基于NPLS的斜激波干扰超声速后台阶流场精细结构实验研究

NPLS Based Experimental Investigation on Fine Flow Structures over Supersonic Backward-Facing-Step under Interference of Oblique Shock Wave

关键词:  斜激波  超声速  后台阶  平面激光散射  边界层  大尺度结构

Key words:  Oblique shock wave  Supersonic  Backward-facing-step  Nano-tracer-based planar laser scattering  Boundary layer  Large scale structure

摘要:

为分析斜激波与超声速后台阶绕流的相互作用规律，在Ma=3.8的小型低噪声风洞中，开展了台阶高度分别为5mm及10mm的实验研究。采用基于纳米示踪的平面激光散射（Nano-tracer-based planar laser scattering，NPLS）技术获取了流场精细结构图像，在流场上游上表面安装斜劈生成斜激波，对比研究了不同入射角度和作用位置的斜激波对不同高度后台阶流动结构的干扰作用。无斜激波干扰时，台阶附近的剪切层流动为大范围的低速区。斜激波作用于边界层再附发展区域时，台阶下游流动逆压梯度较大，激波后的边界层大幅抬起，形成大量的大尺度/拟序涡结构，斜激波作用点前的大尺度结构运动速度约比来流速度低22%，发展周期大于20μs，作用点后速度加快到接近主流速度；斜激波在边界层发生反射，反射激波形状扭曲严重，同时诱导产生大量不规则的小激波结构。

Abstract:

In order to analyze the law of interaction between oblique shock wave and flow around steps after supersonic speed， in a small low-noise wind tunnel with Ma=3.8， experimental studies with step heights of 5mm and 10mm were carried out. Nano-tracer-based planar laser scattering （NPLS） technology was used to obtain the fine flow structure images. An oblique wedge is installed on the upper surface of the upstream flow field to generate oblique shock waves. The interference effects of oblique shock waves with different incident angles and acting positions on the backward-facing-step（BFS） flow structure of different heights were compared. Without oblique shock interference， the shear layer near the steps flows into a wide range of low-velocity areas. However， when the oblique shock wave acts on the redeveloping boundary layer， the flow inverse pressure gradient downstream of the step is significant. As a result， the boundary layer after the shock wave significantly lifts， forming a large number of large-scale/coherent vortex structures. The movement speed of the large-scale structure is about 20% lower than that of the incoming flow， and the developing period is larger than 20μs. After the point of action the flow accelerates almost to the mainstream speed. The oblique shock wave reflects on the boundary layer. The shape of the reflected shock wave is severely distorted， and many irregular small shock wave structures are induced simultaneously.