Vortical structure in an impeller-diffuser blade configuration of a radial flow pump.  The wake of a rotating impeller blade interacts with a stalled, stationary diffuser blade, to produce a complex pattern of positive (red) and negative (yellow) vorticity. The upper left image shows superposition of (white) streamlines on contours of instantaneous vorticity. The wake of the impeller blade exhibits concentrations of vorticity, and the existence of a stall zone on the diffuser blade generates large-scale, positive coherent vortical structures, distinct from the negative vorticity along and from the trailing-edge of the blade. The lower left image shows superposition of the instantaneous velocity and vorticity fields. This representation clearly shows the extent of the low velocity stall region adjacent to the blade. After encounter of the wake of the impeller blade, represented by the image at the upper right, the scale of the vortical structures along the edge of the stall zone of the diffuser blade has decreased; correspndingly, the width of the stall region decreases as well. Reynolds number based on inlet diameter of impeller is Re = 6.25 x 10³. [37] A related study is described in [38]. Akin and Rockwell