Turbulence contains a wide range of scales which form the turbulent spectrum. In low-pressure turbines (LPT) these scales of the turbulent free-stream influence large-scale mixing, the decay of turbulence kinetic energy and the transition of boundary layers through their reception of the small scales. Although these mechanisms are known in principle, the effect of turbulent scales on LPT aerodynamics has not been quantified and analyzed in detail yet.
By means of Large Eddy Simulations (LES) applying the Incompressible Divergence-Free Synthetic Eddy Method (I-DFSEM) — introduced in Part A of this two-part paper — at the domain inlet to impose any desired turbulent boundary condition, the MTU-T161 LPT cascade is investigated under low-speed conditions. The simulations are successfully validated by the experimental results of the turbulent spectrum. In order to separate the effect of turbulence intensity and length scale on the cascade aerodynamics, the turbulent length scale is systematically varied while ensuring similar turbulence intensity at the profile’s leading edge.
The results show an influence of the turbulent spectrum on separation-induced boundary layer transition. It is shown that the separated shear layer is amplified by integral length scales corresponding to frequencies close to the Kelvin-Helmholtz instability. Consequently it affects the turbulent mixing such that the transition point and lengths differ.