Zhaoyuan Wang

A new volume-of-fluid method with a constructed distance function on general structured grids

A second-order volume-of-fluid method (VOF) is presented for interface tracking and sharp interface treatment on general structured grids. Central to the new method is a second-order distance function construction scheme on a general structured grid based on the reconstructed interface. A novel technique is developed for evaluating the interface normal vector using the distance function. With the normal vector, the interface is reconstructed from the volume fraction function via a piecewise linear interface calculation (PLIC) scheme on the computational domain. Several numerical tests are conducted to demonstrate the accuracy and efficiency of the present method. In general, the new VOF method is more efficient than both the high-order level set and the coupled level set and volume-of-fluid (CLSVOF) methods. The results from the new method are better than those from the benchmark VOF method, particularly in the under-resolved regions, and are comparable to those from the CLSVOF method. Breaking waves over a submerged bump and around a wedge-shaped bow are simulated to demonstrate the application of the new method and sharp interface treatment in a two-phase flow solver on curvilinear grids. The computational results are in good agreement with the available experimental measurements.

Computational ship hydrodynamics: Nowadays and way forward

Computational fluid dynamics for ship hydrodynamics has made monumental progress over the last ten years, which is reaching the milestone of providing first-generation simulation-based design tools with vast capabilities for model- and full-scale simulations and optimization. This is due to the enabling technologies such as free surface tracking/capturing, turbulence modeling, 6 degree of freedom (DoF) motion prediction, dynamic overset grids, local/adaptive grid refinement, high performance computing, environmental modeling and optimization methods. Herein, various modeling, numerical methods, and high performance computing approaches for computational ship hydrodynamics are evaluated thereby providing a vision for the development of the next-generation high-fidelity simulation tools. Verification and validation procedures and their applications, including resistance and propulsion, seakeeping, maneuvering, and stability and capsizing, are reviewed. Issues, opportunities, and challenges for advancements in higher-fidelity two-phase flow are addressed. Fundamental studies for two-phase flows are also discussed. Conclusions and future directions are also provided.

Recent progress in CFD for naval architecture and ocean engineering

An overview is provided of CFDShip-Iowa modeling, numerical methods and high performance computing (HPC), including both current V4.5 and V5.5 and next generation V6. Examples for naval architecture highlight capability and needs. High fidelity V6 simulations for ocean engineering and fundamental physics describe increased resolution for analysis of physics of fluids. Uncertainty quantification research is overviewed as the first step towards development stochastic optimization.

Short note: A simple and conservative operator-splitting semi-Lagrangian volume-of-fluid advection scheme

A semi-Lagrangian, operator-splitting, high-performance volume-of-fluid advection scheme (SLOSH-VOF) is developed. In the backward time integration a directional splitting strategy is adopted, which greatly simplifies the definition and locating of the departure volume and reduces it to a grid cell expanded or compressed in one grid direction. The VOF value in the departure cell is estimated using a geometrical interpolation algorithm with a piecewise linear interface calculation (PLIC) scheme for the interfacial cells. The proposed SLOSH-VOF method is unconditionally stable and very large time steps can be used to significantly speed up the overall computations. It is conceptually simple and can be very easily implemented due to the direction-split advection. Its performance is evaluated through several benchmark advection tests. The SLOSH-VOF results are comparable to those from an Eulerian VOF method in terms of interface position errors, and improved with regard to mass conservation, even with CFL numbers increased up to one order of magnitude.

Comparison of Particle Level Set and CLSVOF Methods for Interfacial Flows

The level set method is susceptible to the numerical dissipation, which usually results in serious mass loss in the under-resolved regions at the interface. In this study, a hybrid particle level set method and a coupled level set and volume-of-fluid method, have been implemented with the aim at improving the mass conservation property of the level set method. A number of standard tests are conducted to evaluate their performances. The two methods are applied to simulate a water drop impact onto a liquid pool, and wave breakings over a submerged bump which is of particular relevance to ship hydrodynamics.

Sharp Interface LES of Breaking Waves by an Interface Piercing Body in Orthogonal Curvilinear Coordinates

A sharp interface LES methodology on orthogonal curvilinear grid for breaking waves produced by an interface-piercing body at high Reynolds number is presented. Both gas and liquid phases are considered for the strong interactions between two phases, such as spray dispersion and bubble entrainment. The level-set based ghost fluid method is adopted for sharp interface treatment and a volume-of-fluid method in orthogonal curvilinear coordinates is coupled with the level set method for enhanced interface tracking. A Lagrangian dynamic Smagorinsky subgrid-scale model is used for the spatially filtered turbulence closure. Several numerical tests are conducted in order to validate the code. Wave breaking around a wedge-shaped bow is simulated with the results compared with the experimental measurement.

Impulsive Plunging Wave Breaking Downstream of a Bump in a Shallow Water Flume

The plunging wave-breaking process for impulsive flow over a bump in a shallow water flume is described using complementary experiments and simulations, which is relevant to ship hydrodynamics since it includes effect of wave-body interactions and wave breaking direction is opposite to the mean flow. Phase averaged measurements (relative to the time at which the maximum wave height is reached just before the first plunge) are conducted, including the overall flume flow and 2D PIV center-plane velocities and turbulence inside the plunging breaking wave and bottom pressures under the breaking wave. A total number of 226 individual plunging wave-breaking tests were conducted, which all followed a similar time line consisting of startup, steep wave formation, plunging wave, and chaotic wave breaking swept downstream time phases. The plunging wave breaking process consists of four repeated plunging events each with three [jet impact (plunge), oblique splash and vertical jet] sub-events, which were identified first using complementary CFD. Video images with red dye display the plunging wave breaking events and sub-events. The first and second plunges take longer than the last two plunges. Oblique splashes and vertical jets account for more time than plunging. The wave profile at maximum height, first plunge, bump and wave breaking vortex and entrapped air bubble trajectories, entrapped air bubble diameters, kinetic, potential, and total energy, and bottom pressures are analyzed. The simulations on four different grids qualitatively predict all four time phases, all four plunging events and their sub-events, and bottom pressure but with reduced velocity magnitudes and larger post-breaking water elevations. The medium grid results are presented and the fine grid simulations are in progress. Similarities and differences are discussed with the previous deep water or sloping beaches experimental and computational studies.Copyright

URANS Study of Air-Layer Drag Reduction in a High- Reynolds-Number Flat-Plate Turbulent Boundary Layer

The air layer formation in a high Reynolds-number flat plate turbulent boundary layer is simulated using a two-phase sharp interface Cartesian grid solver. The interface is tracked by a coupled level set and volume-of-fluid method (CLSVOF) and turbulence is modeled by a Spalart-Allmaras (SA) turbulence model with a wall function (WF) approach. The air layer along the entire test plate is successfully achieved and the drag reduction is approximately 100%, which agrees very well with the experimental findings. With reduced air flow rate, bubble drag reduction (BDR) is also observed; the computational results also qualitatively match the experiments. The transitional region from BDR to ALDR is also observed in the present simulation. However, the critical air flow rate to form the ALDR is lower in the simulations than in the experiments. Several possible reasons are likely accounting for the low critical air flow rate in the simulations, such as SA-WF turbulence model, three-dimensional instability and surface tension effects. The critical air flow rate does not change much with grid refinement.