How stalled diffuser/wing decrease drag?

[hollus]

Obligatory Willem Toet reference:
https://www.racetechmag.com/2017/08/wil ... diffusers/

[Vanja #66]

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?

I don't think anyone would complain, you can always post a general aero topic here in Off topic chat --> viewforum.php?f=3

I'm only present on this forum, don't really have time to visit other places to be honest, so I can't help you with evaluating them, sorry...

[Fluido]

How much is gap between rear wing and flap, few mm or 2, 3cm?

[Vanja #66]

How much is gap between rear wing and flap, few mm or 2, 3cm?

Try 10mm, should be ok. If the slot chokes, increase to 15mm but no more than that.

[Fluido]

How much is gap between rear wing and flap, few mm or 2, 3cm?

Try 10mm, should be ok. If the slot chokes, increase to 15mm but no more than that.

Ok thanks. If I write @username, will he get bell notification?
Because I start new topic in off topic chat but I dont know if you are notified..

[Vanja #66]

Ok thanks. If I write @username, will he get bell notification?
Because I start new topic in off topic chat but I dont know if you are notified..

Unfortunately, no. Only by replying to a post you trigger a notification. I will check this topic now

[Fluido]

Try 10mm, should be ok. If the slot chokes, increase to 15mm but no more than that.

Is flow in slot between main wing and flap faster then freestream?

[Vanja #66]

Is flow in slot between main wing and flap faster then freestream?

The slot should be a converging channel, with inlet on pressure side and outlet on suction side. The velocity is lower than free stream on inlet and significantly higher on outlet. How much higher depends on the entire wing geometry, but it can be more than 2x free stream velocity right on the outlet "surface" and this is what drives the local load (suction and negative Cp increase) and flow attachment on the flap.

[Fluido]

Is flow in slot between main wing and flap faster then freestream?

The slot should be a converging channel, with inlet on pressure side and outlet on suction side. The velocity is lower than free stream on inlet and significantly higher on outlet. How much higher depends on the entire wing geometry, but it can be more than 2x free stream velocity right on the outlet "surface" and this is what drives the local load (suction and negative Cp increase) and flow attachment on the flap.

You wrote that rear wing operate at -5Cp and lower,but this is not so much suction compare to -17 Cp.
Why is that? Rear wing design cant get lower Cp?

[12.84.F1]

Is flow in slot between main wing and flap faster then freestream?

The slot should be a converging channel, with inlet on pressure side and outlet on suction side. The velocity is lower than free stream on inlet and significantly higher on outlet. How much higher depends on the entire wing geometry, but it can be more than 2x free stream velocity right on the outlet "surface" and this is what drives the local load (suction and negative Cp increase) and flow attachment on the flap.

You wrote that rear wing operate at -5Cp and lower,but this is not so much suction compare to -17 Cp.
Why is that? Rear wing design cant get lower Cp?

https://i.stack.imgur.com/1aDSM.png

That's an aircraft wing with leading edge slats. Not an F1 wing without a leading edge device. Also, the aircraft wing isn't constrained by a legality box. Way, way, less camber for one thing.

[Vanja #66]

You wrote that rear wing operate at -5Cp and lower,but this is not so much suction compare to -17 Cp.
Why is that? Rear wing design cant get lower Cp?

https://i.stack.imgur.com/1aDSM.png

Well first, all of these arguments are valid:

That's an aircraft wing with leading edge slats. Not an F1 wing without a leading edge device. Also, the aircraft wing isn't constrained by a legality box. Way, way, less camber for one thing.

Second, I'm not sure that diagram is more than an illustration. Cp of -17 is insane and would mean a negative pressure (not vacuum, negative static pressure) on the slat during aircraft pre-landing approach. Approach is around 200 kts, so 370kmh, so dynamic pressure is around 6400 Pa at sea level. Multiply by -17 and you get -108 800, which is of course way less than atmospheric pressure.

[Fluido]

You wrote that rear wing operate at -5Cp and lower,but this is not so much suction compare to -17 Cp.
Why is that? Rear wing design cant get lower Cp?

https://i.stack.imgur.com/1aDSM.png

Well first, all of these arguments are valid:

That's an aircraft wing with leading edge slats. Not an F1 wing without a leading edge device. Also, the aircraft wing isn't constrained by a legality box. Way, way, less camber for one thing.

Second, I'm not sure that diagram is more than an illustration. Cp of -17 is insane and would mean a negative pressure (not vacuum, negative static pressure) on the slat during aircraft pre-landing approach. Approach is around 200 kts, so 370kmh, so dynamic pressure is around 6400 Pa at sea level. Multiply by -17 and you get -108 800, which is of course way less than atmospheric pressure.

"
This wind tunnel test was done at 67 m/s and very nearly at sea level conditions .That works out to M=0.197. Which means the flow is sonic at Cp=-16.84 -- so the pressure peak in this case is actually supersonic. The peak velocity is more than 5x freestream. "

[Vanja #66]

This wind tunnel test was done at 67 m/s and very nearly at sea level conditions .That works out to M=0.197. Which means the flow is sonic at Cp=-16.84 -- so the pressure peak in this case is actually supersonic. The peak velocity is more than 5x freestream.

Thanks, cool, learned something new. I read that there can be supersonic flow on that type of flap deign, but I never though it could mean such a low Cp The flow around slats is very different from other parts of the wing, the transition form stagnation point means the air goes completely around a sharp leading edge and stays attached.



This is what leads to such a big velocity peak and doesn't happen on F1 wings today.

[Fluido]

This wind tunnel test was done at 67 m/s and very nearly at sea level conditions .That works out to M=0.197. Which means the flow is sonic at Cp=-16.84 -- so the pressure peak in this case is actually supersonic. The peak velocity is more than 5x freestream.

Thanks, cool, learned something new. I read that there can be supersonic flow on that type of flap deign, but I never though it could mean such a low Cp The flow around slats is very different from other parts of the wing, the transition form stagnation point means the air goes completely around a sharp leading edge and stays attached.

https://i.ibb.co/nLH5ZYn/Snimak-ekrana- ... 095822.png

This is what leads to such a big velocity peak and doesn't happen on F1 wings today.

But if atmospheric pressure is 101 325 Pa, we can't go below -101 325 Pa.
Pressure can't be below 0. Zero is minimun.

So how much -17 Cp will be?

[Vanja #66]

But if atmospheric pressure is 101 325 Pa, we can't go below -101 325 Pa.
Pressure can't be below 0. Zero is minimun.

So how much -17 Cp will be?

Well you did confirm there is a local supersonic flow, so at one point there will be a normal shock wave and the flow is slowed down and back to subsonic. There are also, of course, sudden local changes in pressure and temperature.