Sharif Digital Repository

Gas embolotherapy (GE) is a developing medical method which can be utilized either as an autonomous therapeutic method to treat vascularized solid tumors, or it can be combined with other medical procedures—such as high-intensity focused ultrasound—to improve their efficiency. This paper is dedicated to investigating the different parameters which influence bubble lodging inside human vasculature via 2D-modeling of bubble dynamics in arteries’ and arterioles’ bifurcations which are potential sticking positions. Values used in the simulations are in accordance with the non-dimensional physiological numbers. It is found out that inlet pressure plays a decisive role in bubble lodging; the lower... 

The process of fluid flow displacement in porous media has recently gained great prominence owing to its widespread usage in a variety of industries, especially in the case of pore scale investigations. Although, many studies have been conducted to address pore-scale investigations in both modeling and experimental approaches, the role of interfacial tension and contact angle on pore-scale phenomena is less focused. In this work, direct pore-scale modeling was used to precisely examine the effect of interfacial tension and contact angle on the fluid flow at the microscale. Also, several pore-scale mechanisms, including Haines jump and dynamic breakup mechanisms, were observed. Therefore, the... 

The friction on the water–solid interfaces continues to be the most important factor for the energy loss in many marine and submarine applications. Therefore, different techniques have been developed and are available to reduce friction and, as a result, the overall cost. In the past decades, the use of structured surfaces has been given considerable attention because of their specific characteristics such as their abilities in pressure drop reduction. However, an appropriate optimization method is required to find the best surface structure. In the present study, we consider a microgrooved substrate and examine the performance of three shapes including rectangular, elliptical, and... 

As expounded, the precipitation and deposition of asphaltene particles in pipelines has been proved to be the most challenging flow assurance problem due to its unknown and complex behaviors. In this work, a new multicomponent, multiphase and dynamic tool was developed to model the aggregation and deposition of asphaltene particles in a bulk medium. The multiphase and dynamic asphaltene deposition tool, shortened as MAD-ADEPT is, in fact, a modified version of the previously developed ADEPT. The new tool was developed to make the asphaltene deposition and aggregation concepts in oil production wells more predictable. To tackle the complexity of the asphaltene problem, a bespoke algorithm was... 

Numerical simulations are performed to provide an in-depth insight into the effect of instabilities on liquid jets discharging from elliptical orifices. The axis-switching phenomenon and breakup are simulated and characterized under the effect of disturbances imposed at the nozzle exit. The simulations are based on the volume of fluid approach and an adaptive meshing. A range of orifice aspect ratios from 1 to 4 at the Rayleigh breakup regime is considered. The evolution of the jet cross section and axis switching under the influence of disturbances is compared with that of nonperturbed elliptical jets. It is found that the axis-switching repetition and breakup length exponentially decrease... 

The Eulerian methods are susceptible to generate the nonphysical spurious currents in the multiphase flow simulations near the interfaces. This paper presents a new Eulerian method to accurately simulate the velocity fields, especially near the multiphase flow interfaces and prevents the numerical results from generating the nonphysical currents. A Eulerian central difference finite-volume scheme equipped with the suitable numerical dissipation terms is used to simulate incompressible multiphase flows. The interface is captured by Flux Corrected Transport-Volume of Fluid method (FCT-VOF). Increasing the accuracy near the sharp gradients, such as interface, the conservative form of... 

This paper presents a developed CFD (Computational fluid dynamics)-DEM (Discrete elements method) model to study the cuttings transportation in aerated mud drilling process for inclined annuli at downhole conditions. The model is conducted to determine the effects of liquid flow rate, air injection rate, annulus inclination angle, elevated temperature and pressure on the cuttings transport efficiency. The motion of the fluid is computed using CFD based approach with gas–liquid interface capturing provided by the volume-of-fluid (VOF) method. The dynamics of cutting phase is studied by DEM using soft sphere approach in order to take into account the particle collision phenomenon. The... 

Stepped planing hulls enable the feasibility of running at relatively low Drag-Lift ratio by means of achieving more optimal trim angle at high speeds. Currently, there is no precise method to analyze these hulls over the full range of operating speeds. In this study, a three-dimensional computational fluid dynamics (CFD) model using volume of fluid (VOF) approach is presented for examining the characteristics of a planing hull having one transverse step. A procedure is presented to transform a series of fixed-position simulations into a free to heave and pitch simulation. Resistance, lift, running draft, dynamic trim angle, and wetted area are compared with available experimental data and... 

A Volume of Fluid (VOF) method is used to stdy the breakup of droplets in T-junction geometries. Symmetric T-junctions, which are used to produce equal size droplets and have many applications in pharmacy and chemical industries, are considered. Two important factors namely "breakup time" and "breakup length" that can improve the performance of these systems have been introduced. In addition a novel system which consists of an asymmetric T-junction is proposed to produce unequal size droplets. The effects of the channel width ratio and the capillary number on the size and length of the generated droplets and also the time of the generation have been studied and discussed. For simulation the... 

A numerical simulation algorithm based on the finite volume discretisation is presented for analyzing ship motions. The algorithm employs a fractional step method to deal with the coupling between the pressure and velocity fields. The free surface capturing is fulfilled by using a volume of fluid method in which the interface between the liquid (water) and gas (air) phases are computed by solving a scalar transport equation for the volume fraction of the liquid phase. The computed velocity field is employed to evaluate the acting forces and moments on the vessel. Using the strategy of boundary-fitted body-attached mesh and calculating all six degrees-of-freedom of motion in each time step,... 

In this paper, a two dimensional numerical model for two phase flow is presented. For interface tracking, the FGVT-VOF (Fine Grid Volume Tracking-Volume Of Fluid) method is selected. For momentum advection, an improved approach is used. In this scheme, a volume tracking step is coupled with steps of computations for the advection of momentum. A Reynolds stress algebraic equation has been implemented in the algorithm of turbulent modeling. Standard test cases are used for the verification of interface tracking and hydrodynamic modeling in laminar and turbulent conditions. The test results show that this methodology can be used in different applications of two-phase flow modeling. © Sharif... 

In some diseases there is a focal pattern of velocity in regions of bifurcation, and thus the dynamics of bifurcation has been investigated in this work. A computational model of blood flow through branching geometries has been used to investigate the influence of bifurcation on blood flow distribution. The flow analysis applies the time-dependent, three-dimensional, incompressible Navier-Stokes equations for Newtonian fluids. The governing equations of mass and momentum conservation were solved to calculate the pressure and velocity fields. Movement of blood flow from an arteriole to a venule via a capillary has been simulated using the volume of fluid (VOF) method. The proposed simulation... 

In this study, transient 2D/axisymmetric simulations of cavitating flows are performed using a modified 'Volume-of-Fluid' (VOF) technique. Simulation of the cavitation is based on a homogenous equilibrium flow model. To predict the shape of the cavity, the Navier-Stokes equations in addition to an advection equation for the liquid volume fraction are solved. Mass transfer between the phases is treated as a sink term in the VOF equation. The numerical method is used for different geometries in a wide range of cavitation numbers. Computed shapes of cavities were found to be in good agreement with those of the reported experiments. The simulation results also compared well with those obtained... 

In the present study, the presence of a baffle and its effect on the hydrodynamics of the flow in a primary settling tank has been investigated experimentally by ADV (Acoustic Doppler Velocimeter). On the other hand, the characteristics of this flow field were simulated by an unsteady two-phase finite volume method, and VOF (Volume of Fluid) model; and results were evaluated by the experimental data. The numerical calculation performed by using k - ε RNG model agrees well with experiments. It depicts the ability of this method in predicting the velocity profile and flow structure. In addition, the optimum position of the baffle to achieve the best performance of the tank was determined by... 

For the reason that flow expansion model (developed in our previous work) for evaluating mass transfer during droplet formation involves with manifest hydrodynamic aspects, in this research computational simulation of this phenomenon was done for characterization of hydrodynamics effects on the mass transfer during droplet formation. For this purpose, an Eulerian volume tracking computational code based on volume of fluid (VOF) method was developed to solve the transient Navier-Stokes equations for the axisymmetric free-boundary problem of a Newtonian liquid that is dripping vertically and breaking as drops into another immiscible Newtonian fluid. The effects of hydrodynamics effects on the... 

Due to shear layer at the interface of density current and ambient fluid, density current disturbs and entrains the surrounding fluid. Most existing analytical and numerical models for density current flows are based on the equations for single-phase flows. In this research, the density current has been modeled with two-phase flow model. The governing equations are continuity, x- momentum, and y- momentum equations for every fluid. The volume-of-fluid (VOF) interface tracking technique which uses a piecewise-linear interface calculation (PLIC) in each cell is used to determine the deformation of free surface in density current, numerically. Surface tension is implemented by the continuous... 

In this study, a new model was developed to calculate gas pressure at the melt/foam interface due to foam degradation during mould filling in the lost foam casting process. Different aspects of the process, such as foam degradation, gas elimination, and permeability of the refractory coating, were incorporated into this model. A computational fluid dynamic code was developed based on the numerical technique of the solution algorithm-volume of fluid (SOLA-VOF) for the simulation and prediction of the melt flow. In order to verify the computational results of the simulation, a thin plate of grey iron at 1350 °C was poured into a transparent foam mould. The mould filling process was recorded... 

Heap leaching is a process extensively used by the mining industry to recover metals from low-grade ores and large quantities of submarginal material resources. Understanding flow through a packed particle bed is important to enhance the performance of heap leaching with respect to design and operating considerations. Complex fluid behavior in porous media, such as film flow, fragmentation, coalescence of droplets, and rivulet flow with or without meandering, has been widely observed in laboratory experiments. In this study, to provide detailed information of momentum and mass transfer phenomena in a granular bed direct numerical simulations (DNS) were performed. In this case the liquid-gas... 

Numerical simulation of floating or submerged body motions is presented based on a Volume of Fluid (VoF)-fractional step coupling. Solving a scalar transport equation for volume fraction of two phases results in a single continuum with a fluid property jump at the interface. In addition, velocity and pressure fields are coupled using the fractional step method. Based on integration of stresses over a body, acting forces and moments are calculated. Using the strategy of non-orthogonal body-attached mesh and calculation of motions in each time step result in time history of hydrodynamic motions. To verify the accuracy of the numerical procedure in simulation of two-phase flow, sloshing and dam...