The spanwise Mach number gradients (i.e. difference in relative Mach number between the hub and shroud), especially from 40% of chord to trailing edge, are the main driving force of secondary flows on the suction surface. TURBOdesign1, being a 3D Inverse Design method, enables direct control of the Mach number distribution in the impeller by specifying the blade loading distribution.
The graph above shows the loading distribution specified in TURBOdesign1. Below we can see the resulting Mach number distribution, which indicates considerable reduction in the spanwise Mach number gradients as compared to the conventional impeller, which should help to control the secondary flows.