Sharif Digital Repository

In this study, an arbitrary Lagrangian-Eulerian (ALE) approach is incorporated with a mixed finite-volume-element (FVE) method to establish a novel moving boundary method for simulating unsteady incompressible flow on non-stationary meshes. The method collects the advantages of both finite-volume and finite-element (FE) methods as well as the ALE approach in a unified algorithm. In this regard, the convection terms are treated at the cell faces using a physical-influence upwinding scheme, while the diffusion terms are treated using bilinear FE shape functions. On the other hand, the performance of ALE approach is improved by using the Laplace method to improve the hybrid grids, involving... 

In this work, we extend a finite-volume pressure-based incompressible algorithm to solve three-dimensional compressible and incompressible turbulent flow regimes. To achieve a hybrid algorithm capable of solving either compressible or incompressible flows, the mass flux components instead of the primitive velocity components are chosen as the primary dependent variables in a SIMPLE-based algorithm. This choice warrants to reduce the nonlinearities arose in treating the system of conservative equations. The use of a new Favre-averaging like technique plays a key role to render this benefit. The developed formulations indicate that there is less demand to interpolate the fluxes at the cell... 

This paper provides a feasibility study on a low cost system called Hydrokinetic Wells turbine for surface wave energy conversion without using plenum chambers. The elimination of the plenum chamber and its complicated valve systems can reduce the expenses of wave energy conversion up to 23%.The feasibility process were done for high and low frequency conditions. For the low frequency waves, we offer the 300 rpm angular velocity as an optimum selection for further studies. For the high frequency condition a reliable analytical approach based on validated methods was developed. The analytical results indicate that a wells turbine with 60 cm diameter could produce up to 1600 Watts power in... 

In this research, effects of superhydrophobic nano-coating on frictional drag force have been investigated. The result of this study could be considered to be used as a method in applications concerned with fuel consumption reduction, less CO2 emission and environmental problems as well as speed increase; while, its significance can also be of great use in marine applications. A rotating disc apparatus was used as the experimental set-up to compare the frictional drag force on an aluminum disc with TiO2 superhydrophobic nano-coating and a smooth coatless aluminum disc. The superhydrophobic nano-coating was prepared using sol-gel method and was shown to be able to produce a contact angle of... 

The pulsating flow in the exhaust gas of a SI engine causes an unsteady flow at the inlet to the turbocharger turbine. In a four cylinder four stroke engine, the pulse frequency varies between 20 and 200 Hz. Three dimensional pulsating flows in a vane-less turbocharger turbine of a 1.7 liters SI engine are simulated numerically and validated experimentally. Simulations are done for 720 degree engine cycle at three engine speeds. The results are shown the inlet pulsating flow has significant effects on several turbine parameters especially the inlet total pressure, the reduced mass flow rate and the efficiency. The results show a very good agreement between the three-dimensional unsteady... 

The comparison of aerodynamic characteristics of circular and noncircular bodies using computational-fluid-dynamics (CFD) code and semi-emperical code was discussed. It was observed that the performance of aerodynamic coefficients was better for squared section body at different angles of attack. It was also observed from the study of the flow physics that the pressure difference between the front and back of the body that produce pressure drag was more in the circular body. Results show that the friction drag is more for the square body than the circular body as the surface area of the square body is large  

Due to the widespread use of rarefied gas flow in micro-porous media in industrial and engineering problems, a pore-scale modeling of rarefied gas flow through two micro-porous media with fractal geometries is presented, using lattice Boltzmann method. For this purpose, square- and circular-based Sierpinski carpets with fractal geometries are selected due to their inherent behavior for real porous media. Diffusive reflection slip model is used and developed for these porous media through this study. With this respect, the planar Poiseuille flow is selected as a benchmark and validated with the literature. The effect of Knudsen number (Kn) on the permeability is investigated and compared in... 

A series of experiments was performed to investigate the interaction of an under-expanded axisymmetric supersonic jet exhausted from a flat plate with a high subsonic crossflow. The goal was to study the effect of boundary layer thickness (δ) and jet to freestream dynamic pressure ratio (J) on flow field pressure distributions. The resulting measurements upstream of the jet showed that with increasing boundary layer thickness, the magnitude of the pressure coefficient decreases, whereas downstream of the jet, the recovery of the back-pressure moved closer to the nozzle exit. Flow field measurements indicated that with increasing boundary layer thickness, the jet plume dissipation rate... 

Due to the widespread use of rarefied gas flow in micro-porous media in industrial and engineering problems, a pore-scale modeling of rarefied gas flow through two micro-porous media with fractal geometries is presented, using lattice Boltzmann method. For this purpose, square- and circular-based Sierpinski carpets with fractal geometries are selected due to their inherent behavior for real porous media. Diffusive reflection slip model is used and developed for these porous media through this study. With this respect, the planar Poiseuille flow is selected as a benchmark and validated with the literature. The effect of Knudsen number (Kn) on the permeability is investigated and compared in... 

In recent years, application of porous media is highlighted among researchers due to their wide range of usability in micro-scale problems, such as gas reservoirs, micro-filtering, heat exchangers, etc. With this respect, the accurate description of flow behavior using governing equations based on the continuum assumption is not valid since the mean free path is comparable to the characteristics length of the problem. For this purpose, a simple methodology for diffusion reflection boundary condition is developed and validated for two valuable benchmarks, namely micro-channel flow and fractal porous media, where the results were in good agreement with literature. Then, pore-scale simulation... 

This paper reports the results of a study of the transient removal of contaminant particle from enclosures containing an obstacle. We study specially a phenomena occur sometimes called the step effect. This phenomenon may occur if the size of the obstacle is small enough in comparison with the length or height of the enclosure. These results are the basic instruments for finding a model for contaminant particle removal from an enclosure containing an obstacle. A numerical CFD code is developed and validated with different cases, and then proper two- and three-dimensional cases are modeled. The size of the obstacle affect the order of magnitude of the convection-diffusion terms in the... 

This paper reports the results of a study of the transient removal of contaminant particle from enclosures. These results are the basic instruments for finding a model for contaminant particle removal from an enclosure containing an obstacle. A numerical CFD code is developed and validated with different cases, then proper two- and three-dimensional cases are modeled and improvements are done. The improvements are done by proper positioning the inlet/outlet vents. The size and position of the obstacle affect the order of magnitude of the convection-diffusion terms in the Navier-Stokes equations, hence results in different phenomena while removing the particles. One of these phenomena, the... 

This paper seeks to make a study on flow control in two-dimensional square cavities having obstacles on their walls. The goal of using these passive controllers is to enhance mixing in an enclosed space. Lattice Boltzmann method is used to simulate the problem. Results are presented for various Reynolds numbers, 400≤Re≤4000 and different arrangements of tiny-obstacles with different heights. To give a perspective on the physics of this problem, time evolution of the flow is studied at Re= 1000. Then, the flow structure is studied for different Reynolds numbers. Findings show that the interaction of the main vortex with the tiny-obstacles inserted on the wall cavity changes the flow pattern... 

This paper seeks to make a study on flow control in two-dimensional square cavities having obstacles on their walls. The goal of using these passive controllers is to enhance mixing in an enclosed space. Lattice Boltzmann method is used to simulate the problem. Results are presented for various Reynolds numbers, 400≤Re≤4000 and different arrangements of tiny-obstacles with different heights. To give a perspective on the physics of this problem, time evolution of the flow is studied at Re= 1000. Then, the flow structure is studied for different Reynolds numbers. Findings show that the interaction of the main vortex with the tiny-obstacles inserted on the wall cavity changes the flow pattern... 

Attempts to simulate compressible flows with moderate Mach number to relatively high ones using Lattice Boltzmann Method (LBM) have been made by numerous researchers in the recent decade. The stability of the LBM is a challenging problem in the simulation of compressible flows with different types of embedded discontinuities. The present study proposes an approach for simulation of inviscid flows by a compressible LB model in order to enhance the robustness using a combination of Essentially NonOscillatory (ENO) scheme and Shock-Detecting Sensor (SDS) procedure. A sensor is introduced with adjustable parameters which is active near the discontinuities and affects less on smooth regions. The... 

Three-dimensional bubble dynamics in rotating flow under an accelerating field such as a centrifugal one is studied in this work. We employ the lattice Boltzmann method in two phase flows to simulate bubble dynamics for different Bond and Morton numbers of 0.1, 1, 10, and 100 and 0.001, 0.01, 0.1, 1, 10, and 100, respectively. Another dimensionless number named as dimensionless force, F∗, which is the ratio of buoyancy force to centripetal force is defined to explain the dynamics of the bubbles. In this work, we consider 5×10-7≤F∗≤5. The results show that bubbles in rotating flows have different kinds of motions such as spinning, rotating, and translating. Based on the ratios of the forces... 

This paper deals with the investigation of the laminar flow past an elliptical cylinder confined in a channel. In this paper, the Lattice Boltzmann (LB) method is used to simulate flow in two dimensions. The present LB method with the used boundary conditions is validated in simulations of the incompressible flow past a circular cylinder. The simulations are carried out in a range of condition, 0 90 (angle of incidence), 5 Re 100, (Reynolds number) for AR=0.25,0.5 (aspect ratio). The effects of those parameters on the drag and lift coefficients and other flow characteristics of the cylinder are examined in detail. The results demonstrate that the drag and lift coefficients increase with the... 

This paper deals with the investigation of the laminar flow past an elliptical cylinder confined in a channel. In this paper, the Lattice Boltzmann (LB) method is used to simulate flow in two dimensions. The present LB method with the used boundary conditions is validated in simulations of the incompressible flow past a circular cylinder. The simulations are carried out in a range of condition, 0≤θ≤90 (angle of incidence), 5≤Re≤100, (Reynolds number) for AR=0.25,0.5 (aspect ratio). The effects of those parameters on the drag and lift coefficients and other flow characteristics of the cylinder are examined in detail. The results demonstrate that the drag and lift coefficients increase with... 

An attempt was made to introduce a new approach for evaluating mass transfer during drop formation via definition of a parameter related to the extent of the convective mixing within the growing drop. For this purpose it was assumed that the entrance of the dispersed flow into the growing drop from the nozzle is analogous to the entrance of the flow from a smaller channel to a larger one. This transfer mechanism has been dubbed the "flow expansion." A global time-dependent Reynolds number of growing drop (Regd) was defined based on the equivalent diameter of growing drop as a length scale and also on a velocity scale, which is obtained using this flow expansion assumption. The results show... 

Recently, very large eddy simulation approach has attracted a great deal of attention among researchers. This approach can be thought of as an intermediate approach in flow field filtering view point compared with Direct Numerical Simulation and Reynolds-Averaged Navier-Stokes. One famous method to this approach is Partially Averaged Navier-Stokes. Early studies have demonstrated the capability of this technique in flow prediction; however, this method still needs to be evaluated under more flow conditions to ensure its reliable performance. In this study, the performance of PANS k-ω method in the simulation of turbulent flow around a cylinder with square cross section close to a flat...