Numerical investigation of distributed roughness effects on separated flow transition over a highly loaded compressor blade

Abstract

Large eddy simulations were conducted to investigate the effects of distributed roughness on separated flow transitions over a highly loaded compressor blade at a Reynolds number (Re) of 1.5 × 105. The distributed roughness elements were located downstream of the velocity peak on the suction surface, and four numerical cases with increasing peak amplitude of the roughness elements (k+ = 0, 23, 50, and 112) were considered. The results showed that low- and high-speed streamwise streaks appeared alternately along the spanwise direction over the distributed roughness elements. The streaks remained steady earlier; however, as the streamwise counter-rotating vortices were induced by a significant spanwise velocity component, the low-momentum fluid in the near-wall region was transported away from the blade surface and interacted with the outer separated shear layers, which caused unsteady merging of streaks and promoted the destabilization of separated shear layers. Compared with the baseline case (k+ = 0), the strong shear effect between the low- and high-speed streamwise streaks near the roughened blade surface accelerated the distortion of spanwise vortices, and three-dimensional hairpin vortex structures broke down into small-scale turbulent eddies at a shorter streamwise distance. With increase in the roughness magnitude, the level of the production term of turbulent kinetic energy was reduced due to weakened vortex dynamics, and the viscous dissipation in turbulent boundary layers also became weaker. Therefore, the profile losses of the three roughness cases, k+ = 23, 50, and 112, were decreased by 7.2%, 10.1%, and 15.5%, respectively.