•A improved VOF interface-sharpening method is developed on the general curvilinear grid for two-phase flows.•The dual-time, preconditioning algorithm was employed to solve the Navier–Stokes ...equations.•An evaluation of mass conservation of the present method is performed.•Validated computations for free surface flows are presented.
A three-dimensional volume-of-fluid (VOF) interface-sharpening method is developed on the general curvilinear grid for two-phase incompressible flows. In this method, a VOF discretization scheme is formulated for the advection of a two-fluid interface. To maintain interface sharpness, a treatment is applied by solving an interface-sharpening equation after each advection time step, thereby reducing the numerical diffusion error in the solution of the discretization scheme. To demonstrate the accuracy and capability of the advection scheme, several numerical experiments involving three benchmark tests of pure advection were conducted. The results show that the method can realize a sharp interface reliably and efficiently, and reasonable mass conservation is obtained. For the flow field of viscous incompressible flows, the Navier–Stokes equations are solved by adopting the dual-time preconditioning method. A fully implicit method with a highly efficient lower-upper symmetrical Gauss–Seidel (LU-SGS) algorithm based on a dual-time stepping technique as a sub-iteration scheme is employed to advance the solution in time. To validate the proposed method for computing incompressible free surface flows, a dam-break flow over a dry horizontal bed and the water entry of a hemisphere with one degree of freedom are simulated. Comparisons of the predicted results with the available experimental data are presented herein.
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•Strong nonlinear interactions between one and two underwater explosion bubbles with a free surface are numerically studied.•Numerical method is based on axisymmetrical fully compressible three-phase ...homogeneous model coupled with two interface advection equations.•Numerical and experimental data agree well regarding the shape of bubbles and free surface.•Detailed pressure and velocity contours during the interaction process are obtained and can better reveal the mechanism underlying the dynamics between the bubbles and free surface.•Motion characteristics of the free surface and bubbles are investigated under different non-dimensional standoff parameters.
In this study, strong nonlinear interactions of one and two underwater explosion bubbles with a free surface are numerically studied using an axisymmetrical fully compressible three-phase homogeneous model. The bubble interfaces and free surface are captured by solving two interface advection equations of vapor and air phases. The numerical results are validated via comparisons with the experimental results. The single-bubble case is validated first, and good quantitative and qualitative agreements are achieved. Special features of the interactions, such as jet impact, toroidal bubble, and primary and secondary water spike, are observed in our numerical model. The detailed pressure and velocity contours during the interaction process are obtained and can better reveal the mechanism underlying the bubbles and free surface dynamics. Afterward, the characteristics of the motion of the free surface and the bubbles are investigated considering different non-dimensional standoff parameters. Four distinct patterns of free surface motion are identified. Next, strong interactions between two bubbles and a free surface are simulated. The essential physical phenomena, such as coalesced bubble, annular residual, and bubble splitting, are reproduced well in the representative experiments. Finally, a parametric study reveals the dependence of the center water spike on the interaction between the bubbles and the free surface.
Evolution of strong nonlinear interaction between a spark-genarated underwater explosion bubble with a free surface. Special characteristic motions including jet impact, toroidal bubble, center and secondary free surface spike are well captured with experimental images. Display omitted
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•A recently developed two-phase flow model was enhanced and extended for the simulation of ricochet and penetration of water entry bodies.•A coupling algorithm for the fluid-body interaction and ...moving overlapping grid modeling strategy is integrated into the flow solver.•Grid-independent studies of water entry problems of an inclined wedge and two cylinders are addressed.•Investigation of the ricochet of a circular cylinder off the water surface at various initial entry velocities and angles was performed.
In this paper, the enhancement and application of the recently developed two-phase flow model for the simulation of ricochet and penetration of water entry bodies are presented. The simple two-phase three-equation model, which is a reduced version of the full two-fluid model for compressible multiphase flows, is extendedly applied. The numerical procedure is advanced by implementing the scheme on moving overset body-fitted grids to facilitate flow simulation of complex geometries and arbitrary motions of objects and improve computational productivity by employing their flexibility. A coupling algorithm for the fluid-body interaction and a moving overset grid modeling strategy are integrated into the flow solver. Validation with numerical grid refinement and convergence studies of water entry problems is addressed, and the capability and robustness of the present model in the accurate simulation of free surface and water-impact flows are demonstrated. The method was subsequently employed for the investigation of the ricochet of a circular cylinder off the water surface, through which the physical picture of the ricochet is more clearly observed.
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•Influence of phase-change on the collapse and rebound stages of spark-generated explosion bubbles was numerically studied.•A reasonable agreement of the bubble radius with the experimental data was ...obtained, particularly at the second cycle.•The condensation process had the highest influence and mainly occurred at the bubble interfaces with a thin boundary layer.•Secondary cavitation regions in the water field induced by rarefaction waves were observed.•The effects of source term by temperature changes on the bubble dynamics were estimated thoroughly.
Influences of phase-change are generally ignored by current numerical models that are used to study spark-generated cavitation bubble dynamics. However, this assumption was limited to predicting bubble behaviors only at the first expansion and collapse stages. In this study, we aimed to explore the phase-change effects on the cavitation bubble dynamics over multiple cycles. A combination of a two-phase homogeneous mixture model and interface-capturing method was adopted to simulate the bubble dynamics. The full compressibility of the water and vapor phases, heat transfer, condensation, and evaporation were involved in our numerical model. Phase-change processes due to pressure changes and temperature changes were evaluated to explore the major influence phenomenon on the bubble dynamics. By comparing with experimental data, a compatible bubble shape and radius evolution under a free-field condition was obtained, particularly at the rebound and collapse stages. Disturbance secondary cavitation regions in the water field induced by rarefaction waves were observed immediately after the second rebound stage. In addition, bubble dynamic behaviors were mainly affected by the condensation phenomenon. The condensation mass transfer rate increases, becomes extremely high at the final collapse stage, and decreases during the rebound stage. Moreover, the condensation phenomenon mainly occurs at the vapor–water bubble interfaces with a thin boundary layer. The evaporation phenomenon occurred purely inside bubbles with a uniform region. Finally, we discuss the effects of phase-change by temperature changes on the cavitation bubble dynamics. In general, these effects were sufficiently small during the first two-cycle bubble oscillations.
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•Simultaneous thermodynamic and hydrodynamic mechanisms of underwater explosion phenomenon are investigated numerically.•Both bubble dynamics and temperature fields agree well with experimental data ...and analytical solution.•Thermal boundary layer inside spherical and non-spherical collapsing bubble is analyzed in detail.•An approximate linear relation is proposed to describe relationship between thermal parameters and bubble dynamics.
In this study, we numerically investigate the simultaneous thermodynamic and hydrodynamic mechanisms of underwater explosion (UNDEX). Bubble explosion in water at the collapsing stage is extremely violent and becomes extraordinarily hot, exceeding 1,000 K. The evolution of the bubbles and temperature fields are simulated using a fully compressible mixture model. The deformable bubble and the heat transfer of the internal explosive gas are captured with higher accuracy compared with published data. First, a spherical bubble that collapses and rebounds without the effects of gravity is computed to verify the accuracy of the model. The numerical results in terms of the bubble radius and temperature fields are consistent with analytical solutions based on the Rayleigh–Plesset equation. Next, a real 5.2 g trinitrotoluene UNDEX experimental case is simulated, in which the formation of a non-spherical bubble with a non-symmetric thermal boundary layer is analyzed. An excellent agreement between bubble motions and experimental data is obtained. The temperature inside the collapsing bubble increased significantly, reached a maximum value of approximately 2,000 K at its final stage, and then decreased rapidly. In addition, a spatially non-uniform temperature field and a thicker thermal boundary layer along the jet direction at the collapse stage are observed. Furthermore, a case study is conducted to estimate the bubble dynamics of non-isothermal and isothermal cases. Finally, the effects of the initial equilibrium gas temperature and water temperature on the thermodynamic and hydrodynamic mechanisms of UNDEX are investigated in detail. An approximate non-linear relation is proposed to describe the relationship among the important parameters.
Evolution process of bubble shapes and temperature fields of non-spherical bubble generated by 5.2 g TNT in free-field condition Display omitted
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In this study, cavitation flow of hydrofoils is numerically investigated to characterize the effects of turbulence models on cavitation-flow patterns and the corresponding radiated sound waves. The ...two distinct flow conditions are considered by varying the mean flow velocity and angle of attack, which are categorized under the experimentally observed unstable or stable cavitation flows. To consider the phase interchanges between the vapor and the liquid, the flow fields around the hydrofoil are analyzed by solving the unsteady compressible Reynolds-averaged Navier–Stokes equations coupled with a mass-transfer model, also referred to as the cavitation model. In the numerical solver, a preconditioning algorithm with dual-time stepping techniques is employed in generalized curvilinear coordinates. The following three types of turbulence models are employed: the laminar-flow model, standard k − ε turbulent model, and filter-based model. Hydro-acoustic field formed by the cavitation flow of the hydrofoil is predicted by applying the Ffowcs Williams and Hawkings equation to the predicted flow field. From the predicted results, the effects of the turbulences on the cavitation flow pattern and radiated flow noise are quantitatively assessed in terms of the void fraction, sound-pressure-propagation directivities, and spectrum.
•An accurate shock- and interface-capturing method for simulations of compressible multiphase flows.•A five-equation two-phase model was transformed into a multi-dimensional general curvilinear ...coordinate system.•High-resolution Godunov-type numerical scheme is constructed using a new eigensystem.•Model is validated by compressible multiphase flow problems under the presence of strong shock waves.•Simulations of 3D complex near-field underwater explosions are presented.
In this study, an accurate shock- and interface-capturing method using curvilinear body-fitted structured grids is introduced to simulate compressible multiphase flows with shockwaves. A five-equation model—proficient in capturing unsteady shocks in compressible multiphase flows without nonphysical spurious oscillations—was enhanced by extending it to a multidimensional general curvilinear coordinate system. A new eigensystem was constructed for a monotonic upstream-centered scheme for conservation laws/Godunov-type finite volume scheme that can capture strong shocks in compressible multiphase flows. The developed method was validated using a typical test involving a free-field underwater explosion that produces a strong shockwave in addition to bubble interfaces. Numerical results were compared and found to agree well with the empirical equation and previously published results. Further, the interaction of a shockwave with a wedge was computed to evaluate the shockwave propagation characteristics and appropriate treatment of solid wall boundary conditions. The numerical results show good agreement with the experimental data. Finally, the impact and propagation characteristics of shockwaves in two more complex cases, involving one and two explosions near a rigid cylinder, were numerically analyzed, and the results indicate high impacts of primary blast waves on the structure.
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•Compressible multiphase model based on a geometrical PLIC-VOF based simulation method.•Fully conservative hyperbolic system is solved based on a Riemann solver.•Reconstruction of the Riemann solver ...for oscillation-free behavior near contact discontinuity.•Dynamic behavior of bubble collapses, high-speed jets, and pressure loads.•3D bubble collapse near a free surface and an oblique wall.
The dynamic behavior of bubble collapses, water jets, and pressure loads during the collapse of the bubble near walls and a free surface were numerically investigated via a geometrical volume of fluid (VOF)-based simulation method. The numerical method is based on the compressible Navier–Stokes equations in a conservative form that describe the flow of compressible viscous fluids. The equations are discretized on a general curvilinear grid using an associated Godunov-type numerical scheme, and a reconstruction of a computational finite-volume Riemann solver is introduced for suppression of oscillation near the interface between fluids. The interface was tracked using the VOF reconstruction method. The VOF method is based on a geometrical tool and Lagrangian propagation of the interface reconstructed by a piecewise linear interface calculation (PLIC), resulting in strictly mass-conserving and sharp interface solutions. The numerical procedure was validated for capturing sharp interface and strong shock waves. For the simulation of bubble collapse, grid dependence studies of a spark-generated bubble were studies of both spherical and non-spherical bubbles were conducted. The results showed good agreement between the simulation and experiment of the bubble dynamics during the collapse process. Subsequently, an investigation of a single bubble near a wall with different standoff distances was performed. The pressure loads induced by the jets impacting the walls were calculated and analyzed. Furthermore, a more complex case of bubble collapse near an oblique wall and free surface was simulated. The resulting bubble dynamics with the jets and free surface shape were compared via photographs of the experiments.
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