The Effect of Rotation on the Boundary Layer Transition over the Rotor Blades

Large-Eddy Simulations of a boundary layer over a rotor blade are performed with and without inclusion of the rotational sources in the code. The numerical setup matches the one of a wind tunnel test available in the literature, and the numerical results are compared to each other and to the experiment. The mean boundary layers obtained from the simulations are studied by means of the linear stability analysis techniques with the aim of reproducing the transition location. It is shown that the non-rotating scenario, even when performed for the matching Mach and Reynolds numbers, predicts the transition location that is much farther downstream than the one seen in the experiment. However, inclusion of the rotational sources in the code moves the transition forward to the location that better agrees with the experiment. This shows that non-inertial forces associated with rotation play a crucial role in the transition in the considered setup. The character of the transition is different in two scenarios, being dominated by the Tollmien-Schlichting waves in the non-rotating case and by the travelling crossflow vortices in the presence of a laminar separation bubble in the rotating case. The linear stability analysis can reproduce the transition location in both cases, although the rotating scenario demonstrates noticeably lower amplification factors.