EXPERIMENTAL STUDY ON PROCESS AND MECHANISMS OF WAVE DRAG REDUCTION DURING PULSED LASER INTERACTING WITH NORMAL SHOCK1)

Abstract

Nanosecond pulsed laser has the prominent advantage of high peak power density, so it is easy to break down air to form plasma. It has an important application value in reducing supersonic wave drag. To deeply reveal the mechanisms of wave drag reduction by nanosecond pulsed laser, in this paper, the basic physical phenomenon of the interaction of pulsed laser plasma with a normal shock is studied by experiments. A high resolved schlieren system is developed to reveal the complex wave structures. Time resolution of the schlieren system reaches up to 30 ns, with a space resolution up to 1 mm. A high speed PIV system is applied to measure the velocity and vorticity of the flow field quantificationally. Time resolution of the PIV system reaches up to 500 ns. Features of the spherical shock wave and high temperature area with low density induced by laser plasma are revealed. The flow features and evolution process of the laser plasma impacted by shock wave are revealed. Simulated results are adopted to prove the basic reason of super sonic wave drag reduced by pulsed laser plasma. Research results show that: the initial Mach number of the shock wave induced by laser plasma increases with the laser energy increasing, and the shape is gradually developed from the droplet shape to the spherical shape. The propagation velocity decreases with time and is close to the sound velocity after 50 $\mu$s. The high temperature with low density region is approximate to sphere at first, and then begins to destabilize from the downstream of the laser incident direction. A sharp spike structure is then formed. 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 increases with the laser energy. The entrainment and contra-flow of the vortex can remodel the shape of the shock wave of the nose, which is an important way of flow field remodel. It causes a notable reducing of the surface pressure of the aircraft. It is the key mechanism that causes the wave drag reduction of supersonic vehicle.