关键词: 超声速混合层, 隔板, 直接数值模拟, 流场结构, 转捩, 湍流特性

Abstract:

The mixing layer flow model can be easily found in super-sonic combustion ramjet propulsion systems when high-speed fuel gas and air encounter. Most high-precision direct numerical simulation on the flow evolution and mechanism of mixing layer are based on the flow model without splitter plate， and few research consider the effect of splitter plate on the mixing layer. Therefore， the three-dimensional supersonic mixing flow model with a splitter plate is taken as the research subject， where the inflow convective Mach number is 0.8 and the thickness of splitter plate is selected as 4 mm. Using the self-developed high-precision direct numerical simulation program， both the evolution of fine flow structure and the turbulence statistics in supersonic mixing flow down-stream the spitter plate are systematically analyzed. The research results show that after the two supersonic incoming flows form a recirculation zone at the trailing edge of the partition， they merge into a supersonic mixing layer. Transition begins after a very short distance and finally the flow reaches the self-similar stage. Various typical vortex structures generate in the flow transition region. It is found that the hairpin vortex structures developed from Λ vortices are the primary vortex structures in the transition development region， and in the late stage of the flow transition， the hairpin vortices begin to shed and twist and finally form the multiple necklace-like vortex. In the self-similar region， slender quasi-streamwise vortices exists in this region， and there are numerous small-scale vortex structures scattered in the flow field. Unlike the case without splitter plate， the mixing flow influenced by the splitter plate transits faster and show intense three-dimensional characteristics. At different spanwise locations， there are notable differences in the evolution speed of the flow field structure. Meanwhile， the vortex structures interact more vigorously in the spanwise direction. Through turbulence statistics analysis， it is found that in the transition region of mixing layer with an splitter plate， the Reynolds shear stress follows a unimodal distribution. Besides， the numerical results of the Reynolds shear stress terms in the self-similar region are consistent with the experimental results， notably， the transverse Reynolds stress term increase significantly， indicating that the presence of the splitter plate introduces considerable lateral disturbances to the flow field. Meanwhile， the flatness factor value approaches 3， indicating that the self-similar region has achieved an isotropic state. The three-dimensional vortex structure evolution， quantitative characterization of the mixing layer growth process， and turbulence statistics results of the supersonic mixing layer with the splitter plate obtained can provide data references for related experimental and numerical simulation studies.

Key words: supersonic mixing layer, splitter plate, direct numerical simulation, flow structure, transition, turbulence characteristic