As micro aerial vehicles (MAVs) find more applications in industry, their unsteady aerodynamic behavior becomes increasingly important. In the literature, stationary airfoils in ground effect have been studied over a wide range of Reynolds numbers (Re), but plunging airfoils in ground effect have been studied mostly at high Re, which may not be directly relevant to small MAVs. In this study, we perform two-dimensional numerical simulations of the flow around a plunging NACA 0012 airfoil in ground effect at a low Reynolds number (Re = 1000) and a moderate angle of attack (α = 10°) in order to better understand the unsteady aerodynamics of MAVs during take-off and landing. We use (i) a non-dimensional ground clearance of 0.2 ≤ H ≡ h/c ≤ ꝏ, where c is the chord length; (ii) a reduc...[ Read more ]

As micro aerial vehicles (MAVs) find more applications in industry, their unsteady aerodynamic behavior becomes increasingly important. In the literature, stationary airfoils in ground effect have been studied over a wide range of Reynolds numbers (Re), but plunging airfoils in ground effect have been studied mostly at high Re, which may not be directly relevant to small MAVs. In this study, we perform two-dimensional numerical simulations of the flow around a plunging NACA 0012 airfoil in ground effect at a low Reynolds number (Re = 1000) and a moderate angle of attack (α = 10°) in order to better understand the unsteady aerodynamics of MAVs during take-off and landing. We use (i) a non-dimensional ground clearance of 0.2 ≤ H ≡ h/c ≤ ꝏ, where c is the chord length; (ii) a reduced plunging frequency of 0 ≤ K ≡ 2πƒ_pc/U_ꝏ ≤ 7, where U_ꝏ is the free-stream velocity; and (iii) a non-dimensional plunging amplitude of A ≡ a/c = 0.001. Our results show the coexistence of two distinct modes of vortex shedding: (i) a forced mode due to the plunging motion, which occurs at frequency K, and (ii) a natural mode due to vortex shedding, which arises from global hydrodynamic instability. We find that within a certain range of K, lock-in can occur between the forced and natural modes, producing synchronized motion in the airfoil wake. We examine this lock-in behavior and the routes to lock-in using nonlinear time-series analysis. Knowledge of the critical plunging conditions required for lock-in is important for understanding and optimizing the design of MAVs.

Keywords: low Reynolds number, plunging airfoil, ground effect