep angle.

Without that there, the flow velocity within the diffuser is very low and because of the steep angle, there's still a large adverse pressure gradient, but with much less KE in the boundary layer. With the lower wing downforce and ride height/rake change, I think you do have chance for a "off design" separation occuring