Plasma for shock wave mitigation: Experiment and theory

Experiments were conducted in a Mach 2.5 wind tunnel to explore the modification effects on the shock wave structure by two types of plasma spikes generated by on-board 60 Hz periodic and pulsed dc electric discharges in front of a truncated 60° cone with a protruding spike, which was used as a shock wave generator. The pronounced influence of plasma on the shock structure is demonstrated by the experimental results. The power measurements exclude the heating effect as a possible cause of the observed shock wave modification. (1) In the periodic discharge case, the nose of the model is a cone-shaped ceramic insulator with a very short protruding spike, which replaces the truncated part of the cone. The results show a transformation of the shock from a well-defined attached shock into a highly curved shock structure, which has increased shock angle and also appears in diffused form. As shown in a sequence with increasing discharge intensity, the shock in front of the model moves upstream to become detached with increasing standoff distance from the model and is eliminated near the peak of the discharge. A theory using a cone model as the shock wave generator is presented to explain the observed plasma effect on the shock wave. The analysis shows that the plasma generated in front of the model can effectively deflect the incoming flow; such a flow deflection modifies the structure of the shock wave generated by the cone model from a conic shape to a curved one. The shock front moves upstream with a larger shock angle, matching well with that observed in the experiment. (2) In the pulsed dc discharge case, the nose of the model is a spike sticking out of the truncated cone to the tip location of a perfect cone. Hollow cone-shaped plasma that envelops the physical spike is produced in the discharge. The results show that this plasma has changed the original bow shock to a conical shock, equivalent to reinstating the model into a perfect cone and to generate a 70% increase in the body aspect ratio. A significant drag reduction in each discharge is inferred from the pressure measurements; at the discharge maximum, the pressure on the frontal surface of the body decreases by more than 30%, the pressure on the cone surface increases by about 5%, while the pressure on the cylinder surface remains unchanged. The energy loss due to wave drag is reduced to make up for the two-thirds of the energy consumed in the electric discharge for the plasma generation. The measurements also show that the plasma effect on shock structure lasts much longer than the discharge period.