A Method for Approximate Prediction of Laminar-Turbulent Transition on Helicopter Rotors

Abstract

An approximate method is presented to predict the laminar-turbulent transition onset within the unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of helicopter rotors. Based on the presented method, the influence of laminar flow on the required rotor power and generated rotor thrust is investigated and compared to fully turbulent results. To compute the laminar boundary layer, an integral method according to Schlichting/Walz and a set of rotating velocity profiles according to Blaser and Velkoff are used. The laminar-turbulent transition onset is predicted by empirical criteria with respect to Tollmien-Schlichting and crossflow instabilities, bypass mechanisms, and attachment line contamination. The method presented accounts for the turbulence-level variation of the simulated rotor wake. A detection of the boundary layer edge within the URANS solution is not required. The approximate method is validated using the experimental onset positions of representative transition test cases. This includes a laminar flow airfoil and a swept wing. To cover a rotor in forward flight, the onset positions on the model rotor of a Mach-scaled helicopter configuration are computed and compared to available hot-film measurements. In general, the predicted laminar flow lengths on the rotor blades reduce the required rotor power by 4-5%. The rotor thrust remains practically unaffected.