Would this be effective at reducing drag of a car + what would be the effect on police speed monitoring radar?
If the front of the car was divided into many small slats, like window blinds. These slats would move backwards at 1/2 the speed of the car, then would turn 90 degrees (as the window blinds turned to allow light in) and move forward at 1.5x the car speed, at which time the would once again turn to block the wind and move backwards. There would be 2 sets of these so there is always a solid surface moving backwards.
In theory this would seem to create a buffer, so the air would strike a surface at perhaps 40 MPH before striking the car at 80 MPH, and since air resistance goes up at the square of the velocity, 2 40MPH strikes should be less resistance then one 80 MPH one, though you do have the sideways parts moving forward at 120 MPH, but these on their side have much less resistance.
I recall seeing something similar - a vibrating element at the base of a metal horn, so basically the strogest radar reflection was a varibale signal; if the reflection dos not appear steady for long enough, the radar wll not lock. Never saw any practical test of it.
Many years ago (around 1980?) Car and Driver had a test of stealth material used to coat cruise missiles. It’s basically foam rubber with metal flecks randomly embedded, about half a wavelength thickness of typical radar signals. Since the signal bounces from all different distances over a wavelength, the cumulative reflections effectively cancel each other out. They put a sheet on the bumper and hood of a slope-front porsche (quick and dirty shield)They tested it on a back country road and found that the car was undetectable until it was about 500 feet away. Of course, they also found if some Bozo in a big Ford comes speeding around the corner a quarter mile away, you could be blamed for his speeding.
The real problem of wind resistance is - OK, where does the air go when it hits that slat, and after it hits that slat? Resistance is also related to how air escapes from an obstacle. That’s why a sleek pointy object will have less wind resistance than a flat surface of the same cross-section. In fact, I read once that even just rounding the corners of a cube box can cut wind resistance significantly. TO determine wind resistance is a fairly complex science, usually done with models in wind tunnels.