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Rotating thermal convection, commonly encountered in natural and industrial environments, is typically influenced by both buoyancy forces and boundary rotation. In this study, we conduct direct numerical simulations to explore the effect of rotating bottom on the flow structure and heat transport of Rayleigh–Bénard convection (RBC) in a cylindrical cavity. This cavity has a radius-to-height ratio of 1 and is filled with an incompressible fluid with a Prandtl number of 0.7. Our results show that the axisymmetric convection pattern, observed in RBC for R a ∈ [ 4000 , 8000 ] without rotation, transitions into a double roll structure at low rotating speeds (ω), while for R a ≤ 4000 , the pattern remains axisymmetric, independent of ω. We then focus on the impact of bottom rotation on heat transport in the axisymmetric regime. Based on the variation in the Nusselt number (Nu) with ω , two distinct regimes are identified: a convection-dominated regime at low ω , where Nu closely resembles that of standard RBC, and a rotation-dominated regime at high ω , where strong shear induced by the rotating bottom intensifies the meridional circulation, significantly boosting global heat flux. The critical rotating speed, ω * , marking the transition between these regimes, follows different power-law relations below and above the buoyant convection onset (R a c ): ω * ∼ R a − 0.64 for R a < R a c and ω * ∼ R a 0.33 for R a > R a c . As ω increases, the exponents for N u ∼ R a λ and R e ∼ R a γ evolve before converging to λ ≈ 0.3 and γ = 0.5 , respectively. Scaling laws for Nu and Re as functions of ω and Ra in the rotation-dominated regime are finally derived: N u ∼ R a 0.3 ω 0.64 and R e ∼ R a 0.5 ω. [ABSTRACT FROM AUTHOR]

We employ the direct numerical simulation to study the heat transfer behavior and flow structures in a vertical convection system with rough sidewalls. The parameters are chosen with Rayleigh number spanning the range of 1 × 10 8 ≤ R a ≤ 3 × 10 10 and Prandtl number fixed at 1.0. The results reveal that the impact of rough walls on the Nusselt number Nu and the Reynolds number Re is influenced by the height of the rough element h. When h is not sufficiently high, the roughness impedes the flows within the boundary layer and traps massive heat between rough elements, and both Nu and Re are lower than those in the smooth-wall case. However, the extent of the Nu and Re reduction regimes decreases as Ra increases. For sufficiently large Ra, the reduction regime for both Nu and Re may vanish, and roughness breaks up the limitation of the thermal boundary layer and facilitates the eruption of thermal plumes from roughness tips, resulting in the enhancement of both Nu and Re. Based on these results, the critical heights h c for Nu and h c r for Re are identified. Both exhibit similar scaling behavior with Ra, with h c consistently being larger than h c r for the same value of Ra. [ABSTRACT FROM AUTHOR]

The interscale transfer of kinetic energy and Reynolds stress in homogeneous shear turbulence (HST) is numerically investigated using three-dimensional bandpass filtering technique. The flow fields of a statistically steady HST are obtained using direct numerical simulation at three Reynolds numbers 2000, 5000, and 12 500 based on box depth. Visualizations of typical flow structures of bandpass filtered fields show that the small-scale structures are nearly isotropic, whereas the large-scale ones show the preferential alignment with the direction of mean shear. Quantitative results of both kinetic energy and Reynolds stress fluxes between two specific scales show the existence of scale locality. In specific, the eddies of a length scale L mostly transfer their energy or Reynolds stress to eddies of size 0.3L to 0.4L, which seems to be independent in the limit of the high Reynolds number. Furthermore, through the analysis on the vortex stretching, it is shown that the small-scale structures of scale L ω are stretched mostly by straining structures of size about 2 L ω , while large-scale structures are stretched mostly by mean shear. Finally, the evaluation of alignment between vortical structures and strain rate shows that small-scale structures are more likely to align with the strain structures of five times their size, and large-scale ones are mainly align with the mean shear strain. These findings can help enhance our comprehension of the interscale transfer and scale interaction of vortical structures in turbulence. [ABSTRACT FROM AUTHOR]

We experimentally study the heat transfer and flow characteristics of thermal convection in a rectangular cell with built-in lattice channels. The working fluid used is water with a Prandtl number of 5.5, and the Rayleigh number ranges from 2.5 × 10 8 to 6.9 × 10 9 . Three proposed models with different channel sizes and positions and the classical Rayleigh–Bénard convection (RBC) are studied, and the heat transfer and flow structure characteristics are analyzed using measured temperature signals. The first model included two short channels placed near the top and bottom plates, which disrupt the mixing zone and enhance heat transport. The second model involves relatively long channels positioned at the center of the cell, but far from the thermal boundary layer, resulting in a more coherent bulk flow that also enhances heat transport. For these two configurations, the heat transfer enhancement rate is approximately 20% compared to standard RBC. The third model uses long lattice channels that almost touches the top and bottom plates. This configuration results in a maximum heat transfer enhancement of about 138% due to the organized boundary layer and bulk flow induced by lattice channels. The presence of channels also results in a two-order smaller standard deviation of temperature, indicating a significant reduction in fluctuations. However, the average temperatures in the center of some channels were significantly different from the mean system temperature, suggesting the existence of cold or hot fluid flow through the channel. Our experimental results show that the inclusion of channels with appropriate lengths and positions can effectively regulate the flow near the boundary layer and in the bulk, leading to significant enhancements in heat transfer. [ABSTRACT FROM AUTHOR]