Large eddy simulation of the separated flow transition on the suction surface of a high subsonic compressor airfoil

A large eddy simulation (LES) was conducted to investigate the separated flow transition on the suction surface of a high subsonic compressor airfoil at two Reynolds number (Re) conditions (1.5 x 10(5) and 0.8 x 10(5)). The detailed vortex evolution in the separated shear layer was revealed. The instability amplification in the transition process and the associated loss mechanism were clarified. At Re = 1.5 x 10(5), the two-dimensional spanwise vortices shed periodically and were further distorted with the interaction of the streamwise evolving vortices, and then, small vortices were generated in the streamwise pairing of the neighboring spanwise vortices. Finally, three-dimensional hairpin vortices broke down into small-scale turbulent structures near the reattachment, along with the ejection-sweeping process near the wall. When the Reynolds number decreased to 0.8 x 10(5), the initial vortex shedding was not periodic, but the subsequent vortex evolution process was very similar to the case of Re = 1.5 x 10(5). The results have demonstrated the importance of the Tollmien-Schlichting (T-S) mechanism for the initial growth of disturbances in the attached boundary layer, but the transition process that occurred in the separated shear layer was dominated by the inviscid Kelvin-Helmholtz (K-H) instability. Moreover, a secondary instability observed in the vortex pairing process was supposed to have a great impact on the onset of transition. With the decrease in Re, the shear layer instability declined to a lower level, leading to a delayed transition. In addition, the deformation works associated with the Reynolds shear stress was found to be mainly responsible for the loss generation in the transitional flow. Compared with the traditional Reynolds average Navier-Stokes method, the LES was more accurate in predicting the profile loss at a low Re.