Investigation of flow separation control over airfoil using nanosecond pulsed plasma actuator

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A detailed investigation of flow separation control over a stalled NACA 0015 airfoil model using pulsed nanosecond dielectric barrier discharge (NS-DBD) plasma actuator is conducted through combined large eddy simulation (LES), dynamic mode decomposition (DMD) and experimental particle image velocimetry (PIV) measurement possibly for the first time. Airfoil flow at chord-based Reynolds number (Re) of Re = 6.3 × 104 is considered. It is found that the interplay of the residual heat due to the nanosecond plasma discharge with external flow causes the generation of vortices through the baroclinic and vortex dilation mechanisms and finally promotes the formation of large-scale vortical structures that dominate the early-stage evolution of the flow control process. One of the most important findings of this work is that the flow control mechanisms are not universal but depend on the forcing frequencies of plasma actuation, and are categorized into three different types. Meanwhile, three-dimensional DMD analysis of LES datasets is performed and the output of modal analysis helps provide more physical insights into flow control and determine the optimum forcing frequency. Depending on the value of forcing frequency, the global or local resonance of the forced flow with the periodic plasma actuation occurs. The plasma forcing at any frequencies in the range 1≤F+≤20 produces pronounced flow control authority except for that at F+=20. Moreover, the optimum forcing frequency is identified and its relationship with the inherent characteristic frequencies of the baseline flow is established based on DMD analysis. It is found that the most favorable control effect is achieved when the plasma excitation frequency is high and very close to the characteristic frequency of the separated shear layer in the unforced baseline flow.