Dynamic Stall modeling using Viscous Vortex Particle Method for Coaxial Rotors

Dynamic stall is an important source of vibrations on a rotor at high advance ratios. Dynamic stall loads are induced by periodic flow separation and reattachment. In this study, the flow separation is modeled as the shedding of concentrated vorticity, known as a vortex particle, from the leading edge of the airfoil. The rotor wake is obtained from the generation of vortex particles over the rotor blade using the Viscous Vortex Particle Method. Blade loads are calculated using a reduced order model obtained from CFD, and dynamic stall loads are calculated using the ONERA dynamic stall model. Results are presented for isolated and coaxial rotors at advance ratios of μ = 0.3 and μ = 0.4. The results indicate that the separated wake modifies the vibratory hub loads by 15 percent-30 percent for an isolated rotor at μ = 0.3. The vibratory hub loads for the coaxial rotor are modified by 10%-65% at μ = 0.4. The separated wake modifies the angle of attack distribution on the rotor and hence influences the loads. The upper and lower rotor tip path planes are tilted such that the blade and wake interaction increases on the retreating side of the upper rotor, and decreases on the advancing side.