Role of Improved Turbulence and Transition Modeling Methods in Rotorcraft Simulations

RANS turbulence models are modified to account for streamwise transition, crossflow transition, and strong adverse pressure gradients effects. Effectiveness of individual improvements are demonstrated through various simple 2-D and 3-D steady and unsteady flows. Simulations of several full-scale rotor configurations in hover are performed using the newly developed transport equations for laminar-turbulent transition onset prediction. The transition model includes effects of freestream turbulence intensity, streamwise pressure gradients, and crossflow instabilities. It was found that large portions of the rotor blades can remain laminar in hover even at high Reynolds numbers and significant adverse pressure gradients. Inclusion of crossflow transition onset criterion resulted in a more realistic transition onset prediction on the suction side of the rotor blade, whereas ignoring crossflow effects resulted in over-prediction of the extent of the laminar region. Rotor power predictions obtained with the transition model are consistently lower by about 1 – 5% compared to those obtained from a fully-turbulent simulation.