Effects of passing wakes on a separating boundary layer along a low-pressure turbine blade through large-eddy simulation

Abstract

The interactions of a migrating wake and a separating boundary layer on the suction surface of a low-pressure turbine blade are described. Large-eddy simulations (LES) of the wake passing over the T106 profile for a Reynolds number Re = 7.8 × 104 (based on the axial chord and inlet velocity) are performed. The wake data extracted from precursor LES of flow past a cylinder are used to replace a moving bar that generates wakes in front of a cascade. The three-dimensional, time-dependent, incompressible Navier-Stokes equations in fully covariant form are solved using a symmetry-preserving finite difference scheme of second-order spatial and temporal accuracy. The present LES are compared with experiments and direct numerical simulation. There are differences in the state of the resolved boundary layer; however, in a qualitative sense, the LES results illustrate a separation-induced transition on the suction surface. Prior to the arrival of a wake, the boundary layer on the rear half of the suction surface is inflexional. The excitation of the separated shear layer by convective wakes leads to the formation of coherent vortices by the rollup of the shear layer via the Kelvin-Helmholtz (K-H) instability. The transition during the wake-induced path is governed by a mechanism that involves the formation and convection of these vortices followed by the production of turbulent kinetic energy inside the K-H rolls.