How stalled diffuser/wing decrease drag?

[Fluido]

At high speed you don't need downforce, so you'd rather shed drag, so stalling the diffuser is desirable.

How stalled diffuser reduce drag?
( Explain aerodynamics behind this. Any study or graph that show what happened with pressure distribution when diffuser is stalled?)
Rear wing stall:https://www.formula1-dictionary.net/f_duct.html

if in aerodynamics stall/separation is always related to increase in drag..

Stall/separation decrease suction at nose to 1/4 and increase suction from 1/4 to trailing edge(approx.), these both effects contribute to increase in drag.




From "Fundamentals of Aerodynamics" by J.D.Anderson Jr Fifth edition

[Fluido]

Is this forum active ?

[Farnborough]

My view is that these two characteristics are very different.

One, the wing, in free air, has the clear traits you've illustrated.

The underfloor is a venturi, with interaction of one plane, the track surface, at huge contrast in speed to the moving vehicle. also a restriction to promote specifically a drop in pressure absolute to give that attraction to the track surface being the whole reason for such device.

Perhaps some "open" use of th e word "stall" may be giving the impression that the two are the same ?

It's well known that accumulation of low pressure in the venturi, if it can be attenuated at highest vehicles speed, will then result in less drag on the vehicle. How they achieve that is something most of the teams are puzzled about now in reference to the RB design, I believe.

[Fluido]

My view is that these two characteristics are very different.

One, the wing, in free air, has the clear traits you've illustrated.

The underfloor is a venturi, with interaction of one plane, the track surface, at huge contrast in speed to the moving vehicle. also a restriction to promote specifically a drop in pressure absolute to give that attraction to the track surface being the whole reason for such device.

Perhaps some "open" use of th e word "stall" may be giving the impression that the two are the same ?

It's well known that accumulation of low pressure in the venturi, if it can be attenuated at highest vehicles speed, will then result in less drag on the vehicle. How they achieve that is something most of the teams are puzzled about now in reference to the RB design, I believe.

Reduction in drag is possible only if we increase static pressure in diffuser.
I would like to see pressure distribution(or CFD) at diffuser, for attached and separated flow.

[12.84.F1]

The issue is that F1 journalists jumped on the word "stall" when it was entirely inappropriate. No one "stalls" a diffuser. It either diffuses or it doesn't. And if it doesn't then it's "choked".

[Greg Locock]

Your graphs in the first post are misleading- a diffuser is not an airfoil.

[Fluido]

Your graphs in the first post are misleading- a diffuser is not an airfoil.

How pressure distribution looks when diffuser is stalled?

[Vanja #66]

How pressure distribution looks when diffuser is stalled?

Here is an example of diffuser stall on Latios' generic 2022 car model



Here the separation is caused by lower ground clearance, as is usually the case with diffuser stall. The diffuser is completely different from an aero foil, so the two can't be compared. To begin with, the entire lower side of a wing is pressurised in high angle of attack, which increases drag more than the stalled upper (suction) side. You also have a bigger frontal area of a wing at higher angle, but you always calculate using planform surface area, so it's natural to have an increase in drag even if the wing side is stalled (people usually forget the importance of this phenomena with wings and aircraft). Therefore, the overall result is about the same drag coefficient even after the aero foil is stalled, although some foils tend to have a slight decrease in drag at post-stall angles.

[Tommy Cookers]

.... Therefore, the overall result is about the same drag coefficient even after the aero foil is stalled, although some foils tend to have a slight decrease in drag at post-stall angles.

A.C.Kermode's 'Mechanics of Flight' seems to show otherwise
ie the Cd always increases in much greater proportion than the decrease in Cl (for AoA increases beyond the stall)

aeroplanes stall primarily because the Cd increases so much

[Fluido]

A.C.Kermode's 'Mechanics of Flight' seems to show otherwise
ie the Cd always increases in much greater proportion than the decrease in Cl (for AoA increases beyond the stall)

aeroplanes stall primarily because the Cd increases so much

Here is an example of diffuser stall on Latios' generic 2022 car model

https://pic2.zhimg.com/80/v2-2a833bc021 ... 1_720w.jpg

Here the separation is caused by lower ground clearance, as is usually the case with diffuser stall. The diffuser is completely different from an aero foil, so the two can't be compared. To begin with, the entire lower side of a wing is pressurised in high angle of attack, which increases drag more than the stalled upper (suction) side. You also have a bigger frontal area of a wing at higher angle, but you always calculate using planform surface area, so it's natural to have an increase in drag even if the wing side is stalled (people usually forget the importance of this phenomena with wings and aircraft). Therefore, the overall result is about the same drag coefficient even after the aero foil is stalled, although some foils tend to have a slight decrease in drag at post-stall angles.

Rear wing stall:https://www.formula1-dictionary.net/f_duct.html

Quote from link:
: "Previously, teams created flexible wing sections which allowed the 'slot gap' to close up under high aerodynamic loads and stall the wing. Once this became evident to the governing bodies it was rapidly outlawed. Wings are now subject to static load tests to ensure that they cannot flex.
So if a team were able to achieve a similar effect within the regulations, considerable straight-line performance gains could be made."

Above text dont make any sense, because wing has higher drag when is stalled, even at fixed AoA, that is case in F1 cars.

One big differnce between aircraft and race cars is that plane stall wing as he increase AoA, but in car racing wing is stalled(purposely) without change AoA.
For sure, stalled wing at 22° AoA has higher drag than stalled wing at 10° AoA.

[Vanja #66]

A.C.Kermode's 'Mechanics of Flight' seems to show otherwise
ie the Cd always increases in much greater proportion than the decrease in Cl (for AoA increases beyond the stall)

aeroplanes stall primarily because the Cd increases so much

To be clear, I'm not talking about the 25°+ AoA cases, where in the end you have a flat 90° board drag and you calculate the coefficient with the planform surface, which is then also frontal area so the coefficient is 1+. I'm talking about the events in the immediate vicinity of the stall angle, where you don't have sudden increase in frontal area but you do have a suction drop on the top side of the foil.

Anyway, the question of the topic is diffuser stall, not aerofoils or wings, so I don't want to go OT

Above text dont make any sense, because wing has higher drag when is stalled, even at fixed AoA, that is case in F1 cars.

One big differnce between aircraft and race cars is that plane stall wing as he increase AoA, but in car racing wing is stalled(purposely) without change AoA.
For sure, stalled wing at 22° AoA has higher drag than stalled wing at 10° AoA.

The text is completely accurate, when the rear wing is working the peak suction Cp can reach -5 and lower. When it stalls, the Cp is around -1 over the entire surface. That's where the drag reduction happens and the same is the case with diffuser stall, the peak low pressure areas have much higher "low pressure" so the overall drag is lower.

[Fluido]

The text is completely accurate, when the rear wing is working the peak suction Cp can reach -5 and lower. When it stalls, the Cp is around -1 over the entire surface. That's where the drag reduction happens and the same is the case with diffuser stall, the peak low pressure areas have much higher "low pressure" so the overall drag is lower.

I understand what you wont to say but resultant force at open slot case has more downward direction because peak suction is most at main wing ,at case with close slot both main wing and upper wing is at -1 so resultant force point more toward back,that mean drag component can be bigger...(i tink it is bigger)



Are 100% sure in you claim?
Did you try find drag for this 2 case in CFD?

(your theory basicaly say that we can purposely separate airflow from roof of road car coupe to reduce drag?)

[Vanja #66]

I understand what you wont to say but resultant force at open slot case has more downward direction because peak suction is most at main wing ,at case with close slot both main wing and upper wing is at -1 so resultant force point more toward back,that mean drag component can be bigger...(i tink it is bigger)



Are 100% sure in you claim?
Did you try find drag for this 2 case in CFD?

(your theory basicaly say that we can purposely separate airflow from roof of road car coupe to reduce drag?)

Peak suction is the biggest on the main wing, but the flap slot energises the flow and generates a local peak and this is additionally amplified by the curvature of the flap. Depending on the aero foils used and their angles, this peak is more or less strong but it's always there and is always a significant drag generator.



When you stall the flap you eliminate this peak and you also reduce the suction on the main wing as a consequence. So basically, a dual drag reduction.

[Fluido]

I understand what you wont to say but resultant force at open slot case has more downward direction because peak suction is most at main wing ,at case with close slot both main wing and upper wing is at -1 so resultant force point more toward back,that mean drag component can be bigger...(i tink it is bigger)



Are 100% sure in you claim?
Did you try find drag for this 2 case in CFD?

(your theory basicaly say that we can purposely separate airflow from roof of road car coupe to reduce drag?)

Peak suction is the biggest on the main wing, but the flap slot energises the flow and generates a local peak and this is additionally amplified by the curvature of the flap. Depending on the aero foils used and their angles, this peak is more or less strong but it's always there and is always a significant drag generator.

https://pic2.zhimg.com/80/v2-1d461c5f89 ... d_720w.jpg

When you stall the flap you eliminate this peak and you also reduce the suction on the main wing as a consequence. So basically, a dual drag reduction.

Did you try in cfd find drag for rear wing with closed slot?
-3000 Pa is the lowest pressure at F1?

How airflow dont separate at this step to diffuser at you graph?

[Vanja #66]

Did you try in cfd find drag for rear wing with closed slot?
-3000 Pa is the lowest pressure at F1?

How airflow dont separate at this step to diffuser at you graph?

This is one of Latios' results, from his thread. I didn't remove the watermark -3000 Pa is his choice for scale limit. I think cars today can easily exceed -30k Pa peak suction at, say, 350kmh... There is no diffuser step, it's just a contour in y-section due to keel contraction, ie diffuser expansion.

I never tested closed slot on any wing (race car or aircraft) but I did do a lot of simulations with aircraft wing with slotted flaps experiencing stall. The suction drop is significant.