How stalled diffuser/wing decrease drag?

[Fluido]

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.

I calculated for 67m/s and sea level pressure 101325 Pa , I get it -63 168 Pa, this is -0.6bar, this sound impossible to get so high suction at the wing!!

[Vanja #66]

I calculated for 67m/s and sea level pressure 101325 Pa , I get it -63 168 Pa, this is -0.6bar, this sound impossible to get so high suction at the wing!!

I got 0.55 bar static pressure on that point, but yeah, it's possible. It's very local, remember that. Cp of -8 is more spread in that example, which is "only" 0.8b static pressure and that's far more reasonable

[Fluido]

I calculated for 67m/s and sea level pressure 101325 Pa , I get it -63 168 Pa, this is -0.6bar, this sound impossible to get so high suction at the wing!!

I got 0.55 bar static pressure on that point, but yeah, it's possible. It's very local, remember that. Cp of -8 is more spread in that example, which is "only" 0.8b static pressure and that's far more reasonable

-17=p-101325 /(0.5x1kg/m3x67m/s^2)
p=63168 Pa is static pressure, barometric pressure
I made mistake..... 63168 Pa -101325 Pa = suction is -38 157 Pa this is -0.38bar, still big

67m/s is 241km/h,F1 car speeds

[hollus]

Am I the only one thinking that by the time (before, actually) your model gets a supersonic region, neither your equations (no longer incompressible fluid) are adequate nor your wind tunnel (different supersonic point) is realistic?
I was under the impression that these models break down well before supersonic anything.

[Vanja #66]

Am I the only one thinking that by the time (before, actually) your model gets a supersonic region, neither your equations (no longer incompressible fluid) are adequate nor your wind tunnel (different supersonic point) is realistic?
I was under the impression that these models break down well before supersonic anything.

Good point. I would have to jog my memory to be honest, but yeah the local supersonic flow is changing things. I haven't been studying transonic nor supersonic flows a lot after my studies. The local supersonic flow leads to drag divergence due to local normal shockwave generation, but I can't tell how it infulences local pressure field and if subsonic coefficient calculations are valid. Supersonic flow (global) has very different coefficient expression, but I don't know what happens with local supersonic cases.

[Fluido]

Am I the only one thinking that by the time (before, actually) your model gets a supersonic region, neither your equations (no longer incompressible fluid) are adequate nor your wind tunnel (different supersonic point) is realistic?
I was under the impression that these models break down well before supersonic anything.

Good point. I would have to jog my memory to be honest, but yeah the local supersonic flow is changing things. I haven't been studying transonic nor supersonic flows a lot after my studies. The local supersonic flow leads to drag divergence due to local normal shockwave generation, but I can't tell how it infulences local pressure field and if subsonic coefficient calculations are valid. Supersonic flow (global) has very different coefficient expression, but I don't know what happens with local supersonic cases.

Do you know which NACA profiles they use for rear main wing and flap?

[jjn9128]

Do you know which NACA profiles they use for rear main wing and flap?

In F1?

[Fluido]

Do you know which NACA profiles they use for rear main wing and flap?

In F1?

Yes.
Or this profiles are not from NACA family?

[jjn9128]

Do you know which NACA profiles they use for rear main wing and flap?

In F1?

Yes.
Or this profiles are not from NACA family?

F1 wing profiles are not from any aerofoil family. They're drawn parametrically by eye and experience using splines/curves.