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 Wed May 04, 2022 1:37 pm, edited 1 time in total.