The influence of kinematics of blades on the flow structure in deep dynamic stall

Abstract

This study considers the effect of kinematics on the aerodynamic loads and flow structure around moving blades of micro air vehicles (MAVs) in deep dynamic stall. The transversal (pure heaving) and rotational (pure pitching) motions are considered distinctly to investigate the dynamic stall. An equivalent effective angle of the attack profile is given to both motions. This method helps to figure out the influence of kinematics on flow structures when all boundary conditions and effective angles of attack profiles are the same. An experiment is conducted in fully turbulent flow at Re = 1.5×104 to avoid any transition regime in the boundary layer, and make the results relatively independent of the flow characteristics. A NACA 0012 airfoil is chosen at high reduced frequencies (k = 0.25 and 0.375) and high angles of attack to reach deep dynamic stall conditions. Additionally, time-resolved particle image velocimetry (PIV) and post-processing are used to compute the aerodynamic loads using a control-volume approach. The flow field is also reconstructed using proper orthogonal decomposition (POD) to separate the flow structures in different modes. It is shown that the kinematics can significantly influence the flow structure and aerodynamic loads. In the pre-stall region, the pure pitching motion usually produces higher lift force, while the pure heaving motion has a higher lift peak. However, in the post-stall region, the pure heaving motion usually has higher lift than the pure pitching motion. The pure heaving motion produced lower drag force than the pure pitching motion. For pure heaving motion, the POD analysis reveals there is a high-energy mode in the flow structure that helps to make the vortices more stable compared to pure pitching motion. Furthermore, the pure heaving motion adds extra kinetic energy to the boundary layer, which decelerates the reversal flow and the transfer of the separation point on suction side of the airfoil.