等温-绝热边界对湍流热对流转捩特性的影响

Through theoretical analysis and numerical simulation, the turbulent convection transition mechanism under different isothermal-adiabatic boundary conditions is studied systematically in this project. The effects of isothermal-adiabatic boundary with different effective isothermal ratios η and periodic width to height ratios φ on plume generation, evolution and intermittency of turbulence during transition are investigated. The time-averaged characteristics and fluctuation characteristics of flow field are analyzed. The law of large-scale motion of turbulent thermal convection under isothermal-adiabatic boundary and the difference of turbulent fluctuation intensity between different regions are revealed. In turbulent thermal convection with temperature-adiabatic boundary, the correlation between effective isothermal ratio_and period width-height ratio_and scaling laws of velocity structure function Su(r) and temperature structure function ST(r). The influence of different effective isothermal ratios η and period width-height ratios φ on Nusselt number, which characterizes heat transfer efficiency in turbulent heat convection. The influence of different effective isothermal ratio and periodic width to height ratio on heat transfer efficiency in turbulent heat convection are clarified. The influence of different effective isothermal ratios and periodic width to height ratios on turbulent RB system when transition from soft turbulence to hard turbulence. This project breaks through the transition mechanism of turbulent heat convection under the traditional isothermal boundary conditions and explains the influence of isothermal-adiabatic boundary on the transition characteristics of turbulent heat convection.

本项目拟通过理论分析与数值模拟对不同的等温-绝热边界下湍流对流转捩机理进行深入系统的研究。探究具有不同有效等温占比η及周期宽高比φ的等温-绝热边界对湍流对流转捩过程中羽流的产生、演化及湍流的间歇性的影响；分析流场的时间平均特性和脉动特性，揭示等温-绝热边界下湍流热对流中的大尺度运动规律以及不同区域间湍流脉动强度的差异；建立等温-绝热边界下湍流热对流中，有效等温占比η以及周期宽高比φ与速度结构函数Su(r)、温度结构函数ST(r)的标度律之间的关联；明确不同的有效等温占比η以及周期宽高比φ对湍流热对流中表征换热效率的Nusselt数的影响规律；揭示不同的有效等温占比η以及周期宽高比φ对湍流RB系统中向湍流转变的转捩区以及“软湍流”向“硬湍流”转捩的影响规律。突破传统等温边界条件下的湍流热对流的转捩机理，诠释等温-绝热边界对湍流热对流转捩特性的影响规律。

对流传热现象广泛存在于自然界及工程应用中，而目前大量的工作集中在等温边界的研究上，对不均匀边界的研究工作不多。本项目采用双分布的格子Boltzmann方法对等温-绝热边界条件下的Rayleigh-Bénard对流进行了模拟。详细讨论了Rayleigh-Bénard系统的失稳、“过渡区”及湍流时，非均匀的等温-绝热边界对流动及传热特性的影响。分析了等温-绝热边界对Nu数与Ra数之间标度律关系的影响。模拟结果显示，瑞利数在107~109区间内，混合边界与均匀边界的标度律指数均为0.3。比较了均匀等温边界和非均匀的等温-绝热边界对平均Nu数的均方根的影响。结果显示，混合边界更有利于抑制努塞尔数的脉动。同时，“过渡区”与湍流对流的平均Nu数脉动的对比可以发现混合边界对“过渡区”传热脉动抑制效果更好。最终，我们结合流场信息提出了“热源核心”成立的条件及其对传热脉动的抑制效果。