激波诱导环形SF6气柱演化的机理

基于可压缩多组分Navier-Stokes控制方程,结合5阶加权本质无振荡格式以及网格自适应加密技术和level-set方法,数值模拟了平面激波(Ma=1.23)与环形SF₆气柱(内外半径分别为8和17.5 mm)界面的相互作用过程。相比于之前的实验结果,数值模拟结果揭示了入射激波在界面内4次透射过程中的复杂波系结构,观察到透射激波在内部界面传播时形成自由前导折射结构并向自由前导冯诺依曼折射结构转换的波系演变过程;另外,界面内的复杂激波结构诱导内部下游界面上的涡量发生了3次反向;在界面演化后期,内部界面形成的“射流”结构与下游界面相互作用,诱导界面形成一对主涡、一对次级涡以及一个反向“射流”结构。定量分析了环形界面长度、宽度、位移、环量以及混合率的变化情况,结果表明,内部气柱的存在减弱了前期小涡结构合并形成大涡结构过程中对界面高度与长度的影响,同时提高了重质气体与环境气体的混合率。

On the evolution mechanism of the shock-accelerated annular SF6 cylinder

Based on the compressible multicomponent Navier-Stokes equations, the interaction of a planar shock wave (Ma=1.23) with an annular SF₆ cylinder whose inner and outer radii were set as 8 and 17.5 mm respectively was numerically studied. The simulation was conducted based on the finite volume method. For capturing the complex shock and vortex structures as well as the interfaces, the adaptive mesh refinement method, level set method, and fifth-order weighted essentially non-oscillatory scheme were used for the simulation. The adaptive mesh refinement method dynamically refined the uniform Cartesian grids around the multiple moving shocks and accelerated interfaces. The level set method tracked the interface, while the fifth-order weighted essentially non-oscillatory scheme captured discontinuities such as shock waves and contact surfaces. Time advancement was achieved with the third-order strong-stability-preserving Runge-Kutta method. Compared with the previous experimental results, numerical results revealed the complex evolution of shock wave structures generated in the process of four shock transmissions in the annular cylinder. It is found that the transition from free precursor refraction to free precursor von Neumann refraction occurs when the transmitted shock wave passes through the inner cylinder. In addition, the complex shock structures that developed between the inner and outer downstream interfaces cause the pressure gradient direction to reverse several times on the inner downstream interface, which eventually leads to three reversals of vorticity on the inner downstream interface. In the later stage, the “jet” structure formed on the inner cylinder would impact the downstream interfaces, and finally induces the interfaces to generate a pair of primary vortices, a pair of secondary vortices and a reverse “jet”. Quantitative analyses of the variation of the length, width, displacement, the circulation and mixing rate of the annular cylinder were conducted. The results demonstrate that the presence of the inner cylinder attenuates the influence on the height and length of the annular cylinder during the process of small vortexes merging into the large vortexes in the early stage, and increases the mixing rate of the heavy gas and the ambient gas.