High-Fidelity Investigation of Vortex Shedding From a Highly Loaded Turbine Blade

Abstract

High-fidelity numerical simulations based on wall-resolved large-eddy simulations (LESs) are used to investigate the vortex shedding dynamics in a linear turbine cascade. The profile geometry is the well-documented LS59 highly loaded rotor blade. The simulation campaign covered several outlet Mach numbers (subsonic and transonic) and several experimental configurations to shed light on the relations between vortex shedding frequency, the laminar or turbulent states of boundary layers, and the resulting cascade losses. A first major result concerned experiments for transonic outlet Mach number. LES without inlet turbulence and LES without tripping the suction-side boundary layer are unable to match the experimental flow field. In the untripped case, the recirculation bubble is shorter leading to a vortex shedding Strouhal number of 0.17, whereas in simulations with trip wire or inlet turbulence, the bubble was longer, with a Strouhal of 0.24. A second finding is that the inlet turbulence intensity is able to drive the switching between two regimes for subsonic outlet Mach numbers: the so-called detached vortex shedding (long bubble, St∼0.24) or a “transonic vortex shedding” (short bubble, St∼0.19). The appearance of shock waves and very coherent rolls in the wake for the “transonic” regime can lead to a doubling of the cascade losses due to the drastic pressure drop near the trailing edge. Such a change in the flow regime, due solely to an increase in freestream turbulence, has not been reported earlier.