Simulating unsteady aerodynamics of helicopter rotor with panel/viscous vortex particle method

The unsteady aerodynamics of a helicopter rotor has been a central issue in the field of rotorcraft aerodynamics. This is because the rotor generates a complex time-dependent pattern of vorticity in its wake, which has significant effects on its performance, stability, loading, and vibration. Conventional free-wake methods used in most of the current comprehensive rotorcraft analysis codes are limited by the potential flow assumption and empirical formulations, such as vortex core size. Based on a numerical solution of the unsteady fluid-dynamic equations governing transportation and diffusion of vorticity, a viscous vortex particle method is coupled with an unsteady panel method to predict the unsteady aerodynamics of helicopter rotor blades with fewer empirical formulations in viscous flow. The coupled method is implemented through the trailing-edge Kutta condition, Neumann boundary condition, and by converting shed-wake doublet panels to wake vorticity. A TreeCode method is also employed to reduce computational cost for practical analysis. Helicopter rotors including the scaled model, Caradonna–Tung, and AH-1G rotors are simulated in hover and forward flight to validate the accuracy of the present approach. The unsteady dynamics of the rotor wake, such as wake contraction, tip-vortex pairing, and vortex roll-up, are well simulated. The predicted inflow distribution is more accurate than that in the conventional free-wake method, and the predicted pressure coefficient distribution and unsteady aerodynamic loads of rotor blades agree well with measured data and computational fluid-dynamics results.