To reduce the whirling air vortices which are normally formed at the disc tip, the outer edge of the disc surface may be gradually thinned down to a sharp tip. An alternative approach, the disc tip is gradually reduced in width and rounded at its outer edge. A flow obstruction may be incorporated in the shroud adjacent the edge of the disc tip. This obstruction may comprise a small substantially rectangular insert extending from the inner edge of the housing. Or, the inner surface of the portion of the housing adjacent the ends of the disc may incorporate grooves therein. These grooves, which are roughly herringbone in shape, and would have their center near either the edge of disc in a single disc environment or mid-way between the disc in a two disc environment.