Supersonic flow around the wheels

[hollus]

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?
Does that mean that subsonic vehicles can be exposed to supersonic conditions at least locally?
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.

Does anyone know? All others that like me do not know, feel free to speculate!

[Giblet]

Good thing we have supersonic cars that have wheels.

http://www.bloodhoundssc.com/car/wheels.cfm

http://www.bloodhoundssc.com/car/facts_ ... m#VTS_13.2

This is F1 related as the motor that drives the fuel pump for the rocket motor is a Cosworth V8 race engine at 750hp. All it does is drive the pump.

I didn't see anything on the aero side of though, but you could email them.

[Jersey Tom]

You'd probably have to adjust your braking point.

[Just_a_fan]

These questions have been looked at by a number of land speed record cars, not least of course, the SSC and Bloodhound teams. These cars have / will go through a speed range at which the questions you have raised will be an issue.

I wonder if, in a wing car such as an F1 car, the issue would be transonic wing flows rather than transonic flows around the wheels. For example, if the front wing were to develop a shockwave on the underside, as might be expected, then the flow might well 'choke' which would massively affect the flow regime below the floor / side pods and rear wing. The car could become hugely unstable even in a straight line.

Look at the longitudinal stability issues that the SSC had in the transonic region - the driver had to put in a full armful of steering lock to hold the thing straight.

[KeithYoung]

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!

[Giblet]

These questions have been looked at by a number of land speed record cars, not least of course, the SSC and Bloodhound teams....

FYI The Bloohound team makes the SSC. They are one and the same, not two teams.

[Twaddle]

Looking into the land speed cars would probably give you the best idea for car specifics. If you just want to get an idea of the kind of issues that can result from a rotating body moving at speed it's probably easier to look for info on rotor design for high speed helicopters.

[Just_a_fan]

@Giblet, true SSC and Bloodhound share some core members but a lot of them are different. The cars are quite different too so will have different issues. Certainly the way they deal with transonic and supersonic flows will be different because of the different car designs.

[olefud]

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]

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.

Supersonic is the word you're looking for - Hypersonic is typically mach 5+ speeds.
We have quite a wait before we see hypersonic cars.

[Ray]

Supersonic is the word you're looking for - Hypersonic is typically mach 5+ speeds.
We have quite a wait before we see hypersonic cars.

Never played F-Zero I take it?

[Greenish]

Bernoulli’s principle says the slipstream will accelerate as it is displaced by an airfoil or car body. .

The basic Bernoulli principle is pretty much out the window by definition once you get to transonic or supersonic flows. Look up a converging-diverging nozzle: the flow accelerates as it expands after being accelerated to mach 1 at the throat; the opposite would happen if it remained subsonic.

Agree with other posters that there would be an ugly mess of interacting shocks and local temperature increases on an open-wheel car where the relative velocity of the tire and free-stream approached mach 1!

[riff_raff]

The transonic flow conditions would be the biggest concern. But as noted, two or three wheeled vehicles (Thrust SSC, Budweiser Rocket) have run at such conditions.

However, dealing with wheel flows at transonic conditions is child's play compared to dealing with flows around a driver's helmet at such speeds :



riff_raff

[hollus]

Thank a lot for all the answers so far.
The departure from Bernoulli is one of the ones I was expecting, and with it goes a lot of what we take to be common sense and intuition, doesn't it?
You keep mentioning heat. I assume that is just from plain vanilla air compression at the shock region?
The analogy of land speed record cars is nice, but those cars are quite slipstreamed (so they carry around their own "shock bubble") and have mostly shrouded wheels, meaning whatever crosses into supersonic does so in a gradual and "controlled" manner. Nothing like the we are both in exposed air but while I am at mach 1.1, my neighbour 30 cm away is only at mach 0.55 of an exposed front wheel.
Come to think of it, all this out of control compression would produce downforce! Let's forbid it in the regulations for 2050.

[gixxer_drew]

If you have something extreme enough you start to get transonic local flow conditions its going to be at the higher end of the spectrum. It's happened to me I just restarted the tests and use a lower speed then scale the numbers back up. Its going to break the non-compressible solvers (compute time skyrockets) or you will get erratic results in the tunnel. It wont happen at a lower speed and when you have any aero bits that extreme if your going fast enough to induce that in the real world you wont care anymore about it. You'll probably be happy for the reduction in load on everything else.

Let me put it like this. If you have 10,000 lbs of downforce and your going 180mph your not going to care if you start choking the flow and reduce downforce because you didn't need it anyway as you are on a long straight.

Just my theory/opinion but I think we will have to worry about human physical limits first.

A friend and I keep joking about having a robotic scale car oval racing series (driven by computer) because we want to see these kind of boundaries getting pushed.