Numerical study of flow reorientations in turbulent convection with a free-slip upper boundary

A typical feature of turbulent convection is the Large Scale Circulation (LSC), which can be described as the principal convective structure and is driven by plume formation at both boundaries. Inside cubical geometries, the LSC orients itself along one of the diagonal planes. However, the plane and direction of circulation are not fixed in time and reorientation events of one or both of them take place. In this paper, we study the dynamic properties of reorientations for a pseudo-Rayleigh--Benard set up that involves a free surface on top instead of a rigid wall. The basic motivation for this study is to better understand the flow phenomena during the early stages of a Loss-of-Coolant Accident in spent fuel pools. More specifically, we report on large-eddy simulations of turbulent thermal convection in a cubical domain with a heated bottom wall, a cooled free-slip boundary and with water as the working fluid. The Rayleigh numbers considered in our study range from 2x10^7 up to 10^8. Since the thermo-physical properties of water vary considerably across the flow domain, the Oberbeck--Boussinesq approximation is not invoked. First, we briefly highlight the influence of the free-slip boundary on the flow structure. The paper continues with a discussion and analysis of the dynamics of the LSC. A study of the properties and frequency of reorientation events is presented herein. Comparisons between the properties of the LSC in the standard Rayleigh--Benard convection and the configuration described above are also provided