Ke-Qing Xia

From laminar plumes to organized flows: the onset of large-scale circulation in turbulent thermal convection

We report an experimental study on the onset of the large-scale coherent mean flow in Rayleigh–Benard turbulent convection. Shadowgraph and particle image velocimetry techniques are used to visualize the motion of thermal plumes and measure the velocity of the plumes and of the ‘background’ flow field, as the fluid motion evolves from quiescent to steady state. The experiment reveals the dynamical origin of the initial horizontal motion required by the large-scale flow: the fluid entrainment caused by the plumes vertical motion generates vortices surrounding the plume itself. These vortices in turn generate the initial horizontal motion of the flow field. Two types of interactions have been identified: (i) direct plume–vortex interaction; and (ii) plume–plume interaction via vortices. These interactions and the interaction and merging of the vortices from neighbouring plumes lead to groupings and/or merging of plumes, which in turn generate vortices of even larger scale. As a result of these interactions, the convective flow evolves into a coherent rotatory motion consisting of mainly the plumes themselves and spanning the whole convection box. This study clearly demonstrates that it is the thermal plumes that initiate the horizontal large-scale flow across the top and bottom conducting plates.

Flow Reversals in Thermally Driven Turbulence

We analyze the reversals of the large-scale flow in Rayleigh-Bénard convection both through particle image velocimetry flow visualization and direct numerical simulations of the underlying Boussinesq equations in a (quasi-) two-dimensional, rectangular geometry of aspect ratio 1. For medium Prandtl number there is a diagonal large-scale convection roll and two smaller secondary rolls in the two remaining corners diagonally opposing each other. These corner-flow rolls play a crucial role for the large-scale wind reversal: They grow in kinetic energy and thus also in size thanks to plume detachments from the boundary layers up to the time that they take over the main, large-scale diagonal flow, thus leading to reversal. The Rayleigh vs Prandtl number space is mapped out. The occurrence of reversals sensitively depends on these parameters.

Flow mode transitions in turbulent thermal convection

We report an experimental study of structures and dynamics of the large-scale mean flow in Rayleigh–Benard convection cells with aspect ratio (Γ)1, 1∕2, and 1∕3. It is found that both a single circulating roll flow structure and one with two vertically stacked counter-rotating rolls exist in the three aspect ratio cells. The average percentages of time that the large-scale mean flow spends in the single-roll mode (SRM) and the double-roll mode (DRM) are 87.1% and 0.8% for Γ=1, 69.5% and 7.9% for Γ=1∕2, and 26.7% and 34.1% for Γ=1∕3. Several routes of transitions among the different flow modes are identified. In addition, different structures for the DRM are found and their relative weights are determined. We also show direct evidence that the SRM is more efficient for heat transfer than the DRM. Although the difference is very small, it shows how changes in internal flow state can manifest in the global transport properties of the system. It is also found that the time interval between successive flow mod...

Heat transport by turbulent Rayleigh-Bénard convection in 1 m diameter cylindrical cells of widely varying aspect ratio

High-precision measurements of the Nusselt number Nu as a function of the Rayleigh number Ra have been made in water-filled 1m diameter cylindrical cells of aspect ratio

Origin of the temperature oscillation in turbulent thermal convection.

\Gamma {=}

Velocity oscillations in turbulent Rayleigh–Bénard convection

0.67, 1, 2, 5, 10 and 20. The measurements were conducted at the Prandtl number

An experimental investigation of turbulent thermal convection in water-based alumina nanofluid

Pr {\approx} 4

Experimental investigation of homogeneity, isotropy, and circulation of the velocity field in buoyancy-driven turbulence

with Ra varying from

Thermal boundary layer profiles in turbulent Rayleigh-Bénard convection in a cylindrical sample

1{\times} 10^7

Horizontal structures of velocity and temperature boundary layers in two-dimensional numerical turbulent Rayleigh-Benard convection

to