基于三角波瓣混合器的超声速流场精细结构和掺混特性

在超声速吸气式混合层风洞中,采用基于纳米粒子的平面激光散射(NPLS)技术对平板混合层和三角波瓣混合器诱导的混合层流场精细结构进行了对比实验研究.上下两层来流的实测马赫数分别为1.98和2.84,对流马赫数为0.2.NPLS图像清晰地展示了Kelvin-Helmholtz涡、流向涡、波系结构以及大尺度涡结构的配对合并过程.通过对比分析时间相关的NPLS流场图像,发现了大尺度拟序结构随时间发展演化的非定常特性.基于流动显示结果,采用分形维数和间歇因子指标对流场结构和混合特性进行了定量分析.实验研究表明,三角波瓣混合器诱导的流向涡结构显著提高了上下两层来流的掺混效率,其流动远场的分形维数突破了平板混合层中完全湍流区的分形维数值,达到了1.88,流场结构表现出明显的破碎性,有利于流动在标量层面的扩散和掺混.流动间歇性分析表明,流向涡与展向涡的相互剪切作用主导着混合层的掺混特性,同时由于流向涡的卷吸作用,三角波瓣混合器诱导的混合层混合区域更大,更多的流质被卷入混合区完成混合.

Fine flow structure and mixing characteristic in supersonic flow induced by a lobed mixer

In a supersonic suction type of mixing layer wind tunnel, by employing nanoparticle-based planar laser scattering (NPLS) method, contrast experiments are carried out with the emphasis on the fine flow structures of planar mixing layer and the mixing layer induced by triangular lobed mixer. The normal-shock equation, isentropic equation and sound speed relationship are utilized to calculate the flow parameters. The calculated Mach numbers are 1.98 and 2.84 for upper and lower airstreams respectively with a convective Mach number of 0.2. The NPLS images clearly shows the Kelvin-Helmholtz vortices, streamwise vortices, shock waves and the pairing processes of large-scale vortex structures. The unsteady properties of development and evolution for large-scale vortices are obtained by contrasting the NPLS images at different times. Also, it has been demonstrated by the present experimental investigation that in supersonic mixing layer with low convective Mach number, the small shock waves are still existing. These small shock waves that occur have negative effects on the mixing process. It is because the convection flow process of upper and lower airstreams is non-isentropic, causing the total pressure to lose. Based on the NPLS results, flow structures and mixing characteristics are analyzed quantitatively by using fractal and intermittency theory. The results show that the mixing efficiency increases obviously with the introducing of large-scale streamwise vortices. The nibbling of vortex clusters induced by large-scale streamwise vortices obviously increases the interface area of mixing. Meanwhile, compared with planar mixing layer, larger spanwise structures roll up in triangular lobed mixing layer, leading to more entrainment of upper and lower airstreams. In the present investigation of supersonic planar mixing layer, the value of fractal dimension of fully turbulent region is stable at 1.55-1.6. Whereas the value of fractal dimension for triangular lobed mixing layer reaches 1.88 at the flow field far away downstream, which breaks through the value of fully developed turbulence for planar mixing layer. Besides, in triangular lobed mixing layer, the shear action between streamwise vortices and spanwise structures plays a leading role in promoting mixing. The mixing flow shows the property of apparent crushability and three-dimensional behavior, which plays a positive role in promoting mixing at a scalar level. The analysis of intermittency indicates that the interaction between streamwise and spanwise vortices dominates the mixing characteristics, and due to the entrainment of streamwise vortices, the mixing region induced by triangular lobed mixer becomes larger, and more fluids are engulfed into the mixing region to complete the mixing process.