The in cowl cooling - is best done from the airflow over the OUTSIDE of the cowl--
this airflow , over the cowl flaps or the outlet hole (whatever) -creates a low pressure in the cowl -- this is what does the cooling on a cowled engine
try as yo might - you can not blow air thru the cowl The air behind the prop is stagnant simply whirlig around as it is rebuffed by the obstruction of the cowl and engine.
However :
Air movement in the cowl is a product of low pressure aft (and below in some cases) the engine , creating a draft to move the air at rear of prop, into and thru th fins .