Investigation of the Dynamics of Secondary Flow Vortex Systems in Low-Pressure Turbines Using Direct Numerical Simulation

Abstract In this work, Direct Numerical Simulation is performed on a low-pressure turbine blade with parallel end-walls, in a linear cascade environment at an exit Reynolds number of 1.5 · 105. Our simulations are performed with Neko, a framework for high-order spectral elements for heterogeneous computing architectures. Secondary flow structures and associated losses are presented in configurations with and without free-stream turbulence and with a Blasius boundary layer inflow profile. Instantaneous and mean flow visualizations validate the classical secondary flow structures reported in the literature. The results highlight strong vortex cores at the outflow and large contributions to losses from the passage vortex and trailing shed vortex (or counter vortex). The application of turbulent structures at the inflow does not affect the formation of the horseshoe vortex nor the vortex cores at the outlet, but still suppresses the shedding at midspan. Proper Orthogonal Decomposition (POD) is applied to provide an overall picture of the flow structures in the entire domain. Without free-stream turbulence, the most energetic modes are found to be linked to the shedding at mid span and the secondary flow structures. Fourier analysis of the POD times series show low frequencies associated with the secondary structures. POD modes for the simulation with free-stream turbulence shows identical secondary flow structures, with additional streamwise-elongated streaky structures in the blade boundary layer and without any modes related to shedding.