Simulation Of Turbulent Natural Convection Research Articles

Simulation of Turbulent Natural Convection in Photovoltaic Solar Panels Based on the Spalart–Allmares (SA) Turbulence Model

Simulation of Turbulent Natural Convection in Photovoltaic Solar Panels Based on the Spalart–Allmares (SA) Turbulence Model

Large-eddy simulation of turbulent natural convection in a cylindrical cavity using an off-lattice Boltzmann method

In the present work, a characteristic-based off-lattice Boltzmann method with the large-eddy simulation (LES) as the turbulence model is developed for the simulation of turbulent natural convection. A double-distribution-function approach is used to resolve flow and thermal fields, and the proposed framework is developed, in three-dimensional curvilinear coordinates. The solver is verified using three benchmark cases, namely, the turbulent Taylor–Green vortex flow, natural convection in a periodic tall cavity, and Rayleigh–Bénard convection. Due to the absence of an inlet in this kind of closed cavity flow, initial perturbations are proposed and verified, which accelerate transition to a turbulent state. The turbulent natural convection in a cylindrical cavity is simulated for a Rayleigh number of Ra=7.5×105, and the flow and thermal characteristics are analyzed. A grid sensitivity study is conducted and an appropriate mesh resolution is selected, that is, further verified using the LES index of quality-of-resolution. The resulting turbulent flow and the associated thermal plume are analyzed using instantaneous and time-averaged mean and second-order statistics, vortical structures, turbulence anisotropy maps, energy budgets, frequency spectra, and the mean and root mean square of temperature and Nusselt numbers. The results indicate that the thermal plume region is highly anisotropic, whereas the rest of the annulus contains single-component axisymmetric turbulence. The production and convection of turbulence are dominant on top of the inner cylinder in the thermal plume region, whereas diffusion is dominant closer to the outer cylinder. The azimuthal profiles of mean Nusselt number for the inner and the outer cylinders are observed to be negatively correlated. Furthermore, natural convection in the cylindrical cavity is simulated for Ra=4×103 to 5×106 and the effect of the Rayleigh number on the mean Nusselt number and flow patterns is studied.

Hybrid Simulation of Turbulent Natural Convection in an Enclosure with Thermally-Conductive Walls

This paper analyzes the interaction of high Rayleigh number flow with conjugate heat transfer. The two-relaxation time lattice Boltzmann is used as a turbulent buoyancy-driven flow solver whereas the implicit finite difference technique is applied as a heat transfer solver. An in-house numerical code is developed and successfully validated on typical CFD problems. The impact of the Biot number, heat diffusivity ratio and the Rayleigh number on turbulent fluid flow and heat transfer patterns is studied. It is revealed that the thermally-conductive walls of finite thickness reduce the heat transfer rate. The temperature of the cooled wall slightly depends on the value of the buoyancy force. The heat diffusivity ratio has a significant effect on thermal and flow behavior. The Biot number significantly affects the mean Nusselt number at the right solid–fluid interface whereas the mean Nusselt number at the left interface is almost insensible to the Biot number variation.

Numerical simulation of turbulent natural convection around spheres

La mayor parte de las investigaciones que se han realizado sobre la convección natural han abordado su estudio en la región laminar y no se ha dirigido mucha atención a la convección natural turbulenta que suele presentarse cuando el número de Rayleigh supera un cierto valor crítico. Estudios recientes han mostrado que las correlaciones existentes para esta región suelen ser poco precisas. Por otra parte, el tratamiento numérico de este problema presenta dificultades relacionadas con el modelo de turbulencia usado, ya que se está en presencia de una capa límite laminar-turbulenta en un entorno laminar, por lo cual, el modelo laminar subestimará la transferencia de calor, mientras los modelos turbulentos la sobreestimarán. Modelos transicionales y otras opciones están disponibles, pero están diseñados para entornos turbulentos. El presente trabajo plantea la solución numérica mediante el programa computacional Ansys Fluent de la convección laminar turbulenta en torno a esferas sin modelo de turbulencia (modelolaminar) y usando un modelo de turbulencia k-ε para bajos números de Reynolds. El trabajo permitió caracterizar los números de Rayleigh a partir de los cuales empieza a ocurrir el cambio de capa límite laminar a turbulenta, así como la posición del desprendimiento de la capa límite. Los valores obtenidos para la transferencia de calor con el modelo laminar muestran muy buena concordancia con experimentos y correlaciones para bajos números de Rayleigh, pero pierde precisión a partir de cierto valor de Rayleigh, aspecto que pudo corregirse satisfactoriamente incorporando el modelo de turbulencia utilizado

Near wall Prandtl number effects on velocity gradient invariants and flow topologies in turbulent Rayleigh\u2013B\xe9nard convection

The statistical behaviours of the invariants of the velocity gradient tensor and flow topologies for Rayleigh–Bénard convection of Newtonian fluids in cubic enclosures have been analysed using Direct Numerical Simulations (DNS) for a range of different values of Rayleigh (i.e. Ra=10^7-10^9) and Prandtl (i.e. Pr=1 and 320) numbers. The behaviours of second and third invariants of the velocity gradient tensor suggest that the bulk region of the flow at the core of the domain is vorticity-dominated whereas the regions in the vicinity of cold and hot walls, in particular in the boundary layers, are found to be strain rate-dominated and this behaviour has been found to be independent of the choice of Ra and Pr values within the range considered here. Accordingly, it has been found that the focal topologies S1 and S4 remain predominant in the bulk region of the flow and the volume fraction of nodal topologies increases in the vicinity of the active hot and cold walls for all cases considered here. However, remarkable differences in the behaviours of the joint probability density functions (PDFs) between second and third invariants of the velocity gradient tensor (i.e. Q and R) have been found in response to the variations of Pr. The classical teardrop shape of the joint PDF between Q and R has been observed away from active walls for all values of Pr, but this behavior changes close to the heated and cooled walls for high values of Pr (e.g. Pr=320) where the joint PDF exhibits a shape mirrored at the vertical Q-axis. It has been demonstrated that the junctions at the edges of convection cells are responsible for this behaviour for Pr=320, which also increases the probability of finding S3 topologies with large negative magnitudes of Q and R. By contrast, this behaviour is not observed in the Pr=1 case and these differences between flow topology distributions in Rayleigh–Bénard convection in response to Pr suggest that the modelling strategy for turbulent natural convection of gaseous fluids may not be equally well suited for simulations of turbulent natural convection of liquids with high values of Pr.

Numerical simulation of turbulent natural convection in an enclosure with a curved surface heated from below

Natural circulation, which is employed in the design of a small modular reactor, influences the removal of decay heat from the upper header of a pressure vessel under severe accident conditions. This phenomenon has been emphasized for a better understanding of passive safety systems. In this paper, a Reynolds-averaged turbulence model is used to simulate turbulent natural convection in a rectangular enclosure with a curved surface heated from below. The simulation results are first compared with the Direct Numerical Simulation (DNS) of a differential heated cavity obtained from the literature. A good agreement demonstrated the ability of the employed computational fluid dynamics (CFD) approach to predict turbulent natural convection flow and its induced heat transfer. Consequently, the turbulent natural convection formed in a rectangular enclosure heated by a curved surface was investigated. The ensemble average was employed to evaluate the local Nusselt number distribution quantitatively as well as the velocity and temperature distributions. Their dependency was also evaluated based on a systematic CFD simulation that covers a wide range of Rayleigh numbers. Least square regression analysis was employed to propose a correlation to predict the averaged Nusselt number as a function of the Rayleigh number. The influence of the thermal boundary conditions and the discrete heater was also determined.

Numerical experiment of turbulent transfer of thermal energy using the immersed boundary method and adaptive mesh refinement

This paper presents the mathematical modeling and numerical simulations of turbulent flows with thermal convection. Cartesian numerical meshes are used for non-Cartesian problems thanks to the dynamic and thermal immersed boundary methods. Turbulence modeling was performed using the LES (Large Eddy Simulation) methodology with dynamic Smagorinsky subgrid-scale modeling. All of the simulations were carried out using a computational code that was developed at home (MFSim - Multiphysic Simulator). Two kinds of thermal immersed boundary conditions were proposed, implemented, and validated. To exemplify the functionalities of this computational tool, simulations of turbulent natural convection were performed on a horizontal cylinder. Consistent qualitative results were obtained, which reveal the dynamic structures characterizing the transition to turbulence. The corresponding statistical results compare well with the reference results that have been published by other authors.

Numerical simulation of turbulent natural convection of nanofluid with thermal radiation inside a tall enclosure under the influence of magnetohydrodynamic

AbstractIn the present study, the natural convective heat transfer in the turbulent flow of water/CuO nanofluid with volumetric radiation and magnetic field inside a tall enclosure has been numerically investigated. The thermophysical properties of nanofluid have been considered variable with temperature and the effects of Brownian motion of nanoparticles have been considered. The main objective of this work is an investigation of the effect of using water/CuO nanofluid and presence of magnetic field on turbulent natural convection in three types of enclosures (vertical, inclined, and horizontal) by considering the volumetric radiation. The governing equations on turbulent flow domain under the influence of the magnetic field and by considering the combination of volumetric radiation and natural convection have been solved by a coupled algorithm. For validating the present research, a comparison has been carried out with the laminar natural convection flow under the influence of the magnetic field and radiation effects and also, the natural turbulent convection flow of previous studies and a proper coincidence has been achieved. The results indicated that by increasing volume fraction and Hartmann number the average Nusselt number enhances and reduces, respectively. By adding 1% CuO nanoparticles to the base fluid, heat transfer improves from 10.59% to 17.05%. However, by increasing the volume fraction from 1% to 4%, heat transfer improves from 1.35% to 4.90%. By increasing Hartmann number from 0 to 600, heat transfer reduces from 9.29% to 22.07%. Also, the results show that the ratio of deviation angle of the enclosure to the horizontal surface has considerable effects on heat transfer performance. Therefore, in similar conditions, the inclined enclosure with a deviation angle of 45° compared to the vertical and horizontal enclosure has better thermal performance.

Large Eddy Simulation of turbulent natural convection in an inclined tall cavity

To a better understanding of buoyancy-driven flows in enclosed tall cavity, Large Eddy Simulations are performed with an original solver, which combines fourth-order compact scheme and parallel computing. The direct numerical simulation of the natural convection flow between two heated parallel plates are provided to validate the present numerical method. Then, the mean and turbulent statistics of the buoyancy-driven in a tall cavity at the moderate Rayleigh number are computed. The results of the vertical configuration flow are successfully compared with the available experimental data. Then, the statistics are discussed with respect to the inclination angle of the tall cavity. Especially, the occurrence of convection rolls in the cavity inclined to the horizontal is shown, as well as their roles in the deterioration of the thermal stratification.

Lattice Boltzmann Simulation of Turbulent Natural Convection: Enclosure Heated from Below

In the present study, the natural convection heat transfer inside a rectangular enclosure is investigated. The bottom and top walls are kept at constant temperatures, and the sidewalls are insulated. A single relaxation time Bhatnagar–Gross–Krook model for the lattice Boltzmann method is used. For low Rayleigh numbers, the results show that the isotherm plots are smooth and weak vortices appear only at the upper part of the cavity; whereas at high Rayleigh numbers, the isotherm plots are disordered and the vortices become strong at the bottom and top parts of the cavity. The results show that the Nusselt number increases with increasing of Rayleigh number. The average Nusselt number at turbulent regime instead of reaching steady state, it undergoes complex transitions to take periodic and chaotic behavior with time. The present numerical results have a good agreement with previous numerical and experimental results in both laminar and turbulent regimes.

Accurate numerical simulation of turbulent natural convection in an enclosed tall cavity

Building simulation tools play an important role in the field of building physics. The performance of these tools relies on the understanding of related physic phenomenon and needs to be ensured by the validation. Nowadays, numbers of building simulation tools have been developed. However, most of these tools lack internationally accepted experimental data for validation. As for the investigation of combined heat, air and moisture (HAM) transfer process, it is limited by the fact that it is difficult to measure air and moisture fluxes and moisture transport potentials, even under laboratory conditions. In this paper, the author established a numerical laboratory to overcome the difficulty of measuring air and mass flux within enclosed cavity. The performance of new developed simulation approach in predicting convective flow characteristics of dry air has been validated. Then convective flow characteristics of moist air within enclosed cavity have been investigated.

Large eddy simulation of turbulent natural convection between symmetrically heated vertical parallel plates for water

The boundary layer interaction effect of the turbulent natural convection in the gap between symmetrically heated vertical parallel plates was evaluated using a numerical simulation in the present study. A large eddy simulation was conducted, and a Vreman model was used as a dynamic subgrid-scale model. The numerical simulation was validated thorough a comparison with the experimental result for the turbulent natural convection adjacent to a single vertical heated plate. The boundary interaction effect was investigated by varying the gap between the parallel plates. The results showed that the flow for vertical parallel plates had a lower heat transfer rate than a vertical plate flow. The boundary layers and vortex structure were evaluated. The heat transfer was reduced as a result of a reduced velocity gradient in the outer region of the velocity boundary layer. The averaged heat transfer was similar to that of the laminar flow.

Simulation of Turbulent Natural Convection in a Local Zone of Largescale Area Under Conditions of the Radiant Energy Supply

This paper presents the results of mathematical modelling of turbulent natural convection under conditions of the conjugate heat transfer in an open cavity heated by infrared emitter. Two-dimensional problem of heat transfer was formulated in the vorticity - stream function - temperature dimensionless variables and solved by means of the finite difference method on a uniform grid. According to the results of numerical analysis, the fields of differential and integral heat transfer characteristics, illustrating the unsteady nature of the thermophysical process under study, were obtained. The nonlinear dependence of the average Nusselt number versus dimensionless time ( τ ) was established. It was shown that the average Nusselt number monotonically increased in a range of 800 τ <2000

Numerical simulation of turbulent natural convection combined with surface thermal radiation in a square cavity

Purpose – The purpose of this paper is to present transient turbulent natural convection with surface thermal radiation in a square differentially heated enclosure using non-primitive variables like stream function and vorticity. Design/methodology/approach – The governing equations formulated in dimensionless variables “stream function, vorticity and temperature,” within the Boussinesq approach taking into account the standard two equation k-ε turbulence model with physical boundary conditions have been solved using an iterative implicit finite-difference method. Findings – It has been found that using of the presented algebraic transformation of the mesh allows to effectively conduct numerical analysis of turbulent natural convection with thermal surface radiation. It has been shown that the average convective Nusselt number increases with the Rayleigh number and decreases with the surface emissivity, while the average radiative Nusselt number is an increasing function of these key parameters. It has been shown that a presence of surface thermal radiation effect leads to an expansion of the eddy viscosity zones close to the walls. Originality/value – It should be noted that for the first time in this paper we used stream function and vorticity variables with very effective algebraic transformation of the mesh in order to create a non-uniform mesh for an analysis of turbulent flow. Such method allows to reduce the computational time essentially in comparison with using of the primitive variables. The considered method has been successfully validated on the basis of the experimental and numerical data of other authors in case of turbulent natural convection without thermal radiation. The used numerical method would benefit scientists and engineers to become familiar with the analysis of turbulent convective heat and mass transfer, and the way to predict the properties of the turbulent flow in advanced nuclear systems, in industrial sectors including transportation, power generation, chemical sectors, ventilation, air-conditioning, etc.

Hybrid LES/URANS simulation of turbulent natural convection by BEM

In this paper we have developed a hybrid LES/URANS turbulent model for a BEM based turbulent fluid flow solver. We employed the unified LES/URANS approach, where the interface between the LES and URANS regions is defined using a physical quantity, which dynamically changes during numerical simulation. The main characteristic of the unified hybrid model is that only one set of governing equations is used for fluid flow simulation in both the LES and URANS regions. Regions where turbulent kinetic energy is calculated by LES and URANS models are determined using a switching criterion. We used the Reynolds number based on turbulent kinetic energy and the Reynolds number based on total turbulent kinetic energy to establish the LES/URANS interface switching criterion. Depending on flow characteristics and with the use of switching criterion, we chose between sub-grid scale viscosity (SGS) and URANS effective viscosity. The SGS or URANS effective viscosity is used in the transport equation for turbulent kinetic energy and in governing equations for fluid flow. The developed numerical algorithm was tested by simulating turbulent natural convection within a square cavity. The hybrid turbulent model was implemented within a numerical algorithm based on the boundary element method, where single domain and sub-domain approaches are used. The governing equations are written in velocity–vorticity formulation. We used the false transient time scheme for the kinematics equation.

Flow and Heat transfer Characteristics in a Square Cavity at High Rayleigh Number

Numerical simulations are performed to investigate the flow and heat transfer characteristics in a air-filled (Pr = 0.71), two-dimensional (2-D) cavity where the flow is induced by a buoyancy force due to differential heating of vertical walls at high Rayleigh numbers (10^9, 1.58 × 10^9, 10^(9.5), and 10^(10)). The governing equations are discretized spatially into a fourth-order-accurate compact form and integrated temporally by the second-order-accurate alternating direction implicit (ADI) method. Numerical results of instantaneous and time-averaged flow structures are presented. The time-averaged temperature and vertical velocity near the hot wall are used to generate the mean temperature and velocity profiles in the turbulent boundary layer, and to illustrate the required spatial resolution for simulations of turbulent natural convection. A universal structure is found to exist in the mean velocity and temperature turbulent boundary layer profiles for x^+ ＜ 10. This finding is also in good agreement with reported experimental data.

Lattice Boltzmann simulation of turbulent natural convection in tall enclosures

In this paper Lattice Boltzmann simulation of turbulent natural convection with large-eddy simulations (LES) in tall enclosures which is filled by air with Pr=0.71 has been studied. Calculations were performed for high Rayleigh numbers (Ra=107-109) and aspect ratios change between 0.5 to 2 (0.5&lt;AR&lt;2). The present results are validated by finds of an experimental research at Ra=1.58x109. Effects of the aspect ratios in different Rayleigh numbers are displayed on streamlines, isotherm counters, vertical velocity and temperature at the middle of the cavity, local Nusselt number and average Nusselt number. The average Nusselt number increases with the augmentation of Rayleigh numbers. The increment of the aspect ratio causes heat transfer to decline in different Rayleigh numbers.

Evaluation of Turbulent Models for Natural Convection of Compressible Air in a Tall Cavity

Numerical simulation of turbulent natural convection of compressible air in a tall cavity is carried out. In order to evaluate the accuracy of turbulent models, various turbulent models are applied to solve the natural convection in a tall cavity that has different temperatures imposed on two opposing vertical walls. For the large-eddy simulation (LES) model, Smagorinsky subgrid scale (SGS) and dynamic Smagorinsky SGS are also applied to the same cases in order to investigate the differences in temperature and velocity caused by different turbulent models. It is found that the k–ϵ model has a high accuracy of predicting velocity distribution at various sampled lines by comparing with experimental data at Rayleigh number of 2.03 × 1010 and 3.37 × 1010, while the LES model has good performance in predicting temperature distributions.

Lattice Boltzmann simulation of turbulent natural convection in a square cavity using Cu/water nanofluid

In this paper, lattice Boltzmann simulation of turbulent natural convection with large-eddy simulations (LES) in a square cavity which is filled by water/copper nanofluid has been investigated. The present results are validated by experimental data at Ra = 1.58×109. This study is conducted for high Rayleigh numbers (Ra = 107–109) and volume fractions of nanoparticles (0 ≤ Φ ≤ 0.06). In this research, the effects of nanoparticles are displayed on streamlines and isotherms counters, local and average Nusselt numbers. The average Nusselt number is enhanced by the augmentation of nanoparticle volume fraction in the base fluid while the manner has an erratic trend toward different Rayleigh numbers.

Lattice Boltzmann Simulation of Turbulent Natural Convection in Tall Enclosures Using Cu/Water Nanofluid

In this article, Lattice Boltzmann simulation of turbulent natural convection with large-eddy simulations (LES) in a square cavity, which is filled by water/copper nanofluid, has been investigated. The present results are validated by the consequences of an experimental research at Pr = 0.71 and Ra = 1.58 × 109. Calculations are performed for high Rayleigh numbers (Ra = 107–109), low volume fractions of nanoparticles (0 ≤ ϕ ≤ 0.06), and three aspect ratios (A = 0.5, 1, and 2). In this investigation, we present that large-eddy turbulent nanofluid flow is modeled by the Lattice Boltzmann method (LBM) with a clear and simple statement. Effects of nanopartcles are displayed on streamlines, isotherm counters, local Nusselt number, and average Nusselt number. The average Nusselt number enhances with the augmentation of the nanoparticles volume fractions in the base fluid for multifarious aspect ratios and the Rayleigh numbers. Heat transfer declines with the increase in the aspect ratios in the base fluid, but the effects of nanopaticles are dissimilar for various aspect ratios at different Rayleigh numbers.