Showing 4 results for Rahmati
A. R. Rahmati, S. Niazi,
Volume 34, Issue 1 (7-2015)
Abstract
In this study, for the first time, a comparison of single-relaxation-time, multi-relaxation-time and entropic lattice Boltzmann methods on non-uniform meshes is performed and application of these methods for simulation of two-dimensional cavity flows, channel flows and channel flows with sudden expansion is studied in the slip and near transition regimes. In this work, Taylor series expansion and least squares based lattice Boltzmann method is utilized in order to apply the lattice Boltzmann models on non-uniform meshes. A diffuse scattering boundary condition and a combination of bounce-back and specular boundary conditions are employed to obtain the slip at the walls. Besides, the relaxation times of lattice Boltzmann methods are computed in terms of Knudsen number. Different lattice Boltzmann methods are used to simulate lid-driven micro cavity flows and their results are compared with each other and with those obtained in the literature. Then, the best model in accuracy and stability, i.e. multi-relaxation-time lattice Boltzmann method, is applied to simulate the micro channel flow in different Knudsen numbers. Results show that the proposed method on non-uniform meshes is capable of simulating micro flows problems in the slip and the transition regimes.
A. R. Rahmati, R. Hajzaman,
Volume 35, Issue 2 (2-2017)
Abstract
In this study, for the first time natural convection heat transfer of Al2O3-water nanofluid with constant and variable
properties is investigated within square shape porous media using the lattice Boltzmann method. The horizontal walls of the
cavity are insulated, and left and right vertical walls are hot and cold, respectively. The Study have been carried out for Rayleigh
numbers of 103, 104, 105, 106, Darcy numbers of 10-2, 10-4, porosity coefficients of 0.4, 0.6, 0.9 and solid volume fraction of 0, 0.01,
0.02 and 0.03. In order to consider the effect of porous media, Darcy-Forchheimer model is used. The results show that the presence of the porous media decreases the velocity of nanofluid and consequently decreases the strength of the flow. With decreasing Darcy number and porosity coefficient, natural convection heat transfer weakens and the mechanism of natural convection of nano-fluids tends to that of thermal conduction. With increasing Rayleigh number, the strength of flow in cavity and average Nusselt number increases. In all cases studied, increase in volume fraction improves heat transfer. In constant properties model, by increasing solid volume fraction, average Nusselt number increases more than that of variable properties model. The results show that Lattice Boltzmann method has the ability to simulate flow in porous media.
P. Rastegar Rajeouni, A. R. Rahmati,
Volume 40, Issue 1 (9-2021)
Abstract
In order to simulate multiphase flow in the presence of dielectric current using the Lattice Boltzmann Method (LBM), three distribution functions are used, two of which for using the He-Chen-Zhang phase field model and one for solving the potential field. Initially, the ability of the code to apply surface tension was tested using the Laplace law and the drop release test. The results show that the present numerical program is capable of modeling well the regulated surface tension force. Then, the Rayleigh–Taylor instability simulation is used to evaluate the code's ability in applying volume forces. The results by the developed numerical program are in good agreement with the numerical results in the references. In this study, for the first time, the effect of electric field on a droplet immersed in another fluid and the presence of droplet in a porous medium is investigated by LBM. For this purpose, first the droplet motion due to the potential difference in the porous and non-porous media is investigated. After modeling the droplet motion due to the potential difference, two electric fields areapplied to the droplet to reverse the droplet deformation. Through various tests, it is shown that at a given potential difference, the droplet breaks down after much deformation and is divided into smaller droplets. The decomposition of droplets in a pre-mixed emulsion is a common technique in the production of monodisperse droplets. The presence of monodisperse droplets in an emulsion improves the physical properties of polymer science experts.
A. R. Rahmati , E. Kashai,
Volume 40, Issue 2 (1-2022)
Abstract
A two-phase lattice Boltzmann model considering the interaction forces of nanofluid has been developed in this paper. It is applied to investigate the flow and natural convection heat transfer of Al2O3–H2O nanofluid in an enclosure containing internal heat generation. To understand the heat transfer enhancement mechanism of the nanofluid flow from the particle level, the lattice Boltzmann method is used because of its mesoscopic feature and numerical advantages. By using a two-component lattice Boltzmann model, the heat transfer enhancement of the nanofluid is analyzed through incorporating the different forces acting on the nanoparticles and the base fluid . The effects of interaction forces, nanoparticle volume fractions (0.0-0.05), and internal and external Rayleigh numbers (103-106) on the nanoparticle distributions and heat transfer characteristics are investigated. The average Nusselt number increases with the increase of nanoparticle volume fraction and Rayleigh number. We also compared and analyzed adding internal heat generation on the nanoparticles and the base fluid separately, and it was found that by considering heat generation on the base fluid, it mostly affects the temperature field, and by considering that on nanoparticles, it mostly affects the stream field.
