Numerical investigations of the separated transitional flow over compressor blades with different loading distributions

Abstract

Large eddy simulations (LESs) were conducted to investigate the separated flow transition process over two compressor blades with different loading distributions at Reynolds numbers (Re) of 1.5×105 and 0.8×105. The baseline airfoil (V103-B) was redesigned to obtain a new front-loaded airfoil (V103-F). At Re=1.5×105, a time-averaged laminar separation bubble (LSB) formed on the suction surface of V103-B and V103-F. The two-dimensional spanwise vortices shed periodically at approximately the same frequency and were further twisted through their interaction with the streamwise evolving vortices. Then, small vortex structures were generated in the vortex pairing, which was related to the onset of transition. The distorted hairpin vortices finally broke down into small turbulent eddies near the reattachment, along with an ejection-sweeping process of the near-wall flow. As Re decreased to 0.8×105, the separated shear layer failed to reattach on the blade surfaces of the two airfoils. The near-wall flow ejection-sweeping disappeared, and there was no distinct periodicity for the two-dimensional spanwise vortex shedding. The spectral analysis indicated that the transition processes in the LSBs of the two airfoils at Re=1.5×105 were both dominated by the Kelvin-Helmholtz (K-H) mechanism; however, the transition onset on the suction surface of V103-F was promoted, and the size of LSB was consequently smaller than that of V103-B. The loss generation mechanism in the LSB was analyzed by comparing the deformation work terms due to the viscous and turbulent dissipation effects, with the results indicating that the largest amount of loss was determined by the Reynolds shear stresses. Due to the suppression of the LSB on the suction surface of V103-F, the profile loss was decreased distinctly by 32.3% at Re=1.5×105 compared with that of V103-B.