Anti-spray wheel brush guards for truck aero?

[aerohead]

Originally Posted by freebeard

Air brakes are perpendicular [in use]. The perforations increase turbulence over a solid plate, thus increasing drag.

For the parallel case, I saw a PDF (that DDG can't find today) on using round holes on the front of the sides of cattle trucks, over horizontal rectangular slot the length of the sides (as is the fashion) to prevent animals suffocating. So location and shape/size matter.

Some cattle trailers have holes produced in clicking-dies which simultaneously stamp and punch the aluminum sheet.
The holes can have a chamfered edge,or a more radius'd edge depending on the maker.
The technique is borrowed from metal aircraft bulkhead fabrication.
If the chamfer or radii are on the windward face,they can increase the efficiency of the hole.
In the 1970s,cattle trailer 18-wheeler rigs were assigned Cd 1.15,the worst in the trucking industry.

[kiwinigma]

Thanks for the theory everyone! Sounds like no-one else has tried it so I might just have to have a go & see. You can buy these starting about $25 for pre-formed truck-wheel-sized half-rounds so it's not an expensive mod...