Flow visualization in the tip region of a rotating blade row

Flow visualization is used to examine the end-wall boundary layer as it passes through a rotating blade row of an axial-flow compressor; it is found that concentrations of vorticity, such as the trailing vortex shed from the blade tip, or the horseshoe vortex from the blade leading edge, which may be expected to be present in the end-wall boundary layer, are swamped and rapidly dispersed by the large-scale motions in the turbulent end-wall layer. The effect of varying tip clearance on the boundary layer is investigated and it is found that increasing tip clearance and blade loading both cause the layer to increase in thickness. The effect on the boundary layer of an end-wall casing treatment of the radial type is also investigated; this leads to an increase in large-scale turbulent motions in the end-wall layer and, for a given clearance and blade loading, to an increase in the thickness of the boundary layer. One vortex is evident with either a treated or solid end-wall: this is the scraping vortex due to the ploughing effect of the blade as it passes through the end-wall boundary layer. This vortex remains evident well downstream.