Thermal convection on rough surfaces

粗糙表面上的热对流是一种通用模型，它比基于光滑表面的其他模型更适合于描述诸如传热装置、电子电路冷却或研究城市地区与大气之间的热交换等物理过程。尽管在这一领域有大量的出版物，但特别是在加热或冷却的粗糙壁附近的局部输运还没有得到很好的理解。这是因为只有很少的对流实验存在，这些实验允许在壁面附近以足够高的空间和时间分辨率对输运量进行局部测量，并且直接数值模拟所提议的实验的高瑞利数实际上需要过度的努力。本课题将在大型对流实验“伊尔梅瑙桶”中测量具有粗糙度的加热/冷却表面附近的局部速度场和温度场。由于试验截面尺寸较大，直径达7.1 m，高度达8.0 m，因此测量的瑞利数可高达Ra = 10^{12}。另一方面，测量的空间分辨率仍然足够高，能够很确定地验证所需要的近壁流体层输运量的函数关系。在项目过程中，将研究各种表面结构，包括均匀元素和高度交错的元素。在他们的研究结果中，科学家们将评估各种流动现象的相关性，如热羽流速率的增加，粗糙元素顶部热边界层的“变薄”，或热边界层向湍流状态的转变，对过去已经观察到的对流热输送的变化。此外，还将对不同表面结构的测量数据进行比较，找出它们对局部流场和热输运影响的共同特征。在理想的情况下，这种搜索会导致一个可能同样普遍适用的物理模型，如管道或剪切流的水力粗糙度模型。

Thermal convection on a rough surface is a general model that is better suited to describe the physical processes in e. g. devices for heat transfer, at the cooling of electronic circuits or studying the heat exchange between an urban area and the atmosphere than other models based on smooth surfaces. Even though, a multitude of publications exists on this field, in particular, the local transport in the close vicinity of a heated or cooled wall with roughness is not well understood. This is due to the fact that only very few convection experiments exist, which permit local measurements of the transport quantities with a sufficiently high spatial and temporal resolution near the wall and that direct numerical simulations modelling the high Rayleigh numbers of the proposed experiments actually require an excessive effort. In the present research project, the local velocity and temperature fields near a heated/cooled surface with roughness will be measured in the large-scale convection experiment "Barrel of Ilmenau". Due to the large dimensions of the test section, the diameter amounts to 7.1 m, the height amounts to 8.0 m, such measurements can be done up to a Rayleigh number as high as Ra = 10^{12}. On the other hand, the spatial resolution of the measurements is still sufficiently high, to be capable to validate the wanted functional relations of the transport quantities in the near-wall fluid layer with a good certainty. In the course of the project, various surface structures including uniform elements and such ones staggered in height will be investigated. In the result of their investigations, the scientists will evaluate the relevance of various flow phenomena, like the increase of the rate of thermal plumes, the “thinning” of the thermal boundary layer on top of the roughness elements, or the transition of the thermal boundary layer towards a turbulent state, on the variation of the convective heat transport already observed in the past. Moreover, measurement data at the various surface structures will be compared and common features with respect to their effect on the local flow field and the heat transport will be found. In the ideal case, this search leads to a physical model that might be applicable similarly universally, like the model of hydraulic roughness at pipe or shear flows.