Thermal convection at high aspect ratio: The local distribution and the dynamics of dissipative processes

大多数包含对流传热的地球物理和工程过程的特征是流体层的横向和垂直范围之间的大比例。然而，热对流的实验和数值工作仍然集中在构型上，其中这个比率相当小（等于或小于1）。特别是，这是由于这样一个事实，即实验达到相当高的瑞利数必须非常高，而且往往这种大的高度不能由相应的宽度来补偿。因此，要完全评价通常基于一维模型而没有横向约束的现象学理论实际上是不可能的。申请人希望通过在实验中同时满足高纵横比（8或更大）和高瑞利数（高达10{12}）的热通量和热耗散率的测量来解决这一问题。与之前的项目不同，他们将在新的工作中关注这两个量的动力学。Scheel和Schumacher从直接数值模拟（DNS）中得知[J]。舒马赫和j·d·谢尔。湍流对流单元中羽流碰撞引起的极端耗散事件。理论物理。Rev. E 94, 043104(2016)]，特别是，热耗散率波动非常强烈，局部波动和时间波动。现在，这将首次在实验中得到验证。由于测量可能覆盖比可比DNS更长的“观察期”，因此测量数据的统计不确定性将显著减小。在第二部分中，申请人将追求以下问题：在整个各自水平面上的单个点（在湍流瑞利- b<s:1> <s:1> <s:1> <s:1> <s:1> <s:1> <s:1> <s:1>）上的本地和时间平均测量数据的代表性如何？对于宽高比小于或等于1的情况，过去一直在研究这个问题，并以“不具有代表性”来回答。对于大宽高比，侧壁只影响流体层的一小部分外部，申请人将通过测量多个水平面上不同位置的散热率来回答这个问题。因此，这些测量也将提供局部湍流热通量的分布。

The majority of geophysical and engineering processes incorporating convective heat transfer is characterized by a large ratio between the lateral and the vertical extent of the fluid layer. However, experimental and numerical work on thermal convection still focuses on configurations, where this ratio is rather small (equal or smaller than one). In particular, this is due to the fact that experiments achieving comparable high Rayleigh numbers must be very high and quite often this large height cannot be compensated by a respective width. Thus, it is actually not possible to fully evaluate phenomenological theories that are usually based one a one-dimensional model without lateral confinement. The applicants wish to contribute to overcome this by measurements of the heat flux and the thermal dissipation rate in an experiment meeting both, high aspect ratio (eight or larger) as well as high Rayleigh number (up to 10{12}). Unlike in the preceding project, they will focus in the new work on the dynamics of these two quantities. It is known from direct numerical simulations (DNS) by Scheel and Schumacher [J. Schumacher and J. D. Scheel. Extreme dissipation event due to plume collision in a turbulent convection cell. Phys. Rev. E 94, 043104 (2016)] that, in particular, the thermal dissipation rate fluctuates very strongly as well locally as in time. Now, this will be validated in an experiment for the first time. Because, measurements potentially cover a much longer “observation period” than comparable DNS, the statistical uncertainty of the measurement data will be significantly smaller In a second part, the applicants will pursue the following question: How representative are local and time-averaged measurement data at a single point (in turbulent Rayleigh-Bénard convection) for the entire respective horizontal plane. For aspect ratios smaller or equal to one, this problem is always studied in the past, and it is answered with “not representative”. For large aspect ratios, where the sidewall affects only a little outer fraction of the fluid layer, the applicants will answer this question by measurements of the thermal dissipation rate at various positions in multiple horizontal planes. These measurements will, thus, provide also the distribution of the local turbulent heat flux.