Abstract: A biasing member (11) having opposed faces (11a, 11b) with a matrix of tapered holes (19) which provide a plurality of closely spaced, inclined biasing surfaces (12) is disposed in the path of a flowing gas. The gas molecules rebound from the biasing surfaces to upset the normal velocity distribution so that more molecules of gas have greater velocity components along the centerline of the holes in the general direction of gas flow when the biasing surfaces are divergingly inclined and more molecules of gas have reduced velocity components in the direction of gas flow when the surfaces are convergingly inclined in relation to the direction of gas flow. The beneficial results are an increase in the velocity of a flowing gas and thrust forces on the biasing member in the opposite direction to gas flow and, in the alternative, a reverse in the direction of gas flow through the biasing member reduces the velocity.