EXPERIMENTAL STUDY ON PROCESS AND MECHANISMS OF WAVE DRAG REDUCTION DURING PULSED LASER INTERACTING WITH NORMAL SHOCK1)
WangDiankai2),, WenMing, WangWeidong, QingZexu State Key Laboratory of Laser Propulsion and Application, Space Engineeing University, PLA Strategic support Force, Beijing 101416, China 中图分类号:V211.1 文献标识码:A
关键词:超声速;波阻;激光;流动控制;激波管 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.
激光与流场的相互作用包含两个基本要素:电离空气时形成的激波和高温低密度区域.激光脉宽为纳秒量级,激光能量沉积和等离子体产生的时间尺度为亚纳秒量级,激光等离子体与流场的相互作用过程的时间尺度为微秒量级,因此,为揭示流动细节机理,重点关注激光等离子体产生以后的流场变化过程.为最大限度地排除其他因素的干扰,本文首先利用单脉冲纳秒 Nd:YAG 激光聚焦电离静止大气,利用高时间空间分辨率纹影技术,实验研究激波传播速度和高温低密度区域形状变化发展规律. 2.1.1 脉冲激光等离子引致激波的特性 单脉冲激光能量电离空气后的流场时序纹影照片如图 4 所示,其中激光能量为 209mJ,脉宽 8 ns,空气为静止、常温、常压大气, 纹影照片每像素代表 45.45$\mu$m,黑色短线代表 10 mm 的长度,激光从左向右入射. 在 $t$ = 100ns 时,电离位置的白色区域为等离子体,由于激光等离子体的屏蔽现象,可以看出空气被激光能量电离的区域并不是一个理想的点、圆球形或者椭球形,而是一个轴对称的类似于水滴的形状,在逆着激光入射的方向能量较高,等离子体区域较大,顺着激光入射的方向形成尖头,此时激光引致的激波尚未与等离子体分离.在 $t$ = 400 ns 时,等离子体已经湮灭,激波阵面已经形成,从中可以清晰看到,激光等离子体引致的激波阵面的初始形状也为水滴形. 在 $t$ = 1$\sim$15 $\mu$s 之间,激波不断向外传播,由初始水滴状逐渐向球状转变,同时在电离位置形成了近似于球形的空气泡,由于激光等离子体具有高温的特点,可以推想,该空气泡被激光等离子体加热,具有高温、低密度的特点.在 $t$ = 20 $\mu$s 之后,激波阵面基本呈球状,高温低密度的空气泡形状仍然近似于球形. 显示原图|下载原图ZIP|生成PPT 图4单脉冲激光电离静止大气后的纹影照片 -->Fig.4Schlieren images of the stationary atmosphere ionized by single pulse laser --> 图5是不同入射激光能量下激波传播马赫数的对比.为了方便,选取纵向传播马赫数 $M_{y}$ 进行研究.入射激光能量越大,激波初始马赫数越大,单脉冲能量达到 152mJ 时,初始马赫数达到 4.1,单脉冲能量达到 56 mJ 时,初始马赫数达到 3.4.不同的入射激光能量引致的激波的发展情况是相似的,传播速度都随着时间降低,在 $t$= 50 $\mu$s 后接近声速. 显示原图|下载原图ZIP|生成PPT 图5单脉冲激光等离子体引致的激波传播马赫数 -->Fig.5Mach number of the shock wave induced by single pulsed laser plasma -->
2.1.2 高温低密度区域的特性 高温低密度区域的纹影实验结果如图 6 所示. 在 $t$ = 18 $\mu$s 时,高温低密度区域近似于球状,值得注意的是逆着激光入射方向的界面比较光滑,而在激光入射方向的下游则开始失稳,出现了褶皱.失稳首先出现在激光入射方向的下游,即右半边球面,其原因是由于激光等离子体的屏蔽作用,引起激光能量沉积不均匀,入射方向的下游沉积激光能量较少,流场状态变化幅度相对较小,流动较弱,更容易受到外界环境气体扰动.外界密度大的气体刺进高温低密度区域,形成尖刺;高温低密度区域的气体进入外界,形成气泡;尖刺和气泡共同表现为褶皱.随着扰动的叠加,失稳更加剧烈,在 $t$ = 22 $\mu$s 时,下游一侧开始塌陷,外界空气逆着激光入射方向刺进高温低密度区域.此后,外界空气逐渐深入高温低密度区域上游,在 $t$ = 60 $\mu$s 时,完全穿透,形成 "等离子体尖刺",并继续向上游流动. 在 $t$ =150 $\mu$s 时,高温低密度区域已经演变形成了上下对称的双涡环结构,外界空气从两个涡环的中间穿过.在外界空气流动的过程中,部分高温低密度气体由于黏性作用被带出,流场最终演化为 "蘑菇云" 形. 显示原图|下载原图ZIP|生成PPT 图6高温低密度区域的纹影照片 -->Fig.6Schlieren images of the high temperature and low density area -->
2.2 激光等离子体在正激波冲击下的流动特性
在激波管中开展实验,单脉冲激光能量 82mJ,正激波马赫数 1.45,实验段为常温常压静止大气.选取两个典型时刻,分析激光等离子体在正激波冲击下的流动特性,如图 7 所示,左侧为纹影试验照片,右侧为数值计算得到的对应时刻的密度梯度图,其中数值计算采用了非对称的激光能量沉积模型和化学反应模型 [33],以获得更加符合实验的模拟结果. 显示原图|下载原图ZIP|生成PPT 图7激光等离子体在正激波冲击下的纹影结果 -->Fig.7Schlieren images of laser plasma under the impact of the normal shock -->
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