Large Eddy Simulation of Boundary Layer Separation and Reattachment in a LPT Blade at Different Incidence Angles

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In order to predict the phenomenon of laminar flow separation, transition and reattachment in a high-lift low-pressure turbine (LPT), a self-developed large eddy simulation program to solve three dimensional compressible N-S equations was used to simulate the flow structures in T106A LPT blade passage. The outlet Mach number is 0.4 and the Reynolds number is 1.1×105 based on the exit isentropic velocity and the axial chord. The distributions of the time-averaged static pressure coefficient, kinetic loss coefficient and wall shear stress on the blade surface at +7.8° incidence angle agree well with the results of experiment and direct numerical simulation (DNS). The locations of laminar separation and reattachment point occur around 83.6% and 97% axial chord respectively. The evolutionary process of spanwise vorticity and large-scale coherent structure near the trailing edge on the suction side in one period indicates that the two-dimensional shear layer is gradually unstable as a result of spanwise fluctuation and Kelvin-Helmholtz (K-H) instability. The boundary layer separates from the suction surface and the hairpin vortex appears in succession, which leads to transition to turbulence. Analysis of the incidence angle effect on the boundary layer separation point as well as separation bubble scale was also performed. A small scale separation bubble exists around the leading edge at positive incidences. As the incidence angle changes from positive to negative, the separation bubble near the leading edge disappears and the boundary layer thickness reduces gradually. The separation point at the rear part of suction side moves downstream, yet the reattachment point barely changes. The Reynolds stress and turbulent kinetic energy profiles change dramatically at zero and positive incidence. This illustrates that the incidence angle has great influence on the development of the boundary layer and the flow field structures.