Experimental Study on Multiobjective Flow Control of Dynamic Stalls Using a Vortex Generator

Abstract

The dynamic stall of the airfoil can adversely affect the performance of helicopter rotor blades, jet engine compressor blades, wind turbine blades, and so on. Therefore, the flow control on the dynamic stall is necessary. This paper conducted an experimental study on the multiobjective flow control effect of the dynamic stall on a NACA0012 airfoil using a passive vortex generator (VG). The control effect of the VG on the dynamic stall is discussed, and the control mechanism is explained. Within the scope of this paper, the VG could significantly increase the maximum normal force coefficient (Cn) by 0.223, reduce the peak negative moment coefficient (Cm) by 0.049, and delay the stall angle of attack of Cn and Cm by Δα=1.8° and 2.9°, respectively. These multiobjective control effects were attributed to the control of the complex vortices on the suction surface. The VG delays stall by suppressing the formation of the shear layer vortex, and enhances the aerodynamic performance during dynamic stall by energizing the dynamic stall vortex (DSV). The effects of the VG on the convection speed and strength of the DSV are additionally investigated from a space-time perspective. Within the reduced-frequency range studied in this paper, the DSV convection speed controlled by VG was smaller than that of the baseline airfoil, and the VG could significantly enhance the strength of the DSV. Dynamic stall phenomena are widely found in nature (wings of flying birds, fins of swimming fish, and so on) and in engineering (bionic flutter wings, helicopter rotors, wind turbine blades, and so on). It is widely believed that the DSV is the main flow structure dominating the dynamic stall. For helicopter rotor blades and large wind turbine blades, dynamic stall is prone to occur in the blade tip under complex realistic conditions. The vortex lift during dynamic stall will significantly raise the blade load, and the motion of the DSV will cause a sharp change in the blade moment, which may damage the blade structure and control system. Therefore, the dynamic stall needs to be controlled. In this paper, passive VGs were used to control the dynamic stall on an airfoil, and ideal multiobjective control effects were obtained: reducing the negative peak moment, increasing the maximum lift coefficient, delaying the stall, and reducing the hysteresis effect. This work is a guideline for the dynamic stall control of helicopter rotor and wind turbine blades.