[freebeard]

Maybe a better term would be quarter cylindrical? As pointed to by the red arrows.

I think it's ineffectual because it just reproduces the existing corner contour and adds interference drag. If the opening faces forward instead of inward (back half of the quarter cylinder), it would function more like an air curtain (and do more on the ends of the bumper at wheel level).

Is the small added frontal area mooted by the cab-trailer gap?

edit: I though some more: It may be effective in cross-wind conditions, ventilating the lee side.