Study of flow transition and boundary-layer asymmetry in non-Oberbeck-Boussinesq thermal convection flow

Thermal convection flow driven by large temperature differences between two vertical walls makes the Oberbeck-Boussinesq (OB) approximation invalid. This flow is governed by the compressible Navier-Stokes-Fourier equations, namely, flow with large density variations. In this study, the discrete unified gas-kinetic scheme (DUGKS) is used to simulate the non-Oberbeck-Boussinesq (non-OB) thermal convection flow. The transition of the flow field from steady to unsteady convection under different normalized temperature differences (0≤ε≤0.9) is studied. The results show that the critical Rayleigh number Ra decreases with increasing ε, due to different magnitudes of the buoyancy force associated with the hot and cold walls. In other words, at a given Ra number, a stable OB convection can become unstable under non-OB treatment. Here we explore this early transition in terms of thermal / momentum boundary-layer asymmetry and the resulting strength of the velocity and integral momentum inside the boundary layer, in order to provide a physical explanation. The DUGKS simulations will be used to quantify the maximum / minimum local velocity, the thermal and momentum boundary-layer thicknesses, and integral jet momentum magnitude, to build up a logical physical interpretation.