How stalled diffuser/wing decrease drag?

[Fluido]

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.

I agree with everything you said but I think we must check this in CFD to see if drag is higher or smaller. Members from aviation stack claim that drag is higher when slot is closed...

[Vanja #66]

I agree with everything you said but I think we must check this in CFD to see if drag is higher or smaller. Members from aviation stack claim that drag is higher when slot is closed...

Teams already did this slot sealing for drag reduction, as recently as 2006. Here's one article, but you'll find more all over the internet

https://www.f1technical.net/development ... -rear-wing

FIA demanded slot gap separators to be added in-season and ever since, since it is a moveable aero device.

[Fluido]

I agree with everything you said but I think we must check this in CFD to see if drag is higher or smaller. Members from aviation stack claim that drag is higher when slot is closed...

Teams already did this slot sealing for drag reduction, as recently as 2006. Here's one article, but you'll find more all over the internet

https://www.f1technical.net/development ... -rear-wing

FIA demanded slot gap separators to be added in-season and ever since, since it is a moveable aero device.

I will do test for my self...

[Vanja #66]

I will do test for my self...

Make sure you use full 400x200mm space for wing and flap allowed by current rules, this will enable to have a big enough camber of the wing. I'd also go for a big flap angle, at least 60-65°, like Red Bull is using.

And do share the results with us!

[Hoffman900]

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.

I agree with everything you said but I think we must check this in CFD to see if drag is higher or smaller. Members from aviation stack claim that drag is higher when slot is closed...

CFD has big time correlation issues, especially when comparing computed drag to measured wind tunnel.

Might be fine for illustration, but don’t believe those numbers, and anticipate 10%+ errors between real life and virtual.

[Fluido]

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.

I agree with everything you said but I think we must check this in CFD to see if drag is higher or smaller. Members from aviation stack claim that drag is higher when slot is closed...

CFD has big time correlation issues, especially when comparing computed drag to measured wind tunnel.

Might be fine for illustration, but don’t believe those numbers, and anticipate 10%+ errors between real life and virtual.

I dont care if numbers are real, I just need do drag comparation for wing with slot vs same wing with closed slot....

[Tommy Cookers]

... CFD has big time correlation issues, especially when comparing computed drag to measured wind tunnel.
.... anticipate 10%+ errors between real life and virtual.

yes ....
I heard from an impeccable source of one major program where the errors were much greater

afaik slotted flaps are the second hardest thing (to model successfully in a wind tunnel)
double (even treble) slotted flaps are the hardest thing - (ie most demanding of Reynolds number similarity)
until you've got the right answer you don't know what answer is/was the right answer etc etc

[Fluido]

I will do test for my self...

Make sure you use full 400x200mm space for wing and flap allowed by current rules, this will enable to have a big enough camber of the wing. I'd also go for a big flap angle, at least 60-65°, like Red Bull is using.

And do share the results with us!

NUMERICAL INVESTIGATION OF THE CONTROLLABLE WINGSTALL CAUSED BY THE AIR INJECTION

https://www.google.com/url?sa=t&source= ... 8Z4Ir3ni


In 2D they get higher profile drag for stalled wing, I expected to be lower because of elimination of pressure peak that you explained..

Do they get profile drag with integration static pressure around wing?

[Vanja #66]

In 2D they get higher profile drag for stalled wing, I expected to be lower because of elimination of pressure peak that you explained..

Do they get profile drag with integration static pressure around wing?

Well, the simulation was done for a Formula Student car, so there are some major differences in wing geometry (the slot is too big and the flap is at a lower than optimal angle, both things reduce pressure drop potential) and simulation boundary conditions - most of all the free stream velocity - compared to an F1 car. I don't agree with some of their conclusions and explanations, but I will keep those details to myself.

That said, there are some cases where they also had lower overall drag with slots at both 11 and 21 degrees. It's not an easy thing to achieve with their wing geometry and I don't think any case of jet velocity above 50% of the free stream velocity is relevant. There were surely some big losses in an F-duct system, so there would be a big drop in jet velocity.



Too bad they didn't put some pressure or velocity plots at 21 degrees, when the wing is actually working the hardest and generating the biggest pressure drops. Those would be much more telling.

[Fluido]

I don't agree with some of their conclusions and explanations, but I will keep those details to myself.

I dont understand why pressure drag or they call it profile drag(isnt this integration of static pressure around wing??) is higher when flow separate?

If we say that suction peaks are smaller when flow separate, why drag is higher?
Are you aerospace engineer or you do aerodynamics for hobby?

[jjn9128]

You all seem to be under the assumption flow separation is the only mechanism. Ground effect - particularly F1 ground effect - is very heavily dependent on vortices to improve/maintain load. If those vortex systems break down your downforce goes too, so your pressure drag reduces (along with that vortex induced drag).

[Vanja #66]

Are you aerospace engineer or you do aerodynamics for hobby?

If I ask someone to help me understand something and I can't understand it even after I was given a few examples and explanations, I would keep thinking about on my own as well and try to put things together. Much more constructive than questioning the person I asked to help me. But hey, that's just me...

You can view my forum profile page and click on my "website" - my LinkedIn account - to see who I am, I don't keep my identity a secret on this forum.

I dont understand why pressure drag or they call it profile drag(isnt this integration of static pressure around wing??) is higher when flow separate?

If we say that suction peaks are smaller when flow separate, why drag is higher?

As I said before, their wing geometry is not adequate, they aren't working the wing hard enough so they could generate benefit of f-duct-type drag reduction over a wider range of parameters they were looking at. They don't generate a Cp which is low enough in the first place. Their flap is too small, the flap angle is too small, their slot is too big and resulting wing camber is relatively small for a high-downforce motorsport rear wing. Compare their design with Latios' (which also doesn't have too much of a camber, but the whole wing is at a higher angle) and 2010 McLaren with actual f-duct.



Do you expect significant suction drop while trying to induce stall on a wing that's not working at the very edge of stall? If we said already that stall induces a Cp of about -1, you need a wing geometry that generates Cp well under -1 to see any benefit of drag reduction by stalling the wing on purpose. After that, you optimise it and introduce a very large flap angle to enhance this effect...

You all seem to be under the assumption flow separation is the only mechanism. Ground effect - particularly F1 ground effect - is very heavily dependent on vortices to improve/maintain load. If those vortex systems break down your downforce goes too, so your pressure drag reduces (along with that vortex induced drag).

Of course, but Fluido did ask only how a stall reduces drag

[Fluido]

Are you aerospace engineer or you do aerodynamics for hobby?

If I ask someone to help me understand something and I can't understand it even after I was given a few examples and explanations, I would keep thinking about on my own as well and try to put things together. Much more constructive than questioning the person I asked to help me. But hey, that's just me...

You can view my forum profile page and click on my "website" - my LinkedIn account - to see who I am, I don't keep my identity a secret on this forum.

I dont understand why pressure drag or they call it profile drag(isnt this integration of static pressure around wing??) is higher when flow separate?

If we say that suction peaks are smaller when flow separate, why drag is higher?

As I said before, their wing geometry is not adequate, they aren't working the wing hard enough so they could generate benefit of f-duct-type drag reduction over a wider range of parameters they were looking at. They don't generate a Cp which is low enough in the first place. Their flap is too small, the flap angle is too small, their slot is too big and resulting wing camber is relatively small for a high-downforce motorsport rear wing. Compare their design with Latios' (which also doesn't have too much of a camber, but the whole wing is at a higher angle) and 2010 McLaren with actual f-duct.

https://i.ibb.co/JFnssQQ/f-duct.jpg

Do you expect significant suction drop while trying to induce stall on a wing that's not working at the very edge of stall? If we said already that stall induces a Cp of about -1, you need a wing geometry that generates Cp well under -1 to see any benefit of drag reduction by stalling the wing on purpose. After that, you optimise it and introduce a very large flap angle to enhance this effect...

Thanks Vanja for help, sadly at this forum I cant ask questions for general aerodynamics(to learn more), only related to F1 aero. What is your opinion about level of aerodynamics knowledge at aviation stack echange, CFD forum and physics forum?

[Hoffman900]

Those strakes at the front of the floors are vortex generators and have been in use in tunnel floors for nearly 30 years. You can find Paul Tracey’s car upside down at Road America in 1996 and see two of them. The Reynards in Champ Car had three of them, like you see in F1, going back to 1997. Using vortex generators to enhance under floor performance has been long known, just as feeding in additional energy from floor edge “leaks” (so sealing is the wrong idea) is also well known.

The problem almost all these “aero analysis” youtuber / social media accounts, save for Kyle Engineers is they draw everything as if flow is laminar. It’s not and there are vortexes all over the place of a F1 car, which makes them even harder to model and for most, to even have a resemblance of an idea of what squigglies to draw on an image. The other is everyone latched onto these ideas of side skirts because that’s what they did near 50 years ago in F1 (and let’s be real, the 2J did it first), and ignored 30 years of development in other series. A bit of an isolationist view, but there is a lot of “if it didn’t happen in F1 it might as well not have happened at all”.

[12.84.F1]

The problem almost all these “aero analysis” youtuber / social media accounts, save for Kyle Engineers is they draw everything as if flow is laminar. It’s not and there are vortexes all over the place of a F1 car, which makes them even harder to model and for most, to even have a resemblance of an idea of what squigglies to draw on an image.

Hamilton even mentioned this in an interview this season when he said he'd seen stuff in development and the flows were really complicated under the floor.