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I was driving over on the interstate in a rainstorm, and was musing over the spray ejected from the sides of cars by the wheels. I could see fairly clearly that the spoked type of wheels had a larger volume of spray/mist coming from around the wheels than the smoother wheels that just had holes around the rim.
The spokers also seemed to have more spray coming from higher on the wheelwells than the smoother rims.
Have you ever looked at the 'airbrakes' that are used on some stationary bicycles to provide a load for the rider to work against? The amount of drag is adjusted by varying the amount of air that is admitted to the spinning flat-bladed fan.
Consider that a spinning wheel is a centrifugal pump as far as air is concerned, and you can visualize that the spoke structure gives more room for air to enter at the hub and be accelerated out to the rim.
The plumes of air ejected to the sides will cool brakes, but will also extract energy from the vehicle.
These are some of the things that come to mind. The longitudinal airflow past the wheels might be of lesser concern than this kind of transverse flow.
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