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The trapped air in the water conveyance systems may cause the obstruction of flow, corrosion and adverse effect on the performance of valves and pumps. This study is part of the Abbas Abad Dam Drinking Water Transmission Line using Ansys Fluent Software, The behavior of air entrapped during the filling and discharging process and the effect of its existence on pressure changes were studied. Entrapped air was simulated at 0.5, 2, 3 m lengths and inlet pressures of 1.2456, 1.7456 and 2.2456 atmospheres for a 35-meter-long metal pipe with a discharge valve embedded in its outlet end. The effects of pressure-time changes were investigated in two ways: a) pressure changes in the pipe and drain valve and b) pressure changes inside the entrapped air. The results showed that the pressure changes in both cases were due to the large fluctuations in confined air and that the fluctuations continued until the drain valve was fully opened and the longer the confined air length was, the greater the amplitude and length of the created fluctuations. Modeling with similar boundary conditions without the presence of air indicates no pressure fluctuations. The results of the verification confirm the research results. Besides the Palau et al, 2019 results, calculation of relative error, RMSE and NRMSE were used in order to validate the numerical model results.

One of the most important factors in pipeline hydraulics and its protection against probable damages is keeping the pressure in a specific limit and also knowing how the pressure functions in the pipe through the pipeline route. Pressure function and pressure amount differ from each other regarding the phase of the fluid (single phase or multiple phase), and when gas phase (air) enters the pipeline, it affects the flow regime (simultaneously when gas-liquid flows there are such regimes as stratified, slug, etc.) and the applied pressure. The present study aims to investigate the effects of flow patterns and phases on maximum pressures along pipelines and to compare it with those of single-phase flows. Results show that the geometry and the percentage of entrained air are the main factors affecting flow patterns and pressure experienced by pipeline systems. For single phase mode, pressure is increased up to 26% in vertical upward flows compared to air-water flows with a different air percentage. Moreover, among various geometries considered in this research for two-phase flows, pressure at most has a 23- percent increase compared to that of single-phase flows.

In the current study, two-phase flow of water and air over a stepped spillway is probed in the form of a two-dimensional incompressible viscous flow. A novel numerical approach is used for the numerical simulation which is a combination of two models: volume of fluid (VOF) which uses an interface tracking algorithm for the simulation of the two-phase flow and two-fluid model which is based on time and space averaged equations and cannot track the interface explicitly. The most important issue in the introduced approach is to couple the two basic methods and select a proper criterion for status change between two basic methods. The latter criterion is based on an approximation from local distribution of the interface at each cell. In the hybrid method. In order to investigate the aeration effect in the stepped spillway, the air suction is generated by designing some holes at the upper edge of the steps and considering atmosphere pressure for these areas. The obtained results divulge the amount of dispersion is low at the beginning part of the step and also the hybrid model take more advantages from VOF, while in the lower steps where the flow disperses two-fluid model has hegemony. The results are compared in the form of pressure contours and streamlines as well as volume fraction counters. The comparison shows that the results of the proposed method is closer to the experimental results with respect to each of the basic model. [ABSTRACT FROM AUTHOR]

In this paper, the three dimensional ventilated cavitating flow in the steady condition around a projectile model is simulated using CFD method combined with a sst k-ω turbulence model and volume-of-fluid technique, With the aid of CFD software ANSYS CFX. The numerical model is validated using comparisons between numerical predictions and existing experimental data and fairly good agreement is revealed. The numerical results show that with increasing the ventilation gas rate at constant Froude number, the cavity length gradually increases to a critical value and then remains fixed upon further increase in gas ventilation rate. Also, it has been observed that rear portion of larger cavity moves upwards due to gravitational effect. With increasing the ventilation gas rate, the gas leakage mechanism at rear portion of ventilated supercavity changes from the re-entrant jet closure mode to twin vortex closure mode. The variation of ventilation gas rate versus cavity length is a function of Froude number and the critical ventilation gas rate increases linearly with Froude number. [ABSTRACT FROM AUTHOR]

A spillway is a hydraulic structure that is provided at storage and detention dams to release surplus or flood water that cannot be safely stored in the reservoir. In this paper, two-dimensional simulation of gas-liquid two-phase flow on stepped spillway in different discharge rate is studied. The VOF model and the two-fluid model are used in order to simulate numerically and then the results of the two models are compared. In order to study the influence of aeration in stepped spillway, two different physics are proposed. In the first geometry it is assumed that there is no air intake via stairs and in second geometry air intake and its effect on the flow over the spillway is studied by embedding hole in the top edge of stair. The air pressure is assumed to be atmospheric. The results showed that VOF model provide more accurate result than that of two-fluid model in low discharge rate. However, in cases where aeration is studied, because of mixing phases, this model is not able to simulate fluid flow as well as two-fluid model. The two-fluid model is more accurate due to solving equations for both phases (air and water). For verification, numerical results have been compared with experimental values and determined that numerical models are able to predict the total energy loss within an error range of %10 compared with the measured experimental data. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study using two-fluid model in order to compare the effect of hydrodynamic and hydrostatic models for pressure correction term in two-fluid model in modeling gas-liquid two-phase flows to provide a more accurate model. Two-fluid model is solved by Godunov Approximate Riemann Solver. The two-fluid model is applied using both hydrodynamic pressure correction term and hydrostatic pressure correction term for four sample examples including Water Faucet Case, Water-Air Separation Case, Toumi's Shock Tube Case, and Large Relative Velocity Shock Tube Case. Hydrostatic pressure correction term is neglected for vertical geometry, therefore, in this geometry; two-fluid model cannot be hyperbolic. Thus, hydrostatic pressure correction term is not a stabilizing term. Also, in horizontal pipe and for atmospheric conditions, hydrostatic pressure correction term presents better results than hydrodynamic pressure correction term. But, in non-atmospheric conditions, hydrodynamic pressure correction term presents better results. Therefore, in order to select a suitable pressure correction term for two-fluid model, we consider geometry (vertical or horizontal) and flow conditions (atmospheric or under-pressure). Also, hydrodynamic pressure correction term in two-fluid equations system is hyperbolic in a boarder range than hydrostatic pressure correction term. [ABSTRACT FROM AUTHOR]

In this study, falling ferrofluid droplet behavior in nonmagnetic viscous fluid under the uniform magnetic field in two-phase flow is studied numerically. To this approach, a hybrid lattice-Boltzmann base shan-chen model and finite-volume method is used. The lattice Boltzmann equation with the magnetic force term is solved to update the flow field while the magnetic induction equation is solved using the finite volume method to calculate the magnetic field. To validate the flow field solution two tests have been considered: the free bubble rising and Laplace law. In order to validate the magnetic field, permeable circle and deformation of static drop under magnetic field is simulated. The comparison of results between present study and previous researches shows that there is a good agreement between the results. The effects of the magnetic Bond number, susceptibility and magnetic field direction on deformation of the falling droplet are investigated. The results show that increase in the magnetic Bond number or susceptibility leads to a larger deformation of the droplet. Also in horizontal magnetic field, the falling process takes more time in compared to the vertical magnetic field. [ABSTRACT FROM AUTHOR]

In the current study, the streamline simulation technique is used for definition of a new objective function to optimize the production rates during water injection process. The streamline simulation technique, in comparison with common numerical methods for simulation of multi-phase flow in porous media, is much faster with less computational memory requirement. This method represents the key parameter of "Time of Flight" which helps to consider complex heterogeneity of porous media in an easier way. In order to optimize oil production rates from reservoir, a function based on averaged time of flight has been introduced in which minimization of this function can be used to have uniform fronts of water for flooding of oil. For this target, two optimization techniques, the Artificial Bee Colony (ABC) and the Sequential Quadratic Programming (SQP) method are employed to optimize the objective function and their results are compared. Advantages and disadvantages of these two methods are investigated and based on their advantages, a new hybrid approach is proposed which utilizes the benefits of both techniques to converge to the optimum solution. In the hybrid approach the SQP algorithm is initialized with the ABC method. In order to validate the mathematical model, a 2D homogeneous model is used for optimization. Next a 2D heterogeneous model and a 3D complex reservoir model are investigated. In all the mentioned problems, it is observed that the hybrid approach, in comparision with the two other methods, can approximate the optimum point with approximately 15% higher speed. [ABSTRACT FROM AUTHOR]

The prediction of distillation zone is very important in steam turbine blades and steam nozzles. In identification of distillery with equilibrium method, as the steam flow contacts the two-phase dome, the second phase forms and flow properties will pass the distillery without any jumping. Hence, after crossing the saturation curve the droplet formation transpires in non-equilibrium method by sudden increase in pressure called "condensation shock". A discontinuity in the flow characteristics is observed and after crossing the saturation curve, the formation of droplets start. In this paper the numerical analysis of vapor-liquid two-phase transonic flow in convergent-divergent nozzle with and without shock is investigated. The effects of stagnation temperature at the nozzle inlet, viscosity and geometry are studied using thermodynamic equilibrium and non-equilibrium methods by comparing the results with experimental data. Roe numerical method is used for vapor-liquid two-phase flow solution. The main properties of the flow at the boundary of elements is extrapolated by MUSCL, which is third order accurate method. The time is discretized using an explicit second order accurate of two-step Laxendroff method. It is observed that the results of non-equilibrium solution are more correlated to the experimental results and condensation starts earlier in the nozzle with further expansion rate. By increasing the temperature at the nozzle inlet, the placement at which condensation starts goes forward. Also the shock location in viscous flow comes closer to the throat in comparing with that of non-viscous flow. [ABSTRACT FROM AUTHOR]

In present paper the Inamuro Model based on free energy approach of the Lattice Boltzmann Method was used to simulate the motion of bubble and coalescence of two bubbles under buoyancy force. By combining the Tanaka and Inamuro models, three-dimensional model of Inamuro was used in two-dimension for decreasing the computational cost. Firstly it was ensured that the surface tension effect and Laplace low for two density ratio 50 and 1000 were properly implemented. Secondly in next step, effect of governing dimensionless numbers problem such as Etvos number and Morton number on Reynolds number and terminal shape of bubble were investigated. Different flow patterns in various dimensionless numbers were obtained and by changing the dimensionless number, terminal change of bubble's shape was seen. Finally, motion of two bubbles and terminal shape of coalescence of two bubbles were studied in different dimensionless number which shape of first bubble was same to single bubble but second bubble experienced various shapes due to its location in wake of first bubble and less difference pressure on two sides of this bubble. [ABSTRACT FROM AUTHOR]