Pulsating turbulent flow in a straight asymmetric diffuser

Abstract

Pulsating turbulent flow is studied for four regimes: steady, quasi-steady, relaxation, and quasi-laminar in a rectangular straight asymmetric diffuser, a generic model of the diffuser found at the end of most Kaplan and Francis type hydropower turbines. The flow entering the diffuser is a developing duct flow at Reynolds number 20 000, based on mean streamwise velocity and hydraulic diameter. The time averaged velocity and turbulent quantities are not affected by the forced oscillation. The regimes prevail in the diffuser, but are shifted due to the decreasing friction velocity. The oscillating quantities are affected by the adverse pressure gradient in the same way as the time averaged quantities, but with a decreasing effect for higher forcing frequencies. The amplitude of the oscillating wall shear stress is found to be signicantly lower than the Stokes solution in the quasi-laminar regime. The regime is confirmed by the behavior of several other quantities. The pressure recovery is found to be 30% higher in the relaxation regime than in the other regimes. Results are compared with experiments in channels and turbulent boundary layers, with and without an adverse pressure gradient, and with large-eddy simulations.