High-Fidelity Simulation of Transitional Flows past a Plunging Airfoil

This investigation addresses the simulation of the unsteady separated flows encountered by a plunging airfoil under low-Reynolds-number conditions (Re-c <= 6 x 10(4)). The flowfields are computed employing an extensively validated high-fidelity implicit large-eddy simulation approach. Calculations are performed first turn SD7003 airfoil section at an angle of attack alpha(0) = 4 deg plunging with reduced frequency k = 3.93 and amplitude h(0)/c = 0.05. Under these conditions, it is demonstrated that, for Re-c = 10(4), transitional effects are not significant. For Re-c = 4 x 10(4), the dynamic-stall vortex system is laminar at its inception, however, shortly afterward, it experiences an abrupt breakdown due to the onset of spanwise instabilities. A description of this transition process near the leading edge is provided. As a second example, the suppression of stall at high angle of attack (alpha(0) = 14 deg) is investigated using high-frequency, small-amplitude vibrations (k = 10, h(0)/c = 0.005). At Re-c = 6 x 10(4), separation is completely eliminated in a time-averaged sense, and the mean drag is reduced by approximately 40%. For larger forcing amplitude (h(0)/c = 0.04, Re-c = 10(4)), a very intriguing regime emerges. The dynamic-stall vortex moves around and in front of the leading edge and experiences a dramatic breakdown as it impinges against the airfoil. The corresponding phased-averaged Now displays no coherent vortices propagating along the airfoil upper surface. This new flow structure is also characterized in the mean by the existence of a strong jet in the near wake which produces net thrust. This study demonstrates the importance of transition for low-Reynolds-number maneuvering airfoils and the suitability of the implicit large-eddy simulation approach for exploring such flow regime .