جستجوی مقالات مرتبط با کلیدواژه « volume of fluid method » در نشریات گروه « مکانیک »

Today, pool boiling heat transfer is used in many industrial equipments and engineering applications. Pool boiling heat transfer compared to single-phase heat transfer has a higher heat transfer capacity due to the use of latent heat evaporation of liquids. Therefore, in this study, the pool boiling heat transfer on a flat vertical plate and plate with triangular and circular grooves have been investigated using numerical simulations. In studies, the effect of adding two types of triangular grooves, two types of circular grooves with different dimensions and four heating surface temperatures has been investigated. In order to simulate the two-phase flow in this study, the volume of fluid method was used and the governing equations were solved using the finite volume method. The obtained results showed that adding triangular and circular grooves on the heating surface will increase the average heat flux. In the triangular grooved surface case, the average heat flux which entered to liquid increased between 128% to 177%. By using circular groove, the average heat flux aguments about 69% to 105%. Also, the water vapor value in the triangular grooved surface case incresead between 160% to 340% and in the circular grooved surface case rises between 101% to 155%.

Two-phase systems are important tools for droplet formation that have received much attention in recent decades due to their vast applications. In the present work, the process of water-in-oil droplet formation, in a coaxial geometry using the fluid volume method, and the impact of effective parameters such as dispersed phase velocity and density and also interfacial tension are investigated. The results are used to produce spherical γ-alumina particles by the oil drop method. In this study, using a laboratory setup, the factors affecting the droplet formation process are investigated. Results are validated against laboratory data. The measurement error is about 5% for droplet size and about 4% for sphericity. Studies show that although the mentioned parameters have a great effect on droplet size and separation time, the dependency of droplets diameter on interfacial tension and dispersed phase density is higher. Increasing the interfacial tension causes increasing droplets size and separation time. Also increasing the density of the dispersed phase reduces the diameter of the droplets and increases separation time. Increasing the velocity also had a small effect, but lead to an increase in size and reduced droplets separation time.

Motion of drops in microchannels is encountered in a wide range of applications, such as encapsulation, DNA analysis, and drug delivery. In this study, the formation of drops in a Y-junction microchannel is numerically investigated. To this approach, the Ansys Fluent software is used. To verify the simulation, results are compared to the previous experimental data. The comparison depicts that the current simulation has a deviation of less than 10% with previous experimental results. Effect of change in the flow rate ratio (0.6≤Q≤25.2) on the flow pattern, size of drops, the distance between them, and the breakup time are studied for the four junction angles of the microchannel (45, 90, 135 and 180 °) in details. Three flow regimes are observed: parallel, squeezing and dripping flow. Results reveal that the junction angle of 135° is the optimum angle for the formation of the smallest drops in the microchannel. It is also observed that, at the optimum angle, increasing the flow rate ratio from 0.6 to 25.2 decreases the size and time of drop formation by factors of 3.83 and 3.33, respectively. However, it has an adverse effect on the distance between the drops which results in 4.33 time larger distance between formed drops.

In the present study, the two-dimensional lock-exchange turbidity current under the influence of wind flow is modeled using open-source software. To solve this, the large eddy simulation method has been used in order to observe turbulent phenomena more accurately. By developing the two-phase solver of the software so that the equations of the volume of fluid method are coupled with the scalar equation of concentration, the three-phase problem is simulated as a phase of a mixture of dense fluid and pure water next to the air phase. The results show that an increase in wind speed reduces the buoyancy force driving the turbidity current and increases the entrainment, which means faster pollution of water areas. This increase in wind speed also increases the wall shear stress, with the difference that the amount of wall shear stress at low wind speeds is not significant. So this prevents a significant change in the deposition behavior of the current. Studying the current's sedimentation behavior, showed that at high wind speeds, the co-current wind flow corresponding to the turbidity current has more harmful effects than the reverse wind flow and its sediment accumulation is getting higher.

In this study, a method for energy harvesting from sloshing of fluids has been proposed. In the first part, voltage and electrical power are measured experimentally. A magnet is floated on the liquid in the coiled container and the system is placed on a shaker table. According to Faraday's law of induction, the movement of the magnet inside the container induces voltage in the coil. The results show that the induced voltage increases with increasing frequency and reaches its maximum value at the natural frequency of the structure and container and decreases again. Also, the inductive voltage has increased with increasing both magnet strength and height of the liquid inside the container. The highest inductive voltage and power output in this study were 850mV and 400µW, respectively. To evaluate the efficiency of the proposed method, the system was installed on a shaper machine and the induced voltage and electrical power were measured. Also, the numerical method is used to simulate and analyze the proposed system. The results show that variations of interface parameters including its pressure, are consistent with experimental data and therefore this method can be used to design and predict the performance of the energy harvester.

Droplet motion in microchannels is observed in versatile industrial and scientific applications, such as biological engineering, drug delivery, and encapsulation. In current investigation, breakup of mother droplet in a T-junction microchannel is simulated using the Gerris open-source software. To validate current simulation, results are appraised by the published literatures. The comparison depicts that the current results are in good agreement with previous studies. The effect of the presence of a semi-circular obstacle on the flow pattern and the breakup time of mother droplet is investigated for different Capillary numbers (0.006 Ca 0.019) and the non-dimensional initial length of droplet (L*=2, 3, and 4). The results reveal that by simple modifications on the symmetry T-junction, the mother droplet splits faster in the comparison of the ordinary symmetry T-junction geometry under the same conditions. For instance, the breakup time of mother droplet at Ca=0.01 for L*=2, 3, and 4 approximately decreases 75%, 45%, and 32% in the presence of obstacle, respectively. Furthermore, the breakup time of the drop decreases by increasing the Capillary number.

This article contains a numerical simulation of polygonal hydraulic jump using the volume-of-fluid (VOF) method. This phenomenon occurs when a circular jet of a high viscous liquid impinges perpendicularly onto a flat surface. In fact, when a liquid jet hits a surface, a circular hydraulic jump appears around the stagnation point. In a fluid with low viscosity (such as water), the shape of this jump is circular and in a high viscosity fluid (e.g., ethylene glycol), a polygonal structure forms. This structure is due to the presence of mechanical waves around the collision area, which is considered in the numerical method. In this paper, the results of the numerical model are validated with available experimental studies for the shape and structure of the generated hydraulic jump and its radius. Finally, based on numerical results, it is observed that a circular hydraulic jump spreads at the beginning, and after its corresponding wave collides with downstream obstacles, a polygonal shape is gradually formed and stabilized. In addition, the streamlines show that the existing of high-speed flows in some points of the solution domain generates corners in the jump shape leading to the formation of a polygonal hydraulic jump.

In the current study impacts of fluid sloshing on the dynamic performance of a partially filled tank vehicle have been investigated. A nonlinear and three-dimensional solver of fluid flow equations were coupled with dynamic equations of a three degrees of freedom moving tank vehicle through an intermediate software to simulate the fluid sloshing behavior inside the tank and the vehicle dynamic performance influenced by liquid sloshing. The fluid sloshing solver is based on corrected Navier-Stokes equations which are included some additional terms owed to taking tank vehicles motions into account. In the present work we used “body weighted method” to consider the effects of accelerating motions of the tank on fluid’s elements. The mentioned method was used to simulate the partially filled container during accelerating horizontal motion which crashed an obstacle after a while from the start point. Computed results were compared to experimental results from literatures to valid the proposed method. The water level evolution on the left wall of the container compared to the experimental one showed a good agreement. Moreover, the two-dimensional rectangular container subjected to periodic external excitation was considered. The pressure history on the tank wall was compared to the measured impact pressure to figure out the ability of the scheme to capture the flow properties to anticipate the exerted forces on the walls. One is observed that there is a good agreement between the computed and measured results. Furthermore, the coupled tank vehicle-fluid simulation has been done during vertical excitations through passing symmetric bumps.

In this paper, flow through a fixed two-dimensional circular cylinder beneath of a free surface at various gap ratios at Reynolds number of 180 and Froude number of 0.2 is numerically investigated. Continuity and momentum equations is resolved with control volume method by commercial software Ansys Fluent 14.00. For the low Froude number case (i.e., gravity-dominated), the results obtained are similar to those for flow past a cylinder close to a no-slip boundary, so that similar vortex shedding mechanism can be observed. As the distance between the surface and the cylinder is reduced, the Strouhal number, as measured from the time varying lift, increases to a maximum at a gap ratio of 0.70. Further gap reduction leads to a rapid decrease in the Strouhal number, with shedding finally ceasing altogether at gap ratios below 0.16. The agreement between the results for a free surface and a no-slip boundary suggests that the mechanism behind the suppression of vortex shedding is common to an adjacent no-slip boundary. In this research, Lift coefficients and velocity contours for various gap ratios has been investigated, also the effect of free surface on flow hydrodynamic for low Froude number s is considered.

Simulation of the flow around propeller is a complex fluid flow problem, especially when the propeller is closed to free surface. In this study, the effect of immersion depth, advance velocity and the ventilation phenomenon on the performance of a B-Wageningen series propeller close to surface of water have been numerically investigated. For this purpose the ANSYS-FLUENT commercial software has been used to solve the viscous, incompressible and two phase flow field. The rotation of the propeller has been implemented using the rotating reference frame model in steady state and the sliding mesh for unsteady flow. For turbulent flow modeling and free surface simulation, the k-ω SST model and the volume of fluid method have been used, respectively. For validation of numerical results due to lack of access to experimental results of propeller close to surface, numerical solution in open water condition have been performed and performance coefficients have been calculated. Comparing the numerical results with the experiment ones, shows good agreement and confirms results of numerical simulation. The results of the numerical solution show that the submergence ratio and ventilation phenomenon affect the performance of propeller so that by reducing submergence ratio from 2.2 to 1.4 in advance ratio J=0.4, ratio of thrust and torsional moment coefficients to open water performance coefficients reduced to 7.7% and 6%, respectively.

In this research two-phase slug flow regime in a T-junction branching divider is examined in two regular and irregular groups. Simulation is accomplished by OpenFOAM™ open source software. Simulation uses single fluid with volume of fluid (VOF) method to follow gas-liquid two-phase flow interface. Constant velocity boundary condition for inlet, constant pressure for outlet boundaries and no slip boundary condition are considered for fixed walls. Since slug flow regimes are one of the most complex two-phase flow regimes which its behavior could result in serious damages to the downward equipments; the present research concentrates on the examination of slug flow behavior in the downstream of the T-junction. This study has concluded that using T junction eliminates flow fluctuation so the pressure and air velocity values decrease. Although the inlet of the vertical branch with cross section of 5×5 cm2 is not fully effective in decreasing upward slugs, but with increasing size of the inlet vertical side-branch from 5×5 cm2 to 10×5 cm2 and 20×5 cm2, pressure value of two-phase flow in the whole duct decreases. The consequences are the slug flow decreases in downstream but the plug flow grows up which means the objectives of the research has been accomplished. To verify the numerical results, comparison was made with the well justified previous works. The agreement was encouraging.

In recent years، many different methods have been developed to simulate free surface flows coupled with moving bodies. In this paper، for simulating ship maneuver، VOF method (CICSAM) and also for coupling velocity field and pressure a fractional step method are used. To increase simulation accuracy، the mesh size should be significantly fine. Unfortunately، fine grid causes increase in simulation time. So، multigrid method was applied for solving the Poisson equation obtained in the Navier-Stokes equations، when using fractional step method. In that case، we generated some coarse grids، using agglomeration method، which needs implemention of a fully unstructured grid. To evaluate the accuracy of the algorithm، some test cases، such as two sided cavity، dam break with obstacle، and barge towing problem، as well as a comparison between V and W cycles in multigrid method have been studied. The results show that implementation of multigrid method can improve the performance of the computer code up to 100%. Moreover، W cycle behaves better than V cycle in most cases.