Investigation of flow separation and its control over rotor blades in forward flight with plasma actuator

Abstract

Flow separation and its control over constantly rotating rotor blades operating at a large angle of attack in forward flight are comprehensively investigated through detailed numerical simulation possibly for the first time. An unsteady three-dimensional Reynolds-Averaged Navier-Stokes calculation with a Reynolds stress turbulence model is utilized to capture the dynamics of airflow. The resolved transient baseline flow pattern around the blade is observed to be heavily influenced by the azimuthal angle, exhibiting significant spatiotemporal characteristics. Two critical indicators, the high-order central moment of pressure (HCMP) and modulated location of peak pressure (MLPP), are generalized to identify the critical flow events involved in the flow evolution. As a result, one typical flow evolution cycle can be classified into three distinct stages: the initial flow separation stage, the flow reattachment stage, and the secondary separation stage. A vorticity transport framework is established within a non-inertial coordinate system attached to the blade, aimed at quantifying the sources and sinks of vorticities crucial for the flow evolution within a chordwise body-fitted control region. It is found that the vorticity transport process over the blade in different flow evolution stages or flow events is influenced by distinct vorticity generation mechanisms. The tip speed ratio (TSR) significantly influences the flow, with more pronounced flow separation occurring at higher TSR values. Moreover, active flow control is realized through the utilization of nanosecond dielectric barrier discharge (NS-DBD) pulsed plasma actuators. Thermal perturbations generated by NS-DBD plasma interact with the separated flow, inducing a series of spanwise vortices that effectively mitigate flow separation. The generation of the spanwise vortices introduces a significant amount of planar convective flux into the separation region and enhances vorticity production through the vortex tilting effect. As a result, the vorticity transport system is reshaped by these spanwise vortices. Under plasma actuation, the aerodynamic performance of the blade is notably enhanced. Across various TSRs, the torque coefficient of the rotor blade can be significantly reduced, with a maximum reduction of up to 21.58 %. Furthermore, thrust enhancement is more pronounced at higher TSRs, with the thrust coefficient of the blade increasing by 10.44 % at TSR = 0.5.