The deposition of laser energy will ionize the air and generate high-temperature and low-density plasma hot core. The interaction of laser-induced plasma with quiescent air and shock waves can change the flow structure, which will change the local pressure. The evolution of the plasma hot core in the quiescent air and the interaction of the hot core and the normal shock at Mach number 1.75 are numerically simulated. The results show that the hot core evolves into a threelobed structure in quiescent air due to the Richtmyer-Meshkov instability; The baroclinic pressure gradient caused by the interaction of shock wave and the hot core boundary leads to the splitting of the hot core into two vortex rings, which are the key reason of the change of the flow field caused by laser.
With the prominent advantage of high peak power density and easy to break down air to form plasma, nanosecond pulsed laser has an important application value in reducing supersonic wave drag. To reveal the mechanisms of drag reduction by nanosecond pulsed laser, bow shock is simply considered as a normal shock in this paper. The phenomenon of the interaction of laser plasma and normal shock is studied by experiment. A high resolved schlieren system is applied to reveal the complex wave structures. A high speed PIV system is built to measure the velocity and vorticity. Time resolution of the PIV system reaches up to 500ns. Firstly, a nanosecond pulsed laser is focused in the experiment section of the shock tube to form laser plasma. Secondly, laser plasma is impacted by a normal shock in the shock tube. The flow features and evolution of the laser plasma during the interaction are revealed. Research results show that: Under the impact of the normal shock, the high temperature and low density region evolves into an upper and lower symmetric double vortex ring structure, and the size of them increases with the laser energy. The entrainment and contra-flow of the vortex can cause the reduction of the surface pressure of the aircraft, and reform the shape of the bow shock. It is the key flow structure that causes the drag reduction of supersonic vehicle.
Pressure sensing and schlieren imaging with high resolution and sensitivity were applied to the study of the interaction of pulsed laser energy with bow shock at Mach 5. A Nd:YAG laser operated at 1.06μm, 100mJ pulse energy was used to breakdown the hypersonic flow in shock tunnel. 3 dimensional Navier-Stokes equations were solved with upwind format to simulate the interaction. Stagnation pressure of the blunt body was measured and calculated to examine the pressure variations during the interaction. Schlieren imaging was used in conjunction with the calculated density gradients to examine the process of the interaction. Results showed that the experimental stagnation pressure and schlieren imaging fitted well with the simulation. Stagnation pressure would increase when the transmission shock approached to the blunt body and decrease with the reflection of the transmission shock. Bow shock was deformed during the interaction. Schlieren imaging supplied important phenomenon to investigate mechanism of the interaction.
Structures which are similar to double wedges usually exist in a supersonic inlet. When a supersonic flow goes
through double wedges, oblique shock waves intersect and shock wave reflection appears. In an off design condition, Mach Reflection (MR), one kind of shock wave reflection, probably appears. The laser energy deposition helps decrease the Mach stem height and reduce the total pressure loss. Numerical simulation on the influence of deposition location and laser energy in reducing the Mach stem height was investigated in Mach 3.45, 5, 6, 7. Mach Reflection was achieved at different Mach number. The simulations and analysis showed that proper increase of distance between energy deposition location and Mach stem where was near by the upstream oblique shock waves would have a better effect on decreasing Mach stem height. When inflow Mach number changed, the influence of laser energy deposition became different at a variety of deposition location
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