Transient separation and fluctuation of turbulent flow in an axisymmetric U-turn channel perturbed by periodically passing gust inflow

The flow in the vane-less diffuser bend and return the channel of radial turbo-machines which is perturbed by the time-periodic and circumferentially passing jet-wake structure exited from the upstream impeller, and the transient characteristics of separation and fluctuation occurring in these stationary components determine the operational stability and performances of the turbo-machines especially for the next stage. In this work, we present a numerical investigation on the transient separation and fluctuation behaviors of turbulent flow in a sector of a 180° bended axisymmetric U-turn channel subjected to time-periodic gust inflow which passes in the circumferential direction. The objective is to explore the effect of period of gust inflow, in simulating the non-dimensional blade passing frequency (BPF*) in turbo-machines, on the characteristics of flow in the whole U-turn channel. Numerical results reveal that flow separation occurs in the curved section for about half the cycle at BPF* = 1 and 2, and earlier separation is observed for BPF* = 2, while there is no separation for BPF* = 5 and 10. There is a phase lag between the local skin friction coefficient in the channel and gust inflow, and the magnitude of the phase lag is related to BPF*. As BPF* increases, the thickness of the boundary layer on both walls decreases.