hollus wrote:A question for the aero experts here and a thought experiment.
This is a hypothetical case spun off from another discussion. We won't see an F1 going at 650Km/h any time soon, but I guess this might apply to some other type of vehicle.
In a hypothetical F1 car with exposed wheels going at 650 Km/h, at any given moment the point of each wheel touching the ground is stationary, and the point exactly opposite is (would be) moving at 1300 Km/h relative to the ground and to the external air.
Incompressibility assumptions would of course break down at such cross-flow speeds, but:
Does that mean that the local flow at that point would become supersonic?
The relative velocity would be greater than the speed of sound if the relative velocity were greater than sqrt(gamma*R*T). Any air particle about to encounter a bump locally on the top of the tire would not "have any warning". I suspect there would be a local shockwave, such as seen developing on the tops of airplane wings at transonic speeds. If the full tire were traveling faster than the speed of sound, a bow shock would form ahead of the tire.
Does that mean that subsonic vehicles can be exposed to supersonic conditions at least locally?
I know this happens in aircraft at transonic speeds (at/near the speed of sound). I highly suspect it would also happen on the top of a spinning tire.
If it means that the local flow indeed becomes supersonic, what exactly happens there? I would expect plenty of counter-intuitive phenomena, other than a fantastic sound.
Nearly instant changes in density/pressure/temperature both stagnation and static. I don't know the process within the shock itself, only how to find the properties on either side of it.
Does anyone know? All others that like me do not know, feel free to speculate!
Just_a_fan wrote:These questions have been looked at by a number of land speed record cars, not least of course, the SSC and Bloodhound teams....
olefud wrote:A car can go hypersonic at a vehicle speed well below the speed of sound. Bernoulli’s principle says the slipstream will accelerate as it is displaced by an airfoil or car body. Thus the displacement of the air stream rather than the line contact at the tire top would seem to be the real concern as a car approaches the speed of sound.
I don’t know if it was the first instance, but the P-38 WWII fighter had a very thick wing that went hypersonic at somewhere just over 450 KN.
Billzilla wrote:Supersonic is the word you're looking for - Hypersonic is typically mach 5+ speeds.
We have quite a wait before we see hypersonic cars.
olefud wrote: Bernoulli’s principle says the slipstream will accelerate as it is displaced by an airfoil or car body. .
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