Vortex Identification and Quantification on Blunt Trailing-Edge Rotor Blades in Reverse Flow

An inherent aerodynamic limitation of high-advance-ratio high-speed helicopters is reverse flow on the retreating blades, which can be a source of several unsteady effects, like vortex formation, that reduce rotor performance. The current study is the first detailed comparison of a rotor with a conventional, sharp trailing-edge NACA 0012 airfoil section with one equipped with a blunt-edge elliptical airfoil section. The aim was to detect and quantify vortex formation in reverse flow using planar particle image velocimetry on a subscale, low-Reynolds-number rotor rig in a water tank. The investigated blade azimuth angles ranged from 240 to 300 deg for advance ratios between 0.40 and 1.00 at pitch angles from 13 to 25 deg. Four main vortex structures were detected. At the aerodynamic leading edge, a strong interference of the tip vortex with the reverse-flow dynamic stall vortex was identified when blade flapping was restricted. Dynamic stall vortices advect closer to the blade surface for the blunt elliptical airfoil, thus reducing the wake area in reverse flow. Blade pitch kinematics govern the reverse-flow entrance-vortex strength and coherency. A coherent vortex street or wake sheet forms at the aerodynamic trailing edge of either airfoil. Overall, the vortex structures that form on the ellipse are more coherent than those on the NACA 0012. The current study highlights the significance of airfoil geometry, specifically the trailing-edge curvature, of an airfoil on the resulting vortex structures in reverse flow, strongly affecting the rotor performance at high advance ratios.