We perform a systematic numerical investigation of the nanosecond-pulsed surface dielectric-barrier-discharge evolution under different electrode polarities. For both positive and negative electrode polarities, two discharge strokes take place corresponding to the leading edge and the trailing edge of the nanosecond voltage pulse. During the first discharge stroke, the positive streamer propagates along the dielectric surface accompanying a thin plasma sheath layer, while the negative streamer stays attached to the dielectric surface. The resultant propagation velocity of the positive streamer is found to be faster than that of the negative streamer. During the second discharge stroke, a plasma sheath layer forms between the negative streamer and the dielectric surface due to the electrons drifting away from the near-surface region, while the sheath layer between the positive streamer and the dielectric surface fades away due to the electrons drifting toward the dielectric surface. For both positive and negative electrode polarities, it is revealed that a strong downstream body force is generated when the plasma sheath layer exists, due to the high net charge density and strong electric field in the near-surface sheath layer.
Two-dimensional numerical simulation of a surface dielectric barrier discharge (SDBD) plasma actuator, driven by a nanosecond voltage pulse, is conducted. A special focus is laid upon the influence of grid resolution on the computational result. It is found that the computational result is not very sensitive to the streamwise grid spacing, whereas the wall-normal grid spacing has a critical influence. In particular, the computed propagation velocity changes discontinuously around the wall-normal grid spacing about 2 μm due to a qualitative change of discharge structure. The present result suggests that a computational grid finer than that was used in most of previous studies is required to correctly capture the structure and dynamics of streamer: when a positive nanosecond voltage pulse is applied to the upper electrode, a streamer forms in the vicinity of upper electrode and propagates along the dielectric surface with a maximum propagation velocity of 2 × 108 cm/s, and a gap with low electron and ion density (i.e., plasma sheath) exists between the streamer and dielectric surface. Difference between the results obtained using the finer and the coarser grid is discussed in detail in terms of the electron transport at a position near the surface. When the finer grid is used, the low electron density near the surface is caused by the absence of ionization avalanche: in that region, the electrons generated by ionization is compensated by drift-diffusion flux. In contrast, when the coarser grid is used, underestimated drift-diffusion flux cannot compensate the electrons generated by ionization, and it leads to an incorrect increase of electron density.
One-zone inhomogeneous phenomenological nanosecond dielectric barrier discharge (NS-DBD) actuation model used for flow control simulation is established to investigate the flow control mechanisms, based on experiments and theoretical analysis. When the inhomogeneous phenomenological model is applied to a plate, the formation of spanwise vorticity is analyzed through the vorticity transport equation, and the spanwise vorticity is mainly engendered due to the baroclinicity of pressure gradient and density gradient, also due to the vorticity transfer by the flow convection in the vicinity of the actuation region. Agreement of the simulation with experiments on a column shows that the inhomogeneous phenomenological NS-DBD actuation model is reasonable. Separation control over NACA 0015 airfoil at high angle of attack indicates that the spanwise vortices induced by plasma actuation make the separated shear layer instable, promote interaction between shear layers, and downstream the separation point. Different excitation frequency has different effect on the lift, with the optimum reduced frequency F+ ≈ 6 in current simulation.
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