具有密度极值流体Rayleigh-Bénard对流结构多样性研究

Rayleigh-Bénard convection driven by the buoyancy in the horizontal fluid layer heated below has received considerable attention not only for its existence in many daily and engineering fields, but also for its fundamental aspects as the rich dynamical behavior and the complex pattern formation determined by the nonlinear effect of this simple system, and therefore, it is one of the research hotspots all over the world. Comparing with the common fluid, the Rayleigh-Bénard convection of fluids with density extremum behaves extreme multiplicity of flow patterns and much more complex transition characteristics. Aiming at the complex Rayleigh-Bénard convection problem of fluids with density extremum, this project studies the flow pattern multiplicity and the transition mechanisms of the complex flow patterns driven by the buoyancy in the horizontal fluid layer heated below by combining the experimental observation, numerical simulation with theoretical analysis. First, various possible flow patterns are explored very carefully using and three-dimensional numerical simulation when the Rayleigh-Bénard convection appears. Then, the evolutions of the flow and temperature fields are discussed with the increase of the Rayleigh number. The physical mechanisms of the instability and flow pattern transition are seeked under the different conditions. Furthermore, effects of geometrical parameters, density extremum characteristic of the fluids, the Rayleigh number and Prandtl number on the flow pattern and transition are analysed. Finally, critical conditions of the flow transition, stability diagram and bifurcation diagram of the flow pattern are obtained by coupling three-dimensional numerical simulation with the linear stability theory. Research contents proposed in this project are of important scientific and academic values for developing the non-equilibrium thermo-dynamic theory of the complex flow and extending the research field of the Rayleigh-Bénard convection.

Rayleigh-Bénard（R-B）对流广泛存在于日常生活及工业生产中，由于在这种简单的物理系统中会孕育出及其复杂多变的流动结构，因此，R-B对流的非平衡热动力学特征一直是国际前沿研究热点之一。相比于常规流体，具有密度极值流体的R-B对流的非平衡流动结构更加丰富、流型演化过程更加复杂，而这方面的研究还很不够。本项目即以具有密度极值流体层的R-B对流为研究对象，采用实验测量、数值模拟和理论分析相结合的方法，研究具有密度极值流体R-B对流的复杂特性，探索可能存在的流动结构，认知流型演变过程的物理机制，分析液层几何特性、流体密度极值特性、Rayleigh数和Prandtl数等对流动结构及其演变过程的影响，获取发生R-B对流的临界条件、流动结构稳定性图和流型分岔图谱。研究内容对于发展复杂流动非平衡热动力学理论、拓展R-B对流研究领域具有重要的科学意义和学术价值。

Rayleigh-Bénard（R-B）对流广泛存在于日常生活及工业生产中，由于在这种简单的物理系统中会孕育出及其复杂多变的流动结构，因此，R-B对流的非平衡热动力学特征一直是国际前沿研究热点之一。相比常规流体，具有密度极值流体R-B对流的非平衡流动结构更加丰富、流型演化过程更加复杂。本项目以具有密度极值流体层的R-B对流为研究对象，采用实验测量、数值模拟和理论分析相结合的方法，研究具有密度极值流体R-B对流的复杂特性，探索可能存在的流动结构，认知流型演变过程的物理机制，分析液层几何特性、流体密度极值特性、Rayleigh（Ra）数等对流动结构及其演变过程的影响，获取发生R-B对流的临界条件、流动结构稳定性图和流型分岔图谱。结果表明，（1）具有密度极值流体R-B对流发生临界Ra数比常规流体高，且临界Ra数随密度倒置参数的增大而增加。（2）流动发生后，随着密度倒置参数的增加，流型数减少，流型之间的转变变得简单。流型不仅与Ra数和密度倒置参数有关，还取决于初始条件和流型的演变过程，同时流型转变过程中存在滞后现象。（3）与常规流体相比，具有密度极值流体R-B对流中会出现一些新的流型。随着Ra数的逐渐增大，各种稳态流型都会转变为非稳态流动，然后转变为混沌流，最终过渡到湍流。（4）对于具有密度极值流体的湍流R-B对流，随着密度倒置参数的增大，冷羽流对大尺度环流的影响逐渐减弱，并最终消失，同时，主流区域温度升高，渗透深度减小。随着Ra数增大，依次观察到了软湍流和硬湍流两种状态，为此，建立了能够覆盖两种状态的Nusselt数和Ra数的指数关联式，并且发现，平均Nusselt数随密度倒置参数线性减小。本项目研究结果对于发展复杂流动非平衡热动力学理论、拓展R-B对流研究领域具有重要的科学意义和学术价值。