湍流热对流中对数温度剖面的实验研究

Thermal convections are ubiquitous in nature and engineering; and they influence our life in various aspects. Rayleigh-Bénard (RB) convection is an idealized model for studying the fundamental problems involved in convection phenomena. It is commonly thought that when the fully developed turbulent RB convection gets into the so-called "ultimate regime" from the "classical regime", the boundary layers (BLs) will become turbulent and the "log layers" are expected to emerge, i.e. the temperature and velocity profiles vary logarithmically with the distance from the plates when this distance is not too large but outside the BLs. However, some recent studies reported the existence of logarithmic temperature profiles in the "classical regime", which came as a surprise and the reason for its origin remains unclear. In the proposed project, we will investigate the mechanism of the formation of logarithmic temperature profiles in the "classical regime" by carrying out a series of experimental measurements of both the temperature and velocity fields in turbulent RB convection. First of all, we will use different convection systems with different mean flow patterns to study the effects of large-scale circulation and corner roll on the temperature profiles. In particular, we will examine quantitatively the role of plumes, including their spatial distribution and strength, in the formation of logarithmic temperature profiles. We will also study whether the shear stress will influence the behavior of temperature profiles (and how if yes). Moreover, we will investigate the detailed properties of temperature and velocity fluctuations, and compare their behaviors with that of the mean profiles. The outcomes of this proposed study will not only give us a comprehensive understanding of the physical origin of the logarithmic temperature profiles in the "classical regime", but will also enrich our existing knowledge of the turbulent BLs and heat transport in turbulent convection.

热对流普遍存在于自然界和工程应用当中，并对人类生活有着重要影响。Rayleigh-Bénard(RB)对流是研究对流问题的理想模型。早期人们认为，当充分发展的RB湍流热对流从“经典区间”进入“终极区间”后，边界层会由层流转变为湍流，相应的温度和速度剖面将出现对数律层。然而，近年来有研究在“经典区间”也观测到了对数温度剖面，但其物理起源尚无定论。本项目将通过系统地实验测量RB湍流在不同流动特性下的温度场和速度场，从三个方面定量考察对数温度剖面的形成机制：(1)通过改变系统宽高比来改变对流的形态结构，研究大尺度环流和角涡对温度剖面的影响；(2)研究羽流空间分布及其强度在形成对数温度剖面时的作用；(3)考察速度和温度剖面的关系,探究近壁剪切对温度剖面的影响。此外，本项目还会考察相应的温度脉动和速度脉动等特性。这些结果将有助于揭示对数温度剖面的起源，并加深人们对湍流边界层以及湍流传热的理解。

热对流普遍存在各类工程应用当中，而温度边界层在很大程度上决定了这些工程系统的传热效率。因此，深入研究热对流的温度边界层特性，对解决工业生产中的传热问题有着重要意义。..在本项目的资助下，我们结合实验测量和数值模拟的方法，对Rayleigh-Bénard对流系统在不同流动状态下的温度边界层特性进行了深入研究，取得了以下三个方面的成果。第一，我们研究了其在不同条件下的演化过程及其对传热效率和流动强度的影响，并在二维系统中发现了由于流态自发转变而出现的湍流能量正逆级串共存现象。第二，我们在很宽的经典区间参数范围内研究了温度剖面的形态，明确了温度剖面的形态并非由羽流特性决定，而是和流场形态以及大尺度环流的动力学特性密切相关。我们还提出了采用最概然温度作为特征温度重构温度剖面的方法，进而利用温度剖面的高阶统计特性获得了流场中不同的动力学区域。第三，我们研究了羽流的生成和演化过程，以及其统计特性对大尺度动力学特性的影响。这些结果不仅加深了人们对于大尺度环流、羽流和温度边界层三者之间相互作用的认识，还为主流的边界层和传热理论提供了支撑。..基于这些成果，我们在流体力学领域的主流期刊Journal of Fluid Mechanics上发表了2篇论文，另有2篇论文也即将发表（1篇Journal of Fluid Mechanics，1篇Physical Review Fluids）。我们的工作在具有影响力的学术会议进行口头报告11人次，得到了国内外同行的关注。本项目培养了1名助理研究员和2名硕士毕业生（其中1名荣获优秀毕业生）。项目负责人应邀担任了2018年美国物理学会流体力学年会以及2019年中国力学大会的分会场主席，并获得了国家自然科学基金委和广东省科技厅其他项目的资助。

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