Vortex dynamics and entropy generation in separated transitional flow over a compressor blade at various incidence angles

The transition process within a Laminar Separation Bubble (LSB) that formed on a com-pressor blade surface was investigated using Large Eddy Simulations (LESs) at a Reynolds number of 1.5 x 10(5) and incidence angles of 0 degrees, +3 degrees, and +5 degrees. The vortex dynamics in the separated shear layers were compared at various incidence angles and its effects on the loss generation were clarified through entropy analysis. Results showed that transition onset, which was accurately identified by the Linear Stability Theory (LST), was significantly promoted at the increased incidence angle. As such, the development of LSB was suppressed and the relative role of viscous instability played in the transition process was weakened. At the incidence angle of 0 degrees, two-dimensional spanwise vor-tices detached from the blade surface and rolled up periodically, which were further stretched and eventually evolved into large-scale hairpin vortices. As time passed, the fully developed hairpin vor-tices broke down into small-scale eddies. Meanwhile, the flow near the wall reversely ejected into the outer separated shear layers and a sweeping process happened subsequently, forcing the separated shear layers to reattach and accelerating the generation of turbulent fluctuations. By comparison, the strength of vortex rolling-up was weakened at higher incidence angles, and the vortex pairing and breakdown of large-scale vortices were less pronounced. Therefore, the level of turbulent fluc-tuations that generated in the separated shear layers was reduced. Detailed entropy analysis showed that the turbulent dissipation effect related to the Reynolds shear stresses determined the largest amount of positive entropy generation, which declined to a lower level as the incidence angle increased from 0 degrees to +5 degrees. Correspondingly, the profile loss was reduced by 50.4%.(c) 2021 Chinese Society of Aeronautics and Astronautics. Production and hosting by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

The research found that the development of LSB was suppressed and the role of viscous instability in the transition process was weakened at higher incidence angles.