It's called Mach choking and is the formation of shock waves due to incompressible flow. In a wind tunnel you can hear it with a surface mike, it sounds like tearing paper or a raspberry (I never have, it isn't an issue on solar cars).
I agree at first sight the speeds in F1 are too probably low for steady state Mach choking to occur, but I'm wondering what the local flow speed is in the venturi when the car drops and squishes all the air that's in there, that is a dynamic pumping effect. I'd have thought one of our Colorful Fun Diagram people would know roughly what the speed is in the throat of the venturi.
1. Downforce increases with speed
2. Downforce compresses tires, storing energy
3. Floor gets closer to the ground
4. Inlet high pressure overpowers central venturi low pressure suction resulting in downforce loss
5. Tires decompress releasing energy, rising vertically
6. Venturi low pressure returns
7. Repeat.
i.e. I suggest the tunnels are not suffering loss of airspeed (since the center floor limits minimum tunnel size), rather they are being suddenly and intermittently overpowered by the lifting force of the front section of the tunnel. If so then a pitching component factors in; the venturi entrance is ahead of the center of mass and the center of lift.
The front of the floor might be a critical part of all this. Also, the effect might be exaggerated if the car is set up for higher peak downforce. More sensitive to porpoising on the limit of ride height.
If I have high angle diffuser and use vortex generators to remain flow attached, where locate them?
Infront diffuser leading edge, where airspeed is maximum or somewhere in diffuser(maybe 1/4 chord line) or maybe just infront where separation begins?
If I have high angle diffuser and use vortex generators to remain flow attached, where locate them?
Infront diffuser leading edge, where airspeed is maximum or somewhere in diffuser(maybe 1/4 chord line) or maybe just infront where separation begins?
Use small VGs and place them ahead of diffuser ramp. Once the air passes the ramp, it's already detached and placing VGs is almost useless.
If I have high angle diffuser and use vortex generators to remain flow attached, where locate them?
Infront diffuser leading edge, where airspeed is maximum or somewhere in diffuser(maybe 1/4 chord line) or maybe just infront where separation begins?
Use small VGs and place them ahead of diffuser ramp. Once the air passes the ramp, it's already detached and placing VGs is almost useless.
Yes this is true if diffuser has sharp turn and too big AoA, but if diffuser has curve shape and curve transition from flat floor to diffuser then doesnt mean 10cm back from leading edge of diffuser flow is detached. Flow can separate at middle of diffuser too in this situation, like on airplane wing does..
But I agree with your suggestion and logic.
Thanks
I agree at first sight the speeds in F1 are too probably low for steady state Mach choking to occur, but I'm wondering what the local flow speed is in the venturi when the car drops and squishes all the air that's in there, that is a dynamic pumping effect. I'd have thought one of our Colorful Fun Diagram people would know roughly what the speed is in the throat of the venturi.
If the throat is 1/5th the size of the inlet and the pressure at the throat drops 1 psi or around 7kpa, the speed at the throat will increase by ~120m/s. If the pressure drops 14kpa that's 240m/s, with a car travelling 80m/s that's 320m/s, that's pretty much M1.
At 50 m/s it looks like the diffuser is pulling around 4-5kpa below ambient, at 75 m/s, I'm getting around 8-9kpa from an unoptimized floor. Teams make floors with much more care regarding the flow field, so maybe 10kpa pressure drop is possible. Add in the fact that it's ground effect and ~250-270 km/h I would imagine the floor enters a steady state choked regime, considering ground effect enhances air compressibility.
I agree at first sight the speeds in F1 are too probably low for steady state Mach choking to occur, but I'm wondering what the local flow speed is in the venturi when the car drops and squishes all the air that's in there, that is a dynamic pumping effect. I'd have thought one of our Colorful Fun Diagram people would know roughly what the speed is in the throat of the venturi.
If the throat is 1/5th the size of the inlet and the pressure at the throat drops 1 psi or around 7kpa, the speed at the throat will increase by ~120m/s. If the pressure drops 14kpa that's 240m/s, with a car travelling 80m/s that's 320m/s, that's pretty much M1.
At 50 m/s it looks like the diffuser is pulling around 4-5kpa below ambient, at 75 m/s, I'm getting around 8-9kpa from an unoptimized floor. Teams make floors with much more care regarding the flow field, so maybe 10kpa pressure drop is possible. Add in the fact that it's ground effect and ~250-270 km/h I would imagine the floor enters a steady state choked regime, considering ground effect enhances air compressibility.
How did you get those Delta p you are using in your calculations?
Last edited by Biggiox on 04 May 2022, 13:38, edited 1 time in total.
Aerodynamics and Aerospace Engineer here. Guys, please, let’s not get confused.
Supersonic chocking, or mass flow chocking or simply chocking is a compressibile phenomenon that, to explain it very quickly and simply, make it impossibile to further increase mass flow through a Venturi by decreasing the outlet pressure as you reach AND ONLY IF YOU REACH sonic speed (Mach = 1) at the throat section. Basically, when the flow becomes supersonic, perturbations can’t propagate upstream and therefore the amount of mass per second flowing though the throat, (and for the mass flow conservation law, though the whole nozzle) becomes indipendent from the downstream conditions thus limiting the amount of speed that can be reached by the flow in the throat. Choking is not reached before Mach 1 at throat, is not something that causes the flow to suddenly stop or decelerate thus increasing pressure and make the car lift… I’ve been reading some very odd things here.
That being said, even though speed does increase considerably inside a F1 Venturi in respect to the freestream velocity, we are well far away from this phenomenon in formula 1 or generally in any automotive applications. The amount of downforce generated by an undertray reaching sonic conditions at throat would be exorbitant, not to mention the fact that you would hear sonic booms as car passing by.
The reason why you don’t get too greedy with inlet cross sectional area is somewhat the same why you don’t want the car to get too close to the ground (porpoising docet): you’re going to cause stall and therefore not only stop gaining downforce, but start to loose it because of separation and a bunch of other very bad stuff happening connected to it such as vortex breakdown. Increasing the inlet section will feed the undertray with more mass flow. This will basically mean more velocity at fixed ground clearance and therefore more suction underneath the car and this is why you don’t see a flat inlet. But remember that the outlet pressure, or base pressure as we aerodynamicist would call it, is fixed and equal to the pressure reigning inside the wake of the car. So decreasing the pressure inside the Venturi’s throat too much, will also entail a bigger pressure recovery needed inside the diffuser; eventually the flow is not going to be able to withstand such a great adverse gradient and stall, making the undertray useless. Same thing happens when the ground clearance gets too small as the suction reached, enhanced by the increased ground effect, will later require a too steep pressure recovery and then stall. This is sometimes referred as flow blockage.
Of course all of this has to be considered as a very simplified explanation, but I can ensure you “supersonic choking” are two words that definitely don’t belong to car aerodynamics.
EDIT: Made some typos
Last edited by Biggiox on 04 May 2022, 14:37, edited 1 time in total.
In the past ground effect era minimizing chocking used to be by either reducing the intake aria or by expanding the outlet. But this time around it is different because of much more sophisticated floors used. the effect of cost cap which effects and limits to a fine balancing act the development of this year chassis and next year one. Teams seems to have went to maximum possible ground effect within their calculations with the aim to adjust/ reduce its effect afterwards in a way to get the fastest time possible around a lap.
The stepped plank era had very interesting structures behind the diffuser, in the area of the rear crash structure.
I looked everywhere for information and discussion on the device, but could not find.
My conclusion was the device was a 'venturi booster', similar to a carburettor, in fact most of the revealing data was in technical papers on carburettors, the diffuser seems to be an identical device in principal.
