ringo wrote:And i presume this thing is on bearings and stays level?
I notice the shape is not circular, so if it were to rotate there would be a spiral wave of air coming out the wheel.
If it does stay level, you could say a steady stream comes out.
One thing though, this hole does not share the same duct as the brakes. The flow would not go through the brakes much if it can easily flow through this big hole in the wheel. The air pressure wont be able to build and pump through the brakes.
Well, it could share the same duct – if the two are essentially connected in series. If the air that's flowed through the brake disk goes on to flow through the axle, and out the end of the wheel, it could work nicely. It may even have the effect of sucking more air through the breaks, and cooling them better, because you have a nice low pressure zone outside the wheel.