Transition mechanism and loss analysis of a separated flow over herringbone riblets in a compressor cascade using the lattice Boltzmann method

Herringbone riblets were regarded as a promising approach to control the separation bubble on the compressor blade. However, the underlying mechanism requires further elucidation. And numerical simulations with body-fitted meshes often face challenges in mesh generation due to the tiny and complex geometries involved. In the present research, high-fidelity simulations using the Lattice Boltzmann Method and Immersed Boundary Method were performed to investigate the effects of herringbone riblets on separated flow in a compressor cascade. At a low Reynolds number of 90 000, a separation bubble appears on the blade suction surface. The application of herringbone riblets on the suction side surface shows that it effectively reduces the bubble length from 0.24c to 0.12c and reduces the loss coefficient by 11%. A counter-rotating mode of secondary flow occurs before the separation, with a near-wall spanwise motion from the divergent region to the convergent region and a compensating flow from the convergent region to the divergent region in the outer layer of the boundary layer. Transition occurs earlier on the suction side surface due to the complex flow patterns. Four different mechanisms are responsible for the earlier transition. Over the divergent region, engulfing of a high momentum fluid from the outer layer to the inner layer of the boundary layer suppresses the separation bubble, forcing a high-momentum passage where an attached boundary layer is observed. This thinner boundary layer leads to an earlier natural transition. Second, the discharge of fluid from the herringbone cusp causes the overflow from the riblet channel beside the divergent line, i.e., overflow transition. Meanwhile, the transition over the converging region is attributed to the accumulation of disturbance. Finally, in the middle region with yawed riblets, transition in a separated shear layer occurs earlier under the influence of adjacent transition mechanisms over the divergent/convergent region. These mechanisms also bring about a serrated structure in the downstream wake. Overall, this research confirms the role of the counter-rotating mode produced by herringbone riblets in separation control and reveals the transition mechanisms for loss production. The findings suggest that proper utilization of herringbone riblets can provide significant improvement on the compressor blade performance.