26 the radial pressure gradient is equal in magnitude and opposite in direction to the radial acceleration acting upon each particle the radial acceleration is expressible in terms of the tangential velocity and radius r is the radius from the center of rotation to the local region equating the radial pressure gradient in the center of the cavity to the radial acceleration the gradient becomes expressible in terms of the local tangential velocity of the air the radial pressure gradient in the central portion of the cavity dp dr is imposed laterally upon the boundary layer adjacent the side surface 24 in contrast to the air in the central portion of the cavity however the air in the boundary layer is subjected to forced vortex phenomenon in forced vortex fields the tangential velocity of the air is equal to the tangential velocity of the adjacent structure summing the radial forces on a particle in the boundary layer the net radial force is shown below f is the net radial force per unit mass on a particle within the boundary layer according to the concepts taught herein air within the cavity is accelerated to a tangential velocity v t which is equal to the local tangential velocity wr of the side surface 24 by flowing purge air through the nozzles 36 resultantly the net radial force in the local region of the boundary layer becomes 0 and the radial outflow of air ceases cessation of the radial outflow in the vicinity of the passage 32 eliminates recirculation patterns which conventionally cause a portion of the working medium gases to be ingested into the cavity and allows a corresponding reduction in the.