Rayleigh-Benard Research Articles

Analysis of bioconvection and oxytactic microorganisms in a porous cavity with nano-enhanced phase change materials and quadrant heater: Application of support vector regression based model

In this numerical study, two-dimensional simulations are performed to obtain the bioconvection flow in a quadrant heater located porous closed space saturated with nano-enhanced phase change material (NePCM). The Galerkin finite element method (GFEM) is employed to compute the governing equations for the considered system by developing a computer code. The study is performed for wide range of parameters as aspect ratio, Darcy number, Hartmann number, nanoparticle fraction, bioconvection Rayleigh number and natural convection Rayleigh number were tested. Also, the analysis is supported via artificial intelligent by using multiple support vector regression based model. The tests were performed with artificial intelligence on temperature values. After the analysis, it is found that radius of the curvilinear shaped heater plays role on both heat transfer and fluid flow, oxygen distribution, density of motile microorganism and heat capacity. Application of support vector machine technique gives good agreement with computational fluid dynamics results. Heat and mass transfer are improved with increasing of nanoparticle addition but decrease with increasing of strength of magnetic field.

ELECTROMAGNETIC WAVE MODEL OF HYDROGEN AND HELIUM DISTRIBUTION AT THE FORMATION OF THE SOLAR SYSTEM

This paper considers the formation process of the protosolar hydrogen-helium cloud in chronological order. From the list of hypotheses about the solar system formation, the electromagnetic hypothesis of Alfvén was selected since it is based on the plasma state of the protosolar hydrogen-helium cloud. It presumably describes a supernova explosion and its contribution to the solar system formation. The spatial-temporal dynamics of changes in the density and velocity of ions in a cylindrical two-component hydrogen-helium plasma is described. It is shown that the motion velocities of two-type particles represent Benard cells oscillating with different periods. The proposed electromagnetic wave model was used to calculate the distribution of hydrogen and helium ions in the solar system at the beginning of its formation and after a supernova explosion.

Effect of aspect ratio on PCM melting behavior in a square cavity

For latent heat thermal energy storage applications with phase change material (PCM) in restricted space, the factors such as container shape and heating surface must be considered. In this paper, a 2D square cavity PCM is taken as the research object, and the melting process of PCM with different square cavity aspect ratio (AR) was simulated under the condition of constant heat flux (60 W) at the left wall/bottom wall. An innovative and effective indicator, i.e., differential liquid fraction curve along with solid-liquid interface evolution is proposed to reveal the melting rate and typical phase change characteristics. The results show that the complete melting time increases with the increase of AR for both left wall and bottom wall heating conditions. For general applications, the bottom wall heating has a higher melting rate compared with left wall, especially for the cavity with AR >1. During the phase change process, the melting rate can reach its maximum value at the peak of differential liquid fraction curve with an observation of the longest solid-liquid interface. In addition, the melting pattern of high AR cavity during the latter part of the left wall heating process would become similar to the low AR case under the “scouring effect” of natural convection on the solid-liquid interface. For bottom wall cases, the natural convection waveform presents Rayleigh-Benard convection cells, and with the increase of AR, the irregularity of the vortex structure becomes more obvious. The results can provide useful guidance for accurately evaluating phase change characteristics and designing thermal energy storage system.

Application of 4D two-colour LIF to explore the temperature field of laterally confined turbulent Rayleigh–Bénard convection

Recent studies show the significant effect of the third dimension and flow unsteadiness of laterally confined Rayleigh–Bénard convection (RBC). However, there are limited studies investigating 4D flow properties. The application of 4D two-colour laser-induced fluorescence to explore a laterally confined RBC at high Rayleigh number of $$Ra=9.9\times {10}^{7}$$ and Prandtl number of $$Pr=6.1$$ is investigated using a scanning laser system. A two-colour, two dyes approach was employed to resolve the laser sheet intensity variations due to refractive index variations caused mainly by the generation of thermal plumes. Two temperature-sensitive fluorescent dyes with opposite sensitivities were used to enhance the overall temperature sensitivity to 7.3%/°C. Details of the experimental procedure and optical system employed to reach such a high sensitivity are demonstrated. From the whole field temperature distribution, the dimensionless heat transfer coefficient, the Nusselt number, and its evolution were calculated for both hot and cold boundaries. Temperature field and Nusselt number obtained from 3D and 2D fields are reported to compare the results for these two scenarios. Thermal plumes were found to have a conical shape in the laterally confined RBC compared to the conventional mushroom shape. From visualization of the time-resolved 3D temperature field and 2D distribution of the Nusselt number, it was also found that only by volumetric measurement, temporal and spatial variations of the temperature and heat transfer can be evaluated. This shows that to evaluate the classic and ultimate theories, volumetric measurement is required for a coherent understanding of the physics.

Single sidewall cooling modulation on Rayleigh–Bénard convection

We experimentally explored the effect of single-sidewall cooling on Rayleigh–Bénard (RB) convection. Canonical RB was also studied to aid insight. The scenarios shared tank dimensions and bottom and top wall temperatures; the single sidewall cooling had the top wall temperature. Turbulence was explored at two canonical Rayleigh numbers, $Ra=1.6\times 10^{10}$ and $Ra=2\times 10^9$ under Prandtl number $Pr=5.4$ . Particle image velocimetry described vertical planes parallel and perpendicular to the sidewall cooling. The two $Ra$ scenarios reveal pronounced changes in the flow structure and large-scale circulation (LSC) due to the sidewall cooling. The density gradient induced by the sidewall cooling led to asymmetric descending and ascending flows and irregular LSC. Flow statistics departed from the canonical case, exhibiting lower buoyancy effects, represented by an effective Rayleigh number with effective height dependent on the distance from the lateral cooling. Velocity spectra show two scalings, $\varPhi \propto f^{-5/3}$ Kolmogorov (KO41) and $\varPhi \propto f^{-11/5}$ Bolgiano (BO59) in the larger $Ra$ ; the latter was not present in the smaller set-up. The BO59 scaling with sidewall cooling appears at higher frequencies than its canonical counterpart, suggesting weaker buoyancy effects. The LSC core motions allowed us to identify a characteristic time scale of the order of vortex turnover time associated with distinct vortex modes. The velocity spectra of the vortex core oscillation along its principal axis showed a scaling of $\varPhi _c \propto f^{-5/3}$ for the single sidewall cooling, which was dominant closer there. It did not occur in the canonical case, evidencing the modulation of LSC oscillation on the flow.

LBM-MHD Data-Driven Approach to Predict Rayleigh–Bénard Convective Heat Transfer by Levenberg–Marquardt Algorithm

This study aims to consider lattice Boltzmann method (LBM)–magnetohydrodynamics (MHD) data to develop equations to predict the average rate of heat transfer quantitatively. The present approach considers a 2D rectangular cavity with adiabatic side walls, and the bottom wall is heated while the top wall is kept cold. Rayleigh–Bénard (RB) convection was considered a heat-transfer phenomenon within the cavity. The Hartmann (Ha) number, by varying the inclination angle (θ), was considered in developing the equations by considering the input parameters, namely, the Rayleigh (Ra) numbers, Darcy (Da) numbers, and porosity (ϵ) of the cavity in different segments. Each segment considers a data-driven approach to calibrate the Levenberg–Marquardt (LM) algorithm, which is highly linked with the artificial neural network (ANN) machine learning method. Separate validations have been conducted in corresponding sections to showcase the accuracy of the equations. Overall, coefficients of determination (R2) were found to be within 0.85 to 0.99. The significant findings of this study present mathematical equations to predict the average Nusselt number (Nu¯). The equations can be used to quantitatively predict the heat transfer without directly simulating LBM. In other words, the equations can be considered validations methods for any LBM-MHD model, which considers RB convection within the range of the parameters in each equation.

Evolution and Features of Dust Devil‐Like Vortices in Turbulent Rayleigh‐Bénard Convection—An Experimental Study

AbstractWe present an experimental study simulating atmospheric dust devils in a controlled laboratory experiment. The experimental facility, called the “Barrel of Ilmenau” (www.ilmenauer-fass.de) represents a classical Rayleigh‐Bénard set‐up and is believed to model the phenomena in a convective atmospheric boundary layer fairly well. Our work complements and extends the numerical work of Giersch and Raasch (2021)https//doi.org/10.1029/2020jd034334 by experiments. Dust devils are thermal convective vortices with a vertical axis of rotation visualized by entrained soil particles. They evolve in the convective atmospheric boundary layer and are believed to substantially contribute to the aerosol transport into the atmosphere. Thus, their evolution, size, lifetime, and frequency of occurrence are of particular research interest. Extensive experimental studies have been conducted by field measurements and laboratory experiments so far. Beyond that, our study is the first attempt of Rayleigh‐Bénard convection (RBC) in air to investigate dust devil‐like vortices in a laboratory experiment. Up to now, this set‐up mimics the natural process of dust devil evolution as closest to reality. The flow measurement was carried out by particle tracking velocimetry using neutrally buoyant soap bubbles. We initially identified dust devil‐like vortices by eye from the Lagrangian velocity field, and in a later, more sophisticated analysis by a specific algorithm from the corresponding Eulerian velocity field. We analyzed their frequency of occurrence, observation time, and size. With our work, we could demonstrate that turbulent RBC is an appropriate model to mimic the natural process of the evolution of dust devils in the convective atmospheric boundary layer without artificial stimulation.

Pattern selection and heat transfer in the Rayleigh–Bénard convection near the vicinity of the convection onset with viscoelastic fluids

The effect of viscoelasticity on the flow and heat transport in the Rayleigh–Bénard convection (RBC), a frequently encountered phenomenon in nature and industry, in a rectangular enclosure with horizontal periodic boundary is investigated via direct numerical simulation. The working fluid is described by a finitely extensible nonlinear elastic-Peterlin constitutive model almost all important features of viscoelastic fluid flow. Numerical simulations are conducted at a low concentration β=0.9, where β=μs/μ0, μs is the solvent viscosity, and μ0=μs+μp is the sum of μs and the polymer viscosity μp. A parametric analysis is performed to understand the influence of the Weissenberg number Wi, the viscosity ratio β, and the extension length L on the oscillating mode of the viscoelastic RBC. The results indicate that both Wi and β weakly inhibit the convection onset and the transition from steady to oscillatory convection. The amplitude and frequency of the oscillations in the oscillatory flow regime are both suppressed. However, the strongly elastic nonlinearity makes the flow transition irregular and even brings about the relaminarization or lead to the convection cells traveling in the horizontal direction. The increasing extension length L induces multiple pairs of roll flow patterns at a specific setting of (Ra, Wi). Heat transport is reduced (up to 8.5%) by elasticity but still obeys the power law with Ra if the flow pattern has one pair of rolls. However, heat transfer enhancement occurs if multiple pairs of rolls are induced.

An Inhomogeneous Steady-State Convection of a Vertical Vortex Fluid

An exact solution of the Oberbeck – Boussinesq equations for the description of the steady-state Bénard – Rayleigh convection in an infinitely extensive horizontal layer is presented. This exact solution describes the large-scale motion of a vertical vortex flow outside the field of the Coriolis force. The large-scale fluid flow is considered in the approximation of a thin layer with nondeformable (flat) boundaries. This assumption allows us to describe the large-scale fluid motion as shear motion. Two velocity vector components, called horizontal components, are taken into account. Consequently, the third component of the velocity vector (the vertical velocity) is zero. The shear flow of the vertical vortex flow is described by linear forms from the horizontal coordinates for velocity, temperature and pressure fields. The topology of the steady flow of a viscous incompressible fluid is defined by coefficients of linear forms which have a dependence on the vertical (transverse) coordinate. The functions unknown in advance are exactly defined from the system of ordinary differential equations of order fifteen. The coefficients of the forms are polynomials. The spectral properties of the polynomials in the domain of definition of the solution are investigated. The analysis of distribution of the zeroes of hydrodynamical fields has allowed a definition of the stratification of the physical fields. The paper presents a detailed study of the existence of steady reverse flows in the convective fluid flow of Bénard – Rayleigh – Couette type.

Scaling transition of thermal dissipation in turbulent convection

Direct numerical simulation (DNS) of non-slip two dimensional (2D) Rayleigh–Benard convection (RBC) is conducted for a wide range of Rayleigh number (Ra up to 1013) at Prandtl number Pr = 0.7 and aspect ratio Γ = 1. The thermal dissipation rate is shown to display an evident scaling transition through the compensated plot, i.e., ⟨εθ⟩∝Raγ with γ≈−0.17 for 106≤Ra≤109, while γ≈−0.19 for Ra=109≤Ra≤1013. To track the transition, separate contributions from the thermal boundary layer (BL) and the bulk flow region are examined, incorporated also with the mean and fluctuation decomposition. It turns out that the mean temperature gradient in the BL is the dominant contribution, and together with other parts (i.e., fluctuations in the BL and bulk, and the mean gradient in the bulk), they all exhibit an obvious transition at Ra≈109. We have further checked the Nusselt number (Nu), which also shows the transition at Ra≈109. Interestingly, Nu∝Ra0.33 is observed for small Ra, while Nu∝Ra2/7 is absent in 2D RBC cases. To understand the physical origin of transition, spatial distributions and probability density functions of thermal dissipation rate are finally discussed, with notable statistical features changed at Ra≈109.

On the thermal effect of porous material in porous media Rayleigh–Bénard convection

We perform a two-dimensional numerical study on the thermal effect of porous media on global heat transport and flow structure in Rayleigh–Bénard (RB) convection, focusing on the role of thermal conductivity $\lambda$ of porous media, which ranges from $0.1$ to $50$ relative to the fluid. The simulation is carried out in a square RB cell with the Rayleigh number $Ra$ ranging from $10^7$ to $10^9$ and the Prandtl number $Pr$ fixed at $4.3$ . The porosity of the system is fixed at $\phi =0.812$ , with the porous media modelled by a set of randomly displayed circular obstacles. For a fixed $Ra$ , the increase of conductivity shows a small effect on the total heat transfer, slightly depressing the Nusselt number. The limited influence comes from the small number of obstacles contacting with thermal plumes in the system as well as the counteraction of the increased plume area and the depressed plume strength. The study shows that the global heat transfer is insensitive to the conduction effect of separated porous media in the bulk region, which may have implications for industrial designs.

Experimental and numerical investigation of Rayleigh-Benard convection in rectangular cavity with motor oil

Naturally flows have been the scope of the scientific research for centuries, Rayleigh-Benard convection being one of the leading. Many researchers have considered the flow patterns, boundary conditions, various cavities, nanofluids, theoretically, numerically, and experimentally. The flow was investigated in atmosphere and in nanofluids, in air, water, molten metals, non-Newtonian fluids. Almost all research focuses on 2-D or 3-D analysis of flow in laterally unlimited enclosures, as parallel plates or coaxial cylinders. In technical practice, only limited enclosures exist. This paper presents numerical and real experimental results for the test chamber with ratio 4?2?1 in x-, y-, and z direction, respectfully. The measurements were taken at fifteen different positions on the faces of the tank. Probes used are PT100 elements. As the chamber is limited in all directions, the results have shown strong influence of the lateral walls. The results are compared with the those obtained by IR camera. Various fluids were tested, and results for motor oil will be presented.

Numerical simulation of tidal synchronization of the large-scale circulation in Rayleigh--Bénard convection with aspect ratio 1

A possible explanation for the apparent phase stability of the 11.07-year Schwabe cycle of the solar dynamo was the subject of a series of recent papers. The synchronization of the helicity of an instability with azimuthal wavenumber m=1 by a tidal m=2 perturbation played a key role here. To analyze this type of interaction in a paradigmatic set-up, we study a thermally driven Rayleigh-B\'enard Convection (RBC) of a liquid metal under the influence of a tide-like electromagnetic forcing. As shown previously, the time-modulation of this forcing emerges as a peak frequency in the m=2 mode of the radial flow velocity component. In this paper we present new numerical results on the interplay between the Large Scale Circulation (LSC) of a RBC flow and the time modulated electromagnetic forcing.

Chemoconvection of miscible solutions in an inclined layer

This paper presents an experimental and numerical investigation of the chemoconvective flow of two miscible reacting solutions that generate a plain layer oriented at some angle to the gravity field. In the experiments, the aqueous solutions of nitric acid and sodium hydroxide were used. The system evolves from an initial state, in which each homogeneous solution occupies half of the layer, and the contact surface between the layers is flat. When the reacting solutions come into contact with each other, an acid-base neutralization reaction occurs, forming salt and water. The system configuration is chosen so that the acid solution of lower density rests on top of the denser base. This excludes the development of the Rayleigh–Taylor instability. The experiments were performed for such initial concentrations of the reagents at which a wave regime is realized in the system. The wave comprises a density jump rapidly advancing in the direction of gravity and separating the immobile alkali solution and the layer of acid and salt mixture, the convective motion of which feeds the frontal reaction with fresh acid. The visualization of the flow in the experiments was performed using a Fizeau interferometer. The numerical study of a complete three-dimensional problem was carried out using the ANSYS CFX hydrodynamic simulation program. We studied the flow evolution at a gradual increase in the angle of inclination from 0° to 70°. We found that the layer inclination leads to a significant change in the structure and intensity of the convective motion. Already at small inclination angles (up to 30°), the flow becomes three-dimensional, which makes the Hele-Shaw approximation inapplicable to this case. We show that there is a spontaneous stratification of salt and acid concentration fields in the cocurrent wave flow. With an increase in the angle of inclination, the wave velocity decreases, and chemoconvection in the cocurrent flow becomes less intense and acquires a certain vortex structure. At larger angles (from 50° to 70°), the wave front is strongly deformed or the wave breaks up. We demonstrate that an up-and-down fluid flow develops in the layer above the density jump. This flow eventually loses its stability with respect to the vertical rolls of solutal Rayleigh convection. There is a good agreement between the experimental measurements and the results of numerical simulation of the 3D problem.

On the energy transport and heat transfer efficiency in radiatively heated particle-laden Rayleigh–Bénard convection

We investigate the energy transport and heat transfer efficiency in turbulent Rayleigh–Bénard (RB) convection laden with radiatively heated inertial particles. Direct numerical simulations combined with the Lagrangian point-particle mode were carried out in the range of density ratio $854.7\le \rho _p/\rho _0 \le 8547$ and radiation intensity $1\le \phi /\phi _{solar}\le 100$ for both two-dimensional (2-D) and three-dimensional (3-D) simulations. The Rayleigh number ranges from $2\times 10^6$ to $10^8$ for 2-D cases, and is $10^7$ for 3-D cases for $Pr=0.71$ . It is found that particles with small density ratio that encounter strong radiation significantly alter the flow momentum transport and fluid heat transfer, so the fluid temperature of bulk is remarkably heated. We then derived the theoretical relation of the Nusselt number for interphase heat transfer in the heated particle-laden RB convection, which reveals that the heat transfer difference between the top and bottom plates stems from the interphase heat transfer. We further found that both the interphase heat transfer and the interphase thermal energy transport exhibit universal properties. They are both increased linearly with the reciprocal of the normalized density ratio. Additionally, both the interphase heat transfer and the interphase thermal energy transport increase linearly with the increase of radiation intensity. The growth rates exhibit specific scaling relations versus Rayleigh number and density ratio. Two different regimes distinguished by the critical density ratio, i.e. the exothermic particle regime and the endothermic particle regime, are observed. We further derived the power-law relation of the critical density ratios versus Rayleigh number and radiation intensity, i.e. $\rho _p/\rho _c \sim (\phi /\phi _{solar})^{1/2}\,Ra^{1/3}$ , which is in remarkable agreement with the 3-D simulations.

Heat flux enhancement by regular surface protrusion in partitioned thermal convection

We investigate the influence of the regular roughness of heated and cooled plates and adiabatic partition boards on the mean heat transport in a square Rayleigh–Bénard (RB) convection enclosure by two-dimensional direct numerical simulations. The roughness is in the form of isothermal protrusions with a rectangular base and triangular tip. The protrusion height varies from 10% to 25% of enclosure height. With increased protrusion height, the large-scale circulation cannot wash out the cavity between two consecutive protrusions. Thus, the overall heat transport of the enclosure impedes. We have inserted the partition boards between two successive protrusions with a gap between the conduction plate and the partition board to wash out the cavity. The partition board height varies from 20% to 99.8% of enclosure height. We have performed the simulations for the range of Rayleigh number 106–108 and at a fixed Prandtl number of 1. The tip of the triangular protrusion acts as an active plume-emitting spot. We observe a single large-scale elliptical roll with counter-rotating corner rolls for small partition board height. With an increase in partition board height, an elliptical large-scale roll breaks down into the number of large-scale rolls horizontally placed one beside the other. Finally, we observe multiple rolls stacked vertically when the partition boards almost touch the conduction walls. Heat flux enhancement strongly depends on large-scale flow structures. We found a maximum heat flux enhancement in protrusion with partitioned RB case approximately up to 4.7 times the classical square RB for an optimal gap between conduction plate and partition board. The maximum heat transport enhancement is due to the strong horizontal flow through the gap between the conduction plate and partition board, which locally reduces the thermal boundary layer's thickness. The interaction between the horizontal jets and the thermal boundary layers enhances heat transport.

On the instability of water-in-oil emulsion drop lamella and rim moving over a heated surface in the film boiling regime

The experimental study deals with the nature of interrelated unstable equilibrium of lamella and rim of an emulsion drop based on n-dodecane and bidistilled water after its collision with a solid surface heated to 350–400 °C at Weber numbers We =160–310 and in the presence of a stable boundary vapor layer. The relationship between the delay time of the onset of emulsion lamella destabilization relative to maximum drop spreading time and the ratio of the characteristics of diffusive and convective heat transfer at unstable equilibrium of the liquid is revealed. The combined effect of We and the emulsion concentration on the critical Rayleigh and Marangoni numbers characterizing the lamella perforation is demonstrated. A mechanism is proposed where the fingering causes the lamella rupture near attachment to the rim. Liquid wettability is partially restored and the liquid periodically contacts the surface, resulting in the vapor bubbles formation and growth, as well as their collapse, which ensures the cascade of the lamella destruction and the liquid bridges formation. The proposed physical model develops ideas about the complexity and interconnectedness of the destabilization mechanisms of the emulsion drop rim and lamella, including not only rim instabilities, but also the long-wave Marangoni instability taking into account the emulsion concentration and, in some cases, Rayleigh-Benard instability.

On convective instabilities in a rotating fluid with stably stratified layer and thermally heterogeneous boundary

The onset of convection in a rotating plane layer due to a vertical temperature gradient is studied in this paper. The background stratification is modulated by lateral temperature variations and stable stratification aimed at understating the Earth's outer core convection subject to thermal core–mantle interaction. At the top boundary, sinusoidal and Gaussian temperature variations are imposed apart from the reference case of isothermal condition used in the classical Rayleigh–Benard convection. The additional modulating conditions break the top–bottom flow symmetry leading to flow localization and asymmetry that exhibit modified temporal dynamics unlike that of the classical Rayleigh–Benard cells. The threshold for convection is lowered with flows occurring in surplus heat flux regions caused by the imposed conditions. Despite flow suppression in the stable layer, rapid rotation favors the penetration of convection rolls with smaller wavelengths. The lateral variations in temperature imposed at the top boundary enhance such axial penetration with a laterally varying penetrative extent resulting in a modified clustered flow structure unlike the reference case. With both modulating conditions imposed, the onset of overstable modes is favored for low Prandtl numbers, a regime which is relevant to the Earth's core conditions. With rapid rotation, a novel mode of traveling wave instability occurs at the onset of convection, the propagation direction of which is controlled by the lateral temperature gradients at the top boundary. The onset of oscillatory modes is suppressed by the imposition of the modulating conditions indicated by the significant lowering of the transition Prandtl number.

Impact of Magnetic Field on the Onset of Double Diffusive Rayleigh-Benard Convection Governed by Local Thermal Non-Equilibrium in a Double Layered System

Onset of Double Diffusive Rayleigh-Benard-Magneto (DDRBM) convection is studied for a double layered system composed of incompressible fluid confined by adiabatic rigid peripheries under the governance of Local Thermal Non-Equilibrium (LTNE). Result of the acquired problem is obtained analytically through the mode of Regular Perturbation. Consequence of physical factors such as solid phase thermal expansion ratio, solid phase thermal diffusivity ratio and inter-phase thermal diffusivity ratio that favours LTNE are being analysed. The outcome of altering the constraints namely fluid phase thermal expansion ratio, solute Rayleigh number, Chandrasekhar number, thermal ratio, concentration ratio, fluid phase solute diffusivity ratio, solute diffusivity ratio in fluid layer and porous layer subject to LTNE set-up are paralleled with that of LTE set-up with diagrammatic representation.

The effect of surface roughness on the Lagrangian coherent structures in turbulent Rayleigh–Bénard convection

We perform direct numerical simulations of turbulent Rayleigh–Bénard (RB) convection in a closed square cell with roughness plates at Rayleigh number fixed at Ra=108 and the Prandtl number fixed at Pr = 1. To gain insight into the effect of surface roughness on material transport in turbulent Rayleigh–Bénard convection, the Lagrangian coherent structures (LCSs) are extracted using the finite-time Lyapunov exponent method in the cases of different roughness heights. First, we find that lobe structures are widely present in RB convection and we elucidate how they play a part in transporting heat from coner-flow rolls to large-scale circulation. Then, we quantify the heat flux along the LCSs, which contributes to 80% of the total flux. This implies that the LCSs play an important role in heat transport regardless of the roughness height. Furthermore, two different mechanisms of heat transport in RB convection induced by roughness heights are explained in the Lagrangian perspective: the decrease in Nu number in the cases of h<hc is caused by the LCSs between the roughness elements which hinders the exchange of material between the fluid in the cavity and the bulk region; whereas, the increase in Nu number in the case of h>hc is produced by the enhanced mixing events of the convection that enhance the contribution of heat transport in the bulk region.