NACA ducts in sport car flat floor

[Belatti]

Do the discs actually 'pump' air?

Could you rig up something so that you could attach a flexible duct to the inside of the disc (just have the free end somewhere you can feel/measure any flow) - I'm sure you could make up a box in cardboard to seal off the driveshaft etc as you would have to if you made the thing properly. Have the car wheels off the ground and run it in gear to a decent roadspeed? If it's pumping anything surely you could measure/feel the flow into the duct.

Great idea! To see what happens with the car off the ground and in gear would give me some feel. I can use whool strings and some tape...

What I do know is that the brake disc dealer tells me: the discs do pump air in the brake testing bench.

I think that with the rim arround they act as a kind of centrifugal air fan but maybe it would be better with an internal rim cover that doesnt have relative speed 0 with the disc like the rim does.

[RH1300S]

Happy to help

Describing it as a centrifugal pump is (IIRC) exactly how Carroll Smith describes it in Tune to Win.

[Belatti]

Happy to help

Describing it as a centrifugal pump is (IIRC) exactly how Carroll Smith describes it in Tune to Win.

I have searched that in the book, but coulndt find it

[ReubenG]

I think your biggest problems will be in getting the air from the periphery of the brake disc out. Normally the exit on the volute of the centrifugal pump is in the plane of rotation. Since your impeller (brake disc) is inside a wheel rim (generally with very little radial clearance), the air exiting the disc is going to have to turn axially (either back towards the body or out through the wheel spokes) while it is already moving radially & tangentially.

I know that multi-stage centrifugal pumps do have this same flow requirement. However they don't have the space constraints on the outer diameter of the volute so the shape of the volute can be optimised for the complicated flow. The only multi-stage pump that I have seen the internals of have a volute diameter at least twice the impeller diameter. Given the space constraints the pumping efficiency will probably be quite low. However I think the torque available to the wheel over the available traction torque will probably be sufficient (at low speeds) that the efficiency of the pump is not really important.