Non-Oberbeck-Boussinesq effects in turbulent convection in cryogenic helium at high Rayleigh numbers

低温氦气中的湍流Rayleigh-Bénard对流（RBC）实验在受控的实验室条件下达到了最大可能的Rayleigh数，并具有接近真实的大气流动的强烈湍流，从而为我们深入了解这些自然系统中热量和动量的湍流传输机制提供了更深入的认识。然而，这些高瑞利数要求在接近相边界或甚至压力-温度相平面的临界点处操作氦工作流体，这导致所有材料参数（例如运动粘度或热导率）的波动。这些波动导致偏离Oberbeck-Boussinesq（OB）制度的热对流与一个明确定义的上下对称的流量及其统计。在这个项目中，我们希望系统地研究这些偏差-所谓的非Oberbeck-Boussinesq（NOB）效应-及其对湍流输运的影响。因此，我们将捷克方面的低温实验专业知识和德国方面的湍流对流大规模直接数值模拟专业知识纳入捷克-德国联合提案。温度和压力的详细参数依赖性将从XHEPAK软件中获得，该软件以其全部复杂性对低温氦气的状态方程进行建模。由此产生的材料参数依赖性进入数值模拟，并可以直接与理想化的OB限制，以及在相同的瑞利数的实验。我们的工作有助于更好地理解在不同的高瑞利数实验室实验中获得的不确定的结果，相对于存在的过渡的RBC流到最终政权（具有显着增强的湍流传热）。

Experiments of turbulent Rayleigh-Bénard convection (RBC) in cryogenic helium gas reach the highest possible Rayleigh numbers with a vigorous turbulence under controlled laboratory conditions that come close to those in real atmospheric flows and can thus provide us deeper insights into the turbulent transport mechanisms of heat and momentum in these natural systems. These high Rayleigh numbers require however to operate the helium working fluid close to the phase boundary or even the critical point of the pressure-temperature phase plane which causes fluctuations of all material parameters such as kinematic viscosity or thermal conductivity. These fluctuations result in deviations from the Oberbeck-Boussinesq (OB) regime of thermal convection with a well-defined up-down symmetry of the flow and its statistics. In this project, we want to investigate these deviations -- the so-called non-Oberbeck-Boussinesq (NOB) effects -- and its impact on the turbulent transport systematically. Therefore we bring the expertise in cryogenic experiments on the Czech side and in massive direct numerical simulations of turbulent convection on the German side in a joint Czech-German proposal together. The detailed parameter dependencies on temperature and pressure will be obtained from the XHEPAK software that models the equation of state of the cryogenic helium gas in its full complexity. The resulting material parameter dependencies enter the numerical simulations and can be directly compared with the idealized OB limit as well as experiments at the same Rayleigh numbers. Our work helps to better understand the inconclusive results obtained in different high-Rayleigh-number laboratory experiments with respect to the existence of a transition of the RBC flow into the ultimate regime (with a significantly enhanced turbulent heat transfer).