In this work, the dual-mesh hybrid RANS-LES approach was applied for the first time to a natural convection flow, namely a high Rayleigh number differentially heated square cavity flow. This approach involves running an unsteady Reynolds Averaged Navier-Stokes (RANS) simulation and a coarse Large Eddy Simulation (LES) simultaneously using two different grids that overlap each other (typically, a highly wall-refined grid is used for the RANS, whereas a more homogenous and isotropic mesh is used for the LES). The two simulations correct each other using a switching criterion that determines the driving and the driven simulations at every point in space. It is demonstrated that the flow unsteadiness and the coexistence of laminar and turbulent regions in the square cavity complicate the task of choosing a suitable switching criterion. Accordingly, a new criterion based on comparing the turbulence lengthscales to the grid size was developed to account for the presence of the aforementioned complex flow features. The behaviour of this criterion and comparisons of the dual-mesh predictions against pure RANS, pure coarse LES and Direct Numerical Simulation (DNS) results are also presented in the current paper.

在这项工作中，首次将双网混合RANS-LES方法应用于自然对流，即高瑞利数差加热方腔流。此方法涉及使用两个相互重叠的不同网格同时运行不稳定的雷诺平均Navier-Stokes（RANS）模拟和粗略的大涡模拟（LES）（通常，将高度精化的网格用于RANS，而使用LES使用更均匀且各向同性的网格。这两个模拟使用切换标准相互修正，该转换标准确定空间中每个点的驱动模拟和从动模拟。结果表明，流动不稳定以及方腔内层流和湍流区域的共存使选择合适的转换准则的任务变得复杂。因此，基于湍流长度尺度与网格尺寸的比较，提出了一个新的标准，以解决上述复杂流动特征的存在。本文还介绍了该准则的行为以及针对纯RANS，纯粗糙LES和直接数值模拟（DNS）结果的双网格预测的比较。