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)实验在接近真实大气流动的受控实验室条件下达到了最高的Rayleigh数和强烈的湍流，从而为我们提供了对这些自然系统中热量和动量的湍流传输机制的更深层次的了解。然而，这些高瑞利数要求氦工质在接近相边界甚至压力-温度相平面的临界点附近工作，这会引起所有材料参数的波动，如运动粘度或导热系数。这些波动导致了与Oberbeck-Boussinesq(OB)热对流体系的偏差，流动及其统计具有明确的上下对称性。在这个项目中，我们想要系统地研究这些偏差--所谓的非Oberbeck-Boussinesq(NOB)效应--及其对湍流传输的影响。因此，我们将捷克方面的低温实验和德国方面的大规模直接数值模拟湍流对流的专业知识结合在一起，形成捷克-德国联合提案。详细的参数对温度和压力的依赖关系将从XHEPAK软件中获得，该软件在完全复杂的情况下模拟低温氦气体的状态方程。得到的材料参数依赖关系进入数值模拟，并可以直接与理想化的OB极限以及相同Rayleigh数下的实验进行比较。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所获得的关于RBC流向最终区域(具有显著增强的湍流换热)转变的存在的非决定性结果。

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).