Improved Initial and Boundary Conditions for Hovering Rotor CFD Simulations

Abstract

The new flow fields originate in a 2012 paper by Spalart (Ref. 1), which addresses the far-field behavior induced by a momentum source, such as a hovering rotor or a static jet. This model incorporates entrainment into the turbulent far wake, and supersedes the time-honored model based on a sink field. In the present study, a new initial condition as well as boundary condition based on a modified version of the above far-field model are investigated by unsteady simulations of the XV-15 rotor in hovering flight condition at collective pitch angle ? = 10?. The idea is to inject an approximation to the rotor's wake under it. From our results, compared to the freestream initial condition, the far-field model based new initial condition can quickly establish the low-speed induced flow and achieve faster convergence of thrust and torque to statistically steady state. The chaotic starting vortex can be effectively flushed out by the new initial condition to prevent the oscillations of thrust due to blade-vortex interaction shown in the simulation from the freestream initial condition. Results of the modified far-field model as a far-field boundary condition to hovering f light in multiple reduced computational domains are also presented. Although the new boundary condition doesn't show evident speedup in convergence of thrust and torque, its wake can be developed much faster than the freestream boundary condition due to its prescribed velocity field at the boundary. This new initial and boundary condition could be beneficial to simulations of multi-rotor aircraft.