Numerical Simulation Of Transient Flow Research Articles

Numerical Simulation of Transient Flow and Heat Transfer in a Steel Ladle During Holding Period

Numerical Simulation of Transient Flow and Heat Transfer in a Steel Ladle During Holding Period

Numerical analysis of transient cavitating pipe flow by Quasi 2D and 1D models

Quasi two dimensional (2D) and one dimensional (1D) numerical simulations of transient flow with column separation are conducted using the discrete vaporous and gaseous cavitation models (DVCM and DGCM) in a simple reservoir-pipeline-valve system. In the quasi 2D transient model, the governing equations are solved by the method of characteristics along the pipe and by finite difference across the pipe cross section to consider the velocity profile, and the final model is coupled with the cavitating models. The comparison between the numerical and experimental results shows that the quasi 2D DGCM is more successful in predicting the maximum head pressures and reproducing the pressure spikes at different water hammer cycles. The quasi 2D model of DGCM correlates better with the experimental data than the quasi 2D DVCM, 1D DVCM and 1D DGCM in terms of pressure magnitude. In sensitivity analysis, the 2D DGCM shows more grid convergence than the 1D models of DGCM and DVCM and 2D DVCM by increasing the number of longitudinal and radial intervals.

Numerical simulation of transient flow with column separation using the lattice Boltzmann method

This study proposes a numerical solution to simulate the transient flow with column separation in pipelines using the lattice Boltzmann method (LBM). By combining the LBM and the discrete vapour cavity model (DVCM), the governing equations and boundary conditions were analysed and derived for numerical simulation. Experiments and actual projects were adopted to verify and examine the new solution. The results indicate that the computational grid of the LBM-DVCM is unrelated to the wave speed, which significantly reduces computational resources and time. Moreover, compared to the method of characteristics (MOC)-DVCM, the LBM-DVCM eliminates virtual pressure peaks during the transient flow. Finally, the LBM-DVCM can simulate the process of transient flow quickly and accurately. It is expected that this method will be useful for practical numerical experimentation and efficient prediction of the pipeline.

Numerical simulation of transient flow with column separation using the Lattice Boltzmann method

Numerical simulation of transient flow with column separation using the Lattice Boltzmann method

Numerical simulation of transient flow in a shaft extension tubular pump unit during runaway process caused by power failure

To explore the load impact and instantaneous flow characteristics of a tubular pump under unconventional operating conditions, the runaway condition caused by the unit power failure are investigated. Unsteady three-dimensional (3D) numerical simulation and model test were executed on whole flow system of the pump, where a 3D VOF method was specifically adopted to simulate water surfaces of the upstream and downstream reservoirs. The results of numerical simulation in terms of system performance parameters and runaway speed presented a quite good agreement with the experimental data. During the transient process, both the rotational speed and flow rate of the pump unit were found to rapidly decrease with time until maximum runaway speed (1.51n0) was reached, where however, a time lag of 0.7s between the rotational speed and flow rate has been noticed. The explored increase of pressure pulsations is generally believed to have taken source from incurred water shock waves, where the main pulsation frequencies throughout the whole flow passage were the blade passing frequency (BPF) and its harmonics. Large flow vortical structures immerged within the rear guide vanes when the flow rate decreased to zero, while the same flow vortices appeared within the front guide vanes when the impeller rotational speed gradually fell to zero. This study’s results provide a meaningful reference about pump transient operations, leading to the prevention of associated structural vibrations and possible blade cracks, for safe operations of the pumping stations.

Estimation of Conservative Contaminant Travel Time through Vadose Zone Based on Transient and Steady Flow Approaches

Estimation of contaminant travel time through the vadose zone is needed for assessing groundwater vulnerability to pollution, planning monitoring and remediation activities or predicting the effect of land use change or climate change on groundwater quality. The travel time can be obtained from numerical simulations of transient flow and transport in the unsaturated soil profile, which typically require a large amount of data and considerable computational effort. Alternatively, one can use simpler analytical methods based on the assumptions of steady water flow and purely advective transport. In this study, we compared travel times obtained with transient and steady-state approaches for several scenarios. Transient simulations were carried out using the HYDRUS-1D computer program for two types of homogeneous soil profiles (sand and clay loam), two types of land cover (bare soil and grass) and two values of dispersion constant. It was shown that the presence of root zone and the dispersion constant significantly affect the results. We also computed the travel times using six simplified methods proposed in the literature. None of these methods was in good agreement with transient simulations for all scenarios and the discrepancies were particularly large for the case of clay loam with grass cover.

Métodos WTF e simulação numérica de fluxo para estimativa de recarga – exemplo Aquífero Rio Claro em Paulínia/SP

A estimativa do volume de recarga dos aquíferos é fundamental para o gerenciamento de recursos hídricos de subsuperfície. Parte da área de ocorrência do aquífero Rio Claro no munícipio de Paulínia/SP é marcada pela presença de hidrocarbonetos na fase residual e influência de poços de bombeamento dos sistemas de remediação em operação, além de incertezas associadas a heterogeneidades geológicas peculiares a essa unidade hidrogeológica. Esses fatores dificultam estimativas seguras de recarga. Para contornar o problema, foram realizadas simulações numéricas de fluxo, em regime transiente, utilizando dados de monitoramento levantados ao longo de 7 anos, para estimar a recarga do Aquífero Rio Claro nessa área. As variações de carga hidráulica da simulação são similares às variações do nível d’água em todos os poços monitorados, indicando que o modelo foi satisfatoriamente calibrado. A comparação entre as estimativas de recarga pelo método Water Table Fluctuation e os valores obtidos pelos modelos matemáticos revelam elevada discrepância entre ambos. Tal discrepância está associada à diferença nos valores do parâmetro Sy (produtividade específica) empregados nos diferentes métodos. Os valores de Sy podem promover taxas de recarga do aquífero superestimadas por não contemplar o efeito da histerese durante a flutuação do nível d’água.

Temporal acceleration of a turbulent channel flow

We report new laboratory experiments of a flow accelerating from an initially turbulent state following the opening of a valve, together with large eddy simulations of the experiments and extended Stokes first problem solutions for the early stages of the flow. The results show that the transient flow closely resembles an accelerating laminar flow superimposed on the original steady turbulent flow. The primary consequence of the acceleration is the temporal growth of a boundary layer from the wall, gradually leading to a strong instability causing transition. This extends the findings of previous direct numerical simulations of transient flow following a near-step increase in flow rate. In this interpretation, the initial turbulence is not the primary characteristic of the resulting transient flow, but can be regarded as noise, the evolution of which is strongly influenced by the development of the boundary layer. We observe the spontaneous appearance of turbulent spots and discontinuities in the velocity signals in time and space, revealing rich detail of the transition process, including a striking contrast between streamwise and wall-normal fluctuating velocities.

Numerical simulation of transient flow in horizontal drainage systems

A numerical simulation model based on the characteristic-based finite-difference method with a time-line interpolation scheme was developed for predicting transient free surface flow in horizontal drainage systems. The fundamental accuracy of the numerical model was first clarified by comparison with the experimental results for a single drainage pipe. Boundary conditions for junctions and bends, which are often encountered in drainage systems, were studied both experimentally and numerically. The numerical model was applied to an actual drainage system. Comparison with a full-scale model experiment indicates that the model can be used to accurately predict flow characteristics in actual drainage networks.

Numerical Simulation of Transient Flows around a 3D Pitching Hydrofoil

The objective of this paper is to investigate the hydrodynamic characteristics of the transient flows around a 3D pitching hydrofoil via numerical studies, where the effects of tunnel wall boundary layer and gap flows are considered. Simulations are performed using an unsteady Reynolds Average Navier-Stokes solver and the k-ω SST turbulence model, coupled with a two-equation γ-Reθ transition model. Hydrodynamic forces and flow structures are compared to the results with the equivalent 2D computations. During the upward pitching stage, the transition phenomenon is accurately captured by both the 2D and 3D simulations. The slightly lower lift and suction side loading coefficients predicted by the 3D simulation are due to the pressure effects caused by the tip gap flow. During the dynamic stall stage, the 2D case exhibits a clear overshoot on the hydrodynamic force coefficients and the 3D simulation results better agree with the experimental results. During the downward pitching stage, the flow transitions back to laminar. As for the effect of gap flow and the wall boundary condition, the gap flow causes disturbances to the formation and development of the vortex structures, resulting in the complex distribution of the three-dimensional streamlines and the particle path.

A Study of the Time Constant in Unsteady Porous Media Flow Using Direct Numerical Simulation

We consider unsteady flow in porous media and focus on the behavior of the coefficients in the unsteady form of Darcy’s equation. It can be obtained by consistent volume-averaging of the Navier–Stokes equations together with a closure for the interaction term. Two different closures can be found in the literature, a steady-state closure and a virtual mass approach taking unsteady effects into account. We contrast these approaches with an unsteady form of Darcy’s equation derived by volume-averaging the equation for the kinetic energy. A series of direct numerical simulations of transient flow in the pore space of porous media with various complexities are used to assess the applicability of the unsteady form of Darcy’s equation with constant coefficients. The results imply that velocity profile shapes change during flow acceleration. Nevertheless, we demonstrate that the new kinetic energy approach shows perfect agreement for transient flow in porous media. The time scale predicted by this approach represents the ratio between the integrated kinetic energy in the pore space and that of the intrinsic velocity. It can be significantly larger than that obtained by volume-averaging the Navier–Stokes equation using the steady-state closure for the flow resistance term.

Numerical Simulation of Transient Flow in Products Pipeline

In this paper, explicit difference scheme, implicit difference scheme and characteristics method are separately used to simulate the transient flow in products pipeline. The simulation result can be used to prevent water hammer in the pipeline of unsteady situation and to improve the efficiency and safety in oil transmission systems. And then, the stability and accuracy of the three methods are compared by adopting different time steps. For explicit difference method, large fluctuation may occur in case of large time step. For implicit method, the result is weakly affected by time step, only if the relaxation factor selected is reasonable. For characteristics method, the results have a high convergence speed and precision. The results show that, in the situation of valve shut down in terminal, it takes about 1.1×104 seconds to return to a new steady state.

Numerical Simulation of Transient Flow in a Gas Pipeline and Tank

Summary Gas transient flow in a gas pipeline and gas tank is critical in flow assurance. Not only does leak detection require a delicate model to simulate the complicated yet dramatically changed phenomena, but gas pipeline and gas tank design in metering, gathering, and transportation systems demands an accurate analysis of gas-transient flow, through which efficient, cost-effective operation can be achieved. Traditionally, there are two types of approaches used to investigate gas-transient flow: one involves treating gas as ideal gas so that the ideal gas law can be applied and the other considers gas as real gas, allowing the gas compressibility factor to come into play. Needless to say, the former method can result in an analytical solution to gas transient flow with a deviation from the real-gas performance, which is very crucial in daily operation. The latter approach requires a numerical method to solve the governing equation, leading to instability issues with a more-accurate result. Our literature review indicated that no study considering the effect of changing gas viscosity on the transient flow was available; therefore, this effect was included in our study. Our investigation showed that viscosity does have a significant influence on gas-transient flow in pipe- and tank-leakage evaluation. In this study, a comprehensive evaluation of all variables was performed to determine the most-important factors in the gas-transient flow. Several case studies were used to illustrate the significance of this study. Engineers can perform a more-reliable evaluation of gas transient flow by following the method we used in our study.

Numerical simulation of transient flows in a vacuum ejector-diffuser system

The objective of the present study is to analyse the transient flow through the vacuum ejector system with the help of a computational fluid dynamics method. An attempt is made to investigate the interesting and conflicting phenomenon of the continuous entrainment into the primary stream with limited mass supply from the secondary chamber. The results obtained show that the one and only condition in which a continuous mass entrainment can be possible in such types of ejectors is the generation of a recirculation zone near the primary nozzle exit. The flow in the secondary chamber attains a state of dynamic equilibrium of pressure at the onset of the recirculation zone. A steady flow assumption in such ejector systems is valid only after the dynamic equilibrium state.

Numerical simulation of transient flows in a rocket propulsion nozzle

A numerical investigation of transient flows in an axisymmetric over-expanded thrust-optimized contour nozzle is presented. These nozzles experience side-loads during start-up and shut-down operations, because of the flow separation at nozzle walls. Two types of flow separations such as free shock separation (FSS) and restricted shock separation (RSS) shock structure occur. A two-dimensional axisymmetric numerical simulation has been carried for a thrust-optimized contour nozzle to validate present results and investigate oscillatory flow characteristics during the start-up processes. Reynolds-Averaged Navier–Stokes equations are numerically solved using a fully implicit finite volume scheme. Governing equations are solved by coupled implicit scheme. The present work is concerned with comprehensive assessment of the flow features by using Reynolds stress turbulence model. Computed pressure at the nozzle wall closely matched with the experimental data. A hysteresis phenomenon has been observed between these two shock structures. The transition from FSS to RSS pattern during start-up process has shown maximum nozzle wall pressure. Nozzle wall pressure and shear stress values have shown fluctuations during the FSS to RSS transition. The oscillatory pressure has been observed on the nozzle wall for high pressure ratio. Present results have shown that magnitude of the nozzle wall pressure variation is high for the oscillatory phenomenon.

Numerical simulation of transient flow around a ship in unsteady berthing motion

Ship berthing is a specific maneuver operation. The flow around a berthing ship and the forces acting on the hull are quite different from those for a ship in normal navigation. By solving the unsteady Reynolds-Averaged Navier-Stokes (RANS) equations, the transient flow field around a ship undergoing unsteady lateral motion is simulated and the varying lateral hydrodynamic force acting on the hull is evaluated in this article. The numerical results obtained with different turbulence models are analyzed and compared with experimental results and other numerical results published in literature, and a turbulence model more suitable for simulation of the viscous flow around a ship undergoing unsteady berthing is determined.

Numerical Simulation of Transient Flows in Viscoelastic Pipes with Vapour Cavitation

A mathematical model has been developed to calculate transient flows in viscoelastic pipes with vapour cavitation. This model takes into account both the viscoelastic behaviour of the pipe wall and the vapour cavitation phenomenon. When the liquid pressure falls below the vapour pressure, a bubbly cavitating flow of homogeneous liquid-vapour mixture occurs in some parts of the pipeline. The mixture density is expressed by means of a non-linear expression of the liquid volume fraction. The pipe wall behaviour is simulated based on the mechanical principle, by introducing additional viscoelastic term into the mass balance fluid equation. The application of mass and momentum conservation laws yields to a system of hyperbolic partial differential equations. The later is resolved by a second order finite differences scheme. The proposed model is tested for transient flows with and without vapour cavitation. The obtained numerical results are discussed and compared with experimental data.

Numerical simulation of transient flow of two immiscible liquids in pipeline

AbstractA two‐fluid model for transient flow of two immiscible liquids in a pipeline was formulated based on two transient continuity equations and a combined momentum equation in a quasi‐steady form. It can be used for simulating both stratified and dispersed flow. Numerical simulations were performed to investigate the behavior of oil–water flow during transients caused by an increase or decrease in the water volume fraction at the entrance to the pipeline. An increase in the water flow rate within the stratified flow regime may result in the formation of a shock (an abrupt change in the water holdup). In transients produced by a decrease in the input water volume fraction, the smoothening of the water holdup wavefront occurs. This behavior is explained by a strong dependence of the speed of the holdup wave propagation in stratified flow on the water holdup. The results obtained for inclined pipelines show that the slope of the pipe affects the flow behavior significantly. The liquid–liquid flow model was used to simulate the transport of corrosion inhibitor in a pipeline conveying an oil–water mixture.

Numerical Simulation of Transient Flow around a Typical Hydrofoil.

When hydraulic machinery start to operate, the flow around the blade elements has to be typically transient. To clarify such flow behavior, we attempt numerical simulations of the viscous transient flow around a typical hydrofoil, by using the QUICK method of high resolution accuracy. For such a viscous flow, our simulations show not only the step response of expectable through potential concept, but also the rather slow viscous response, which is almost linear to the elapse time t.

Numerical simulation of transient hypervelocity flow in an expansion tube

Several numerical simulations of the transient flow of helium is an expansion tube are presented in an effort to identify some of the basic mechanisms which cause the noisy test flows seen in experiments. The calculations were performed with an axisymmetric Navier-Stokes code based on a finite-volume formulation and upwinding techniques. Although laminar flow and ideal bursting of the diaphragms was assumed, the simulations showed some of the important features seen in the experiments. In particular, the discontinuity in the tube diameter at the primary-diaphragm station introduced a transverse pertubation to the expanding driver gas and this perturbation was seen to propagate into the test gas under some flow conditions. The disturbances seen in the test flow can be characterized as either small-amplitude, low-frequency noise, possibly introduced during shock-compression, or large-amplitude, high frequency noise, associated with the passage of the reflected-head of the unsteady expansion.