Wake-boundary layer interaction on the vaned diffuser of a centrifugal stage

An experimental investigation has been carried out by means of a two-component laser Doppler velocimeter (LDV) to survey the unsteady flow entering in the vaned diffuser of a centrifugal stage and to analyse into details the vane boundary layer development perturbed by the rotor flow. Instantaneous velocity data have been acquired in phase with the rotor blade passage and have been processed in ensemble-averaged form to separate periodic and random velocity fluctuations. Periodic incidence variations cause boundary layer transient separations near the vane leading edge, which alternate with opposition of phase on the two sides of the diffuser vane. Owing to the high turbulence energy conveyed by the wakes within the diffuser, which penetrates into the boundary layers, the periodic separation bubbles remain very short and the separated shear layers reattach early, giving rise to turbulent boundary layers capable of withstanding the adverse pressure gradients imposed by the diffuser.