The apparatus comprises means forming a plurality of mutually parallel blades extending outward at the first surface at an acute angle with respect to {HEAD ~ a direction} normal to {TAIL ~ the first surface}, and means forming a plurality of mutually parallel slots extending inwardly at the second surface at the acute angle with respect to a direction normal to the second surface 
normal to
The apparatus comprises means forming a plurality of mutually parallel blades extending outward at the first surface at an acute angle with respect to { HEAD ~ a direction } normal to { TAIL ~ the first surface } , and means forming a plurality of mutually parallel slots extending inwardly at the second surface at the acute angle with respect to a direction normal to the second surface
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH