Richtmyer-Meshkov（RM）混合流动广泛存在于自然（如超新星爆发）及工程界（如惯性约束聚变，ICF）中。特别地，在ICF中，RM混合及其后续转捩至湍流会显著降低热核性能并直接影响点火成败。其中，关键的混合转捩过程一直是制约基础研究及工程应用的瓶颈问题。目前，虽然学术界已在转捩准则及湍流发展的临界状态方面有所认识，但对于混合转捩的具体演化过程尚不清楚。本项目将以中等密度比平面RM混合流动为研究对象，从物质混合及流体动力学两方面性质来综合研究混合转捩的演化机制。主要内容为：（1）研究转捩过程中的物质混合特性，包括物质界面结构、混合宽度、混合度及体积分数等剖面的演化特征；（2）分析混合转捩过程中流体的动力学性质，重点关注流动涡结构、长度尺度（能谱）、雷诺应力等统计脉动量剖面的演化特征。此项研究进一步将为发展混合转捩模型奠定基础，对发展实验室规模的天体物理预测平台给予启示。

Richtmyer-Meshkov (RM) instability induced mixing and subsequent transition to turbulence broadly occur in both natural phenomena, such as supernova explosions, and high-energy-density applications, such as inertial confinement fusion (ICF). Specifically, in ICF, the mixing zone will interact with multiple waves reflected from the capsule center, resulting in increased mixing and transition to turbulence. This leads to the quenching of fuel and magnificently reducing the efficiency of the ignition. Understanding mixing transition aids in clearing of important scientific and engineering bottlenecks. However, previous researchers in this field mainly concentrate on the study of transition criteria and minimum state of turbulence, while the detailed mixing transition process is still unclear. Our research aims at uncovering the mixing transition mechanism from two aspects-material mixing and flow dynamics, by the direct numerical simulation (DNS) of RM mixing with medium density ratio. The detailed items included are as follows: (1) investigating the evolution of mixing in mixing transition process after the reshock, including mixing structure, mixing width, profiles of mixedness and volume fraction along with other mixing quantities; (2) analyzing the characteristic flow dynamics during mixing transition, emphasizing on the evolution of vortex structure, length scales (based on energy spectrum), Reynolds stresses and other statistics. Finally, the physical mechanism of mixing transition will be revealed, which will further shed light on the development of mixing transition models and guiding the design activities for creating turbulent flows of astrophysical phenomena in the laboratory.