Aerodynamic loads and flow structures on a simultaneously rotating and pitching flat plate

Abstract

Unsteady flow separation and leading-edge vortices (LEV) profoundly impact the aerodynamics of insect wings, helicopter rotor blades, and similar systems. The dynamics of such systems depend on wing pitch rate and pivot location. This study employs direct force measurement and particle image velocimetry to analyze a rotating and pitching flat plate that pitches from 0^◦ to 90^◦. The effects of changing reduced pitch rates (K = 0.02, 0.03, 0.05, 0.075, 0.1, 0.2), five non-dimensional pitch pivot locations (x_p/c = 0, 0.25, 0.50, 0.75, 1) and two Reynolds numbers (Re = 5,000 and 10,000) on the aerodynamic lift (C_L), drag (C_D) and moment (C_M) coefficients, as well as on the LEV system are investigated. The results show that the combined effect of K and x_p/c governs the resultant aerodynamic response of the rotating and pitching flat plate. Increasing K at x_p/c = 0 results in highest C_Lmax, C_Dmax and C_Mmax across all cases. The increase in the respective coefficients starts to decrease for the aft pivot locations and eventually becomes nonexistent at x_p/c = 1. A higher K results in a delayed pitch angle for LEV formation and an increase in LEV strength. Moving x_p/c towards the trailing edge also delays the pitch angle for LEV formation, but results in a decrease in LEV strength. Based on the induced camber effect of pitching motion, the study introduces a new trailing edge velocity-based scaling analysis that found to collapse the C_L and C_D trends for all K and x_p/c cases. Furthermore, the study discusses a noteworthy phenomenon – the dynamic convective time shift. Across various x_p/c values, a successful temporal shift leads to the convergence of time histories of C_L and C_D evolution at a same non-dimensional effective convective time (t^∗_eff ). Lastly, changing Reynolds number from Re = 5,000 to 10,000 reveals a noticeable decrease in C_L and C_D, which is accompanied by a lower vorticity growth rate at Re = 10,000.