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result(s) for
"Xu, Ben-Rui"
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Wall-sheared thermal convection: heat transfer enhancement and turbulence relaminarization
We studied the flow organization and heat transfer properties in two-dimensional and three-dimensional Rayleigh–Bénard cells that are imposed with different types of wall shear. The external wall shear is added with the motivation of manipulating flow mode to control heat transfer efficiency. We imposed three types of wall shear that may facilitate the single-roll, the horizontally stacked double-roll, and the vertically stacked double-roll flow modes, respectively. Direct numerical simulations are performed for fixed Rayleigh number $Ra = 10^{8}$ and fixed Prandtl number $Pr = 5.3$, while the wall-shear Reynolds number ($Re_{w}$) is in the range $60 \\leqslant Re_{w} \\leqslant 6000$. Generally, we found enhanced heat transfer efficiency and global flow strength with the increase of $Re_{w}$. However, even with the same magnitude of global flow strength, the heat transfer efficiency varies significantly when the cells are under different types of wall shear. An interesting finding is that by increasing the wall-shear strength, the thermal turbulence is relaminarized, and more surprisingly, the heat transfer efficiency in the laminar state is higher than that in the turbulent state. We found that the enhanced heat transfer efficiency at the laminar regime is due to the formation of more stable and stronger convection channels. We propose that the origin of thermal turbulence laminarization is the reduced amount of thermal plumes. Because plumes are mainly responsible for turbulent kinetic energy production, when the detached plumes are swept away by the wall shear, the reduced number of plumes leads to weaker turbulent kinetic energy production. We also quantify the efficiency of facilitating heat transport via external shearing, and find that for larger $Re_{w}$, the enhanced heat transfer efficiency comes at a price of a larger expenditure of mechanical energy.
Journal Article
Particle transport and deposition in wall-sheared thermal turbulence
We studied the transport and deposition behaviour of point particles in Rayleigh–Bénard convection cells subjected to Couette-type wall shear. Direct numerical simulations (DNSs) are performed for Rayleigh number ($Ra$) in the range $10^{7} \\leq Ra \\leq ~10^9$ with a fixed Prandtl number $Pr = 0.71$, while the wall-shear Reynolds number ($Re_w$) is in the range $0 \\leq Re_w \\leq ~12\\,000$. With the increase of $Re_w$, the large-scale rolls expanded horizontally, evolving into zonal flow in two-dimensional simulations or streamwise-oriented rolls in three-dimensional simulations. We observed that, for particles with a small Stokes number ($St$), they either circulated within the large-scale rolls when buoyancy dominated or drifted near the walls when shear dominated. For medium $St$ particles, pronounced spatial inhomogeneity and preferential concentration were observed regardless of the prevailing flow state. For large $St$ particles, the turbulent flow structure had a minor influence on the particles’ motion; although clustering still occurred, wall shear had a negligible influence compared with that for medium $St$ particles. We then presented the settling curves to quantify the particle deposition ratio on the walls. Our DNS results aligned well with previous theoretical predictions, which state that small $St$ particles settle with an exponential deposition ratio and large $St$ particles settle with a linear deposition ratio. For medium $St$ particles, where complex particle–turbulence interaction emerges, we developed a new model describing the settling process with an initial linear stage followed by a nonlinear stage. Unknown parameters in our model can be determined either by fitting the settling curves or using empirical relations. Compared with DNS results, our model also accurately predicts the average residence time across a wide range of $St$ for various $Re_w$.
Journal Article
Particle transport and deposition in wall-sheared thermal turbulence
We studied the transport and deposition behaviour of point particles in Rayleigh-Bénard convection cells subjected to Couette-type wall shear. Direct numerical simulations (DNSs) are performed for Rayleigh number (\\(Ra\\)) in the range \\(10^7 Ra 10^9\\) with a fixed Prandtl number \\(Pr = 0.71\\), while the wall-shear Reynolds number (\\(Re_w\\)) is in the range \\(0 Re_w 12000\\). With the increase of \\(Re_w\\), the large-scale rolls expanded horizontally, evolving into zonal flow in two-dimensional simulations or streamwise-oriented rolls in three-dimensional simulations. We observed that, for particles with a small Stokes number (\\(St\\)), they either circulated within the large-scale rolls when buoyancy dominated or drifted near the walls when shear dominated. For medium \\(St\\) particles, pronounced spatial inhomogeneity and preferential concentration were observed regardless of the prevailing flow state. For large \\(St\\) particles, the turbulent flow structure had a minor influence on the particles' motion; although clustering still occurred, wall shear had a negligible influence compared with that for medium \\(St\\) particles. We then presented the settling curves to quantify the particle deposition ratio on the walls. Our DNS results aligned well with previous theoretical predictions, which state that small \\(St\\) particles settle with an exponential deposition ratio and large \\(St\\) particles settle with a linear deposition ratio. For medium \\(St\\) particles, where complex particle-turbulence interaction emerges, we developed a new model describing the settling process with an initial linear stage followed by a nonlinear stage. Unknown parameters in our model can be determined either by fitting the settling curves or using empirical relations. Compared with DNS results, our model also accurately predicts the average residence time across a wide range of \\(St\\) for various \\(Re_w\\).
Paper
Pore-scale statistics of temperature and thermal energy dissipation rate in turbulent porous convection
We report pore-scale statistical properties of temperature and thermal energy dissipation rate in a two-dimensional porous Rayleigh-Bénard (RB) cell. High-resolution direct numerical simulations were carried out for the fixed Rayleigh number (\\(Ra\\)) of \\(10^9\\) and the Prandtl numbers (\\(Pr\\)) of 5.3 and 0.7. We consider sparse porous media where the solid porous matrix is impermeable to both fluid and heat flux. The porosity (\\(\\)) range \\(0.86 0.98\\), the corresponding Darcy number (\\(Da\\)) range \\(10^-4
Paper
Wall-sheared thermal convection: heat transfer enhancement and turbulence relaminarization
We studied the flow organization and heat transfer properties in two-dimensional and three-dimensional Rayleigh-Bénard cells that are imposed with different types of wall shear. The external wall shear is added with the motivation of manipulating flow mode to control heat transfer efficiency. We imposed three types of wall shear that may facilitate the single-roll, the horizontally stacked double-roll, and the vertically stacked double-roll flow modes, respectively. Direct numerical simulations are performed for fixed Rayleigh number \\(Ra = 10^8\\) and fixed Prandtl number \\(Pr = 5.3\\), while the wall-shear Reynolds number (\\(Re_w\\)) is in the range \\(60 Re_w 6000\\). Generally, we found enhanced heat transfer efficiency and global flow strength with the increase of \\(Re_w\\). However, even with the same magnitude of global flow strength, the heat transfer efficiency varies significantly when the cells are under different types of wall shear. An interesting finding is that by increasing the wall-shear strength, the thermal turbulence is relaminarized, and more surprisingly, the heat transfer efficiency in the laminar state is higher than that in the turbulent state. We found that the enhanced heat transfer efficiency at the laminar regime is due to the formation of more stable and stronger convection channels. We propose that the origin of thermal turbulence laminarization is the reduced amount of thermal plumes. Because plumes are mainly responsible for turbulent kinetic energy production, when the detached plumes are swept away by the wall shear, the reduced number of plumes leads to weaker turbulent kinetic energy production. We also quantify the efficiency of facilitating heat transport via external shearing, and find that for larger \\(Re_w\\), the enhanced heat transfer efficiency comes at a price of a larger expenditure of mechanical energy.
Paper
Production and transport of vorticity in two-dimensional Rayleigh-Bénard convection cell
We present a numerical study of vorticity production and transport in the two-dimensional Rayleigh-Bénard (RB) convection. Direct numerical simulations are carried out in the Rayleigh number (\\(Ra\\)) range \\(10^5 Ra 10^6\\), the Prandtl number (\\(Pr\\)) of 0.71, and the aspect ratio (\\(\\)) of the convection cell range \\(0.75 6\\). We found that the flow structure and temperature distribution vary with \\(\\) greatly due to multiple vortices interaction. Further investigation on the vorticity production and transport reveals that, in the RB convection, in addition to the vorticity production due to wall shear stress, buoyancy produces significant vorticity in the bulk region. The produced vorticity is transported via advection and diffusion. An interesting finding is that the main vortices and the corner vortices can be visualized via the contour of buoyancy-produced vorticity. Although a vigorous definition of the vortex is still lacking in the community, our efficient vortex visualization approach in the RB convection may shed light on further research toward vortex identification. We also found that the spatial distribution of vorticity flux along the wall is positively correlated with that of the Nusselt number (\\(Nu\\)), suggesting the amount of vorticity that enters the flow is directly related to the amount of thermal energy that enters the flow.
Paper
Revisit eddy viscosity in pressure-driven wall turbulence at high Reynolds number
We investigate eddy-viscosity distributions in pressure-driven wall turbulence for three canonical configurations: plane closed-channel flow, open-channel flow with a free-slip surface, and pipe flow. Using direct numerical simulation (DNS) databases spanning friction Reynolds numbers \\(Re_=\\) 2000--12000, we infer the eddy viscosity from one-point statistics through the Boussinesq relation. The DNS-inferred eddy viscosity displays configuration-dependent behavior in the outer region, indicating that a single full-depth expression is not uniformly accurate for all three configurations. Building on the interpretation of eddy viscosity as the product of a velocity scale and a length scale, we extend the log-law scaling into the outer region. Specifically, we adopt a stress-based velocity scale and introduce an outer correction function to capture the remaining dependence on the outer coordinate. We then embed a compact parametric form of this correction into a Cess-type framework with van Driest near-wall damping, yielding a full-depth eddy-viscosity model. We assess the model using eddy-viscosity profiles, the log-law indicator function, and skin friction. The results show that the proposed model yields noticeable improvement for open-channel flow while remaining comparable to the classical Cess model for closed-channel flow and pipe flow. These findings underscore the role of outer boundary conditions in shaping the outer-region eddy viscosity and, consequently, mean-flow predictions.
Paper
Velocity dip in turbulent mixed convection of an open Poiseuille-Rayleigh-Bénard channel
We study the emergence of a velocity-dip phenomenon in turbulent mixed convection in open Poiseuille-Rayleigh-Bénard (PRB) channels with a free-slip upper boundary. Three-dimensional direct numerical simulations (DNS) are performed for Rayleigh numbers in the range \\(10^5 Ra 10^8\\), at a fixed Prandtl number \\(Pr = 0.71\\) and a bulk Reynolds number \\(Re_b = 2850\\). In the shear-dominated regime, the flow is characterised by small-scale structures such as near-wall streaks. As buoyancy becomes comparable to shear, streamwise-oriented large-scale rolls emerge and span the full channel height. At higher Rayleigh numbers, buoyancy dominates and the rolls fragment, giving rise to a convection-cell-dominated regime. Short-time-averaged flow fields show that streamwise rolls transport low-speed fluid from the bottom wall towards the upper boundary, forming laterally extended low-speed regions, while roll fragmentation induces upstream low-speed regions near the upper boundary. Both mechanisms locally reduce the near-surface mean velocity, leading to a velocity dip in which the maximum mean streamwise velocity is located below the upper boundary. Consistent with the mean momentum budget, the near-surface region exhibits a large-scale Reynolds shear stress that exceeds the local total shear stress, implying a negative viscous contribution and a reversal of the mean velocity gradient. To model this behaviour, we propose a model based on a balance between buoyancy and shear production with dissipation, incorporating a linear wall-normal profile for the Reynolds shear stress, a wall-normal-independent buoyancy-production term, and a decomposition of the dissipation into shear-induced and buoyancy-induced contributions. Our model accurately reproduces the DNS mean velocity profiles across the explored \\(Ra\\) range.
Paper
Interpolation-supplemented lattice Boltzmann simulation of thermal convection on non-uniform meshes
We present a systematic evaluation of an interpolation-supplemented lattice Boltzmann method (ISLBM) for simulating buoyancy-driven thermal convection on non-uniform meshes. The ISLBM extends the standard lattice Boltzmann framework by incorporating quadratic interpolation during the streaming step, enabling flexible mesh refinement near solid boundaries while maintaining algorithmic simplicity and parallel scalability. The method is implemented for a two-dimensional side-heated cavity at high Rayleigh numbers \\(10^6 Ra 10^8\\), and for a three-dimensional side-heated cavity at \\(10^5 Ra 10^7\\), with the Prandtl number fixed at \\(Pr=0.71\\). Benchmark results show that the ISLBM accurately captures thermal and velocity boundary layers, yielding Nusselt and Reynolds numbers in close agreement with high-fidelity reference data. Grid-convergence studies demonstrate nearly third-order accuracy for global quantities and about second-order for local fields. We further assess the computational performance of the in-house LBM solver against two open-source solvers: Nek5000 based on the spectral element method, and OpenFOAM based on the finite volume method. Performance metrics, including million lattice updates per second (MLUPS) and wall-clock time per dimensionless time unit (WCTpDT), indicate that the ISLBM offers one to three orders of magnitude higher efficiency in large-scale simulations. On GPU architectures, the ISLBM retains high computational performance: throughput on non-uniform meshes reaches 60-70% of that on uniform meshes in terms of MLUPS, while the cost in WCTpDT is about three times higher. These results highlight the potential of interpolation-based LBM approaches for high-fidelity simulations of thermal convection on non-uniform meshes, providing a robust foundation for future extensions to turbulent flows.
Paper
Learning to traverse convective flows at moderate to high Rayleigh numbers
We study the navigation of a self-propelled inertial particle in two-dimensional Rayleigh-Bénard convection at Prandtl number \\(Pr=0.71\\) and cell aspect ratio \\(=4\\) for Rayleigh numbers \\(Ra\\) ranging from \\(10^7\\) to \\(10^11\\). A reinforcement-learning (RL) controller selects the propulsive acceleration, subject to an upper bound \\(A_\\), to achieve a prescribed horizontal displacement. We find that the success rate increases abruptly with \\(A_\\) at moderate \\(Ra\\), whereas at higher \\(Ra\\) the transition becomes more gradual and shifts to larger \\(A_\\). Moreover, although the completion time increases with \\(Ra\\), the propulsion energy required for successful traversal decreases. Proper orthogonal decomposition indicates that these performance differences are associated with reorganisation of the carrier flow. At moderate \\(Ra\\), the dominant large-scale circulation partitions the domain through persistent transport barriers, requiring a finite thrust surplus to cross them; at higher \\(Ra\\), energy is distributed across many modes, the barriers fragment and transient plume-assisted pathways emerge. Compared with a constant-heading baseline, the learned policy aligns with local currents and consumes significantly less energy. Lagrangian coherent structure analysis further suggests that the RL agent tends to cross repelling barriers and surf along attracting pathways. Finally, by mapping these behaviours onto the local Eulerian flow topology using Voronoi tessellation and the \\(Q\\)-criterion, we distil an interpretable, physics-based heuristic strategy that retains robust navigability. These results connect turbulent-flow organisation with autonomous navigation under bounded actuation.
Paper