We report an experimental and numerical study of the effect of spatial confinement in turbulent thermal convection. It is found that when the width of the convection cell is narrowed, the heat-transfer efficiency increases significantly despite the fact that the overall flow is slowed down by the increased drag force from the sidewalls. Detailed experimental and numerical studies show that this enhancement is brought about by the changes in the dynamics and morphology of the thermal plumes in the boundary layers and in the large-scale flow structures in the bulk. It is found that the confined geometry produces more coherent and energetic hot and cold plume clusters that go up and down in random locations, resulting in more uniform and thinner thermal boundary layers. The study demonstrates how changes in turbulent bulk flow can influence the boundary layer dynamics and shows that the prevalent mode of heat transfer existing in larger aspect ratio convection cells, in which hot and cold thermal plumes are carried by the large-scale circulation along opposite sides of the sidewall, is not the most efficient way for heat transport.

Many natural and engineering systems are simultaneously subjected to a driving force and a stabilizing force. The interplay between the two forces, especially for highly nonlinear systems such as fluid flow, often results in surprising features. Here we reveal such features in three different types of Rayleigh-Bénard (RB) convection, i.e. buoyancy-driven flow with the fluid density being affected by a scalar field. In the three cases different stabilizing forces are considered, namely (i) horizontal confinement, (ii) rotation around a vertical axis, and (iii) a second stabilizing scalar field. Despite the very different nature of the stabilizing forces and the corresponding equations of motion, at moderate strength we counterintuitively but consistently observe an enhancement in the flux, even though the flow motion is weaker than the original RB flow. The flux enhancement occurs in an intermediate regime in which the stabilizing force is strong enough to alter the flow structures in the bulk to a more organised morphology, yet not too strong to severely suppress the flow motions. Near the optimal transport enhancements all three systems exhibit a transition from a state in which the thermal boundary layer (BL) is nested inside the momentum BL to the one with the thermal BL being thicker than the momentum BL.

Coherent structures are ubiquitous in turbulent flows and play a key role in transport. The most important coherent structures in thermal turbulence are plumes. Despite being the primary heat carriers, the potential of manipulating thermal plumes to transport more heat has been overlooked so far. Unlike some other forms of energy transport, such as electromagnetic or sound waves, heat flow in fluids is generally difficult to manipulate, as it is associated with the random motion of molecules and atoms. Here we report how a simple geometrical confinement can lead to the condensation of elementary plumes. The result is the formation of highly coherent system-sized plumes and the emergence of a new regime of convective thermal turbulence characterized by universal temperature profiles and significantly enhanced heat transfer. It is also found that the universality of the temperature profiles and heat transport originate from the geometrical properties of the coherent structures, i.e., the thermal plumes. Therefore, in contrast to the classical regime, boundary layers in this plume-controlled regime are being controlled, rather than controlling.

We report an experimental study of the large-scale circulation (LSC) reversal in quasi-2D turbulent thermal convection, in which the aspect ratio Γ (= height/length of a rectangular box) is used as a parameter to perturb the stability of the LSC. It is found that the mean time interval τ between two successive reversals increases strongly with increasing Γ . A stochastic model is proposed to incorporate the effect of the corner rolls. In the model, the aspect ratio serves as a tuning parameter for the relative weight of the corner rolls that damp the LSC. The model predictions for the shape of the bistable states of the system and τ agree excellently with the experimental results, with τ having an unexpected stretched exponential Rayleigh number dependence, ∼exp(Ra α ). We further show quantitatively that the main damping force of the LSC in a quasi-2D system is from the corner rolls rather than the viscous drag from the sidewalls, which bridges the difference found in quasi-2D and 3D systems.

We report the first experimental study of Lagrangian acceleration in turbulent Rayleigh–Bénard convection, using particle tracking velocimetry. A method has been developed to quantitatively evaluate and eliminate the uncertainties induced by temperature and refraction index fluctuations caused by the thermal plumes. It is found that the acceleration p.d.f. exhibits a stretched exponential form and that the probability for large magnitude of acceleration in the lateral direction is higher than those in the vertical directions, which can be attributed to the vortical motion of the thermal plumes. The local acceleration variance $\langle {a}^{2} \rangle $ was obtained for various values of the three control parameters: the Rayleigh number $\mathit{Ra}$ ($6\ensuremath{\times} 1{0}^{8} \leq \mathit{Ra}\leq 1\ensuremath{\times} 1{0}^{11} $), the Prandtl number $\mathit{Pr}$ ($\mathit{Pr}= 4. 4, 5. 5$ and 6.1) and the system size $L~(L= 19. 2\text{~and~} 48. 6~\mathrm{cm} )$. These were then compared with the theoretically predicted dependence on these parameters for buoyancy-dominated turbulent flows and for homogeneous and isotropic turbulence, respectively. It is found that $\langle {a}^{2} \rangle $ in the central region is dominated by contributions from the turbulent background rather than from the buoyancy force, and the Heisenberg–Yaglom relation holds in this region. From this, we obtain the first experimental results of the constant ${a}_{0} $ of the acceleration variance in the micro-scale Reynolds number range $20\leq {R}_{\lambda } \leq 120$, which fills a gap in this constant in the lower ${R}_{\lambda } $ end from the experimental side, and provides possible constraints for its high ${R}_{\lambda } $ behaviour if a certain fitting function is attempted. In addition, acceleration correlation functions were obtained for different $\mathit{Ra}$. It is found that the zero crossing time of acceleration correlation functions is at $\tau \approx 2. 2{\tau }_{\eta } $ (${\tau }_{\eta } $ is the Kolmogorov time scale) over the range of $\mathit{Ra}({R}_{\lambda } )$ spanned in our experiments, which is the same as the simulation results in isotropic turbulence, and the exponential decay time ${\tau }_{1/ e} = (1. 12\pm 0. 05){\tau }_{\eta } $, which is larger than $(0. 73\ensuremath{\sim} 0. 80){\tau }_{\eta } $ found experimentally for other types of turbulent flows with larger ${R}_{\lambda } $.