Numerical Investigation of Turbulent Junction Flows

Turbulent junction flows are known to exhibit a bimodal behavior of the horseshoe vortex that can be described by a random switching between a zero-flow and a backflow mode. The physical mechanism that causes the bimodal behavior is not well understood. Large-eddy simulations of a canonical junction flow geometry, the Rood wing, were carried out for a Reynolds number based on approach flow velocity and maximum thickness of 7000, and the junction flow physics were analyzed. The approach boundary-layer profile, mean flow data, and turbulent statistics obtained from the simulation are in good agreement with measurements at Penn State University. The present results for Re=7000 clearly exhibit a bimodal behavior that is characterized by a forward and backward motion and intermittent loss of coherence of the horseshoe vortex. Instantaneous flow visualizations reveal that the interaction of pockets of elevated upstream boundary-layer turbulence with the horseshoe vortex can both strengthen and weaken the horseshoe vortex. This suggests that the bimodal behavior may be triggered by the turbulent boundary layer. In addition, simulations with two different circular endwall fillets were carried out. In accordance with the literature, the fillets were found to suppress the bimodal behavior.