Kelli Hendrickson

Simulation of steep breaking waves and spray sheets around a ship: the last frontier in computational ship hydrodynamics

Breaking ship waves are one of the most challenging problems in the field of free-surface hydrodynamics. We aim to produce and implement unique, scalable parallel-computing capabilities for simulating turbulent breaking waves and the resultant formation of spray and entrainment of air. SAIC has developed a Cartesian-grid method for simulating breaking waves around ships. Body-force and finite-volume formulations are used to model the hull and an interface capturing method is used to model the free surface. As a result, minimal user input is required to simulate breaking waves, which makes the tool ideal for ship design studies. At MIT, a suite of codes has been developed based on advanced large-eddy simulation of coupled air-water flows. This suite uses time accurate interface capturing and interface tracking methods to model turbulent free-surface flows. The results of the numerical simulations are used to guide the development of turbulence models for the SAIC code and other codes, such as Reynolds averaged Navier-Stokes (RANS), which are currently being used by the Navy. Through large-scale computations on the IBM SP3 and Cray T3E using both Cray SHMEM and MPI and hybrid techniques, the numerical results and their analyses provide us with the framework to develop models of wave breaking and spray formation and air entrainment. Numerical simulations of various ship-like geometries moving with forward speed and spilling breaking waves have been performed. With these promising results, which were achievable only through high-performance computations, the last frontier of computational ship hydrodynamics will be breached in the near future.

Air entrainment and multiphase turbulence in the bubbly wake of a transom stern

Accurate prediction of the highly-mixed flow in the near field of a surface ship is a challenging and active research topic in Computational Ship Hydrodynamics. The disparity in the time and length scales and the scales of entrainment dictates the use of bubble source and mixed-phase flow models in which the current state of the art models are ad hoc. This paper presents the air entrainment characteristics and multiphase turbulence modeling of the near-field flow of a canonical stern with the inclusion of simple geometry effects. Using state of the art Cartesian-grid numerical methods with the full field equations, high-resolution two-phase flow data sets of a canonical stern with three different half-beam to draft ratios are simulated down to the scales of bubble entrainment. These data sets are used as the foundation for: (1) characterization of wake structure and near-wake air entrainment of the stern; (2) analysis of turbulent mass flux in the wake of the stern; and (3) a priori testing of multiphase turbulence models for turbulent mass flux. Results are obtained to show that these techniques enable analysis and physics-based parameterization of near-field air entrainment about surface ships for use in Computational Ship Hydrodynamics.

Nonlinear Effects on Interfacial Wave Growth Into Slug Flow

The objective of this study is to understand the effects of nonlinearity on the interfacial stability of a two fluid stratified flow through a horizontal channel. An efficient perturbation expansion based high-order spectral method is developed, for the simulation of the generation and nonlinear evolution of interfacial waves. The method is capable of accounting for the nonlinear interactions of a large number of wave components in a broadband spectrum, and obtains an exponential convergence of the solution with grid refinement and interaction order. The method is applied to investigate the role that nonlinear effects have on the initial development of Kelvin-Helmholtz (KH) instabilities and nonlinear wave-wave interactions with the purpose of gaining insight into the physical mechanisms which cause slug flow. It will also be demonstrated that nonlinearity can introduce unstable interfacial wave growth in regions predicted to be stable by linear analysis. In addition, it is shown that energy transfer from short (KH unstable) waves to long (KH stable) waves due to nonlinear resonant wave-wave interactions is an effective mechanism for the development of slug flow.

Computation of three-dimensional multiphase flow dynamics by Fully-Coupled Immersed Flow (FCIF) solver

Abstract This work presents a Fully-Coupled Immersed Flow (FCIF) solver for the three-dimensional simulation of fluid–fluid interaction by coupling two distinct flow solvers using an Immersed Boundary (IB) method. The FCIF solver captures dynamic interactions between two fluids with disparate flow properties, while retaining the desirable simplicity of non-boundary-conforming grids. For illustration, we couple an IB-based unsteady Reynolds Averaged Navier Stokes (uRANS) simulator with a depth-integrated (long-wave) solver for the application of slug development with turbulent gas and laminar liquid. We perform a series of validations including turbulent/laminar flows over prescribed wavy boundaries and freely-evolving viscous fluids. These confirm the effectiveness and accuracy of both one-way and two-way coupling in the FCIF solver. Finally, we present a simulation example of the evolution from a stratified turbulent/laminar flow through the initiation of a slug that nearly bridges the channel. The results show both the interfacial wave dynamics excited by the turbulent gas forcing and the influence of the liquid on the gas turbulence. These results demonstrate that the FCIF solver effectively captures the essential physics of gas–liquid interaction and can serve as a useful tool for the mechanistic study of slug generation in two-phase gas/liquid flows in channels and pipes.

Computational Naval Ship Hydrodynamics

The primary purpose of our research efforts is to improve naval design and detection capabilities. Our current research efforts leverage high performance computing (HPC) resources to perform high-resolution numerical simulations with hundreds-of-millions to billions of unknowns to study wave breaking behind a transom stern, wave-impact loading, the generation of spray by high-speed planing craft, air entrainment by plunging breaking waves, forced-motion, and storm seas. This paper focuses on the air entrainment and free-surface turbulence in the flow behind a transom-stern and wave-impact loading on marine platforms. Two codes, Numerical Flow Analysis (NFA) and Boundary Data Immersion Method (BDIM), are used in this study. Both codes are Cartesian-based Large-Eddy Simulation (LES) formulations, and use either Volume-of-Fluid (VOF) (NFA) or conservative Volume-of-Fluid (cVOF) BDIM treatments to track the free-surface interface. The first project area discussed is the flow behind the transom stern. BDIM simulations are used to study the volume of entrained air behind the stern. The application of a Lagrangian bubble-extraction algorithm elucidates the location of air cavities in the wake and the bubble-size distribution for a flow that has over 10 percent void fraction. NFA simulations of the transom-stern flow are validated by comparing the numerical simulations to experiments performed at the Naval Surface Warfare Center, Carderock Division (NSWCCD), where good agreement between simulations and experiments is obtained for mean elevations and regions of white water in the wake. The second project area discussed is wave impact loading, a topic driven by recent structural failures of high-speed planing vessels and other advanced vehicles, as well as the devastation caused by Tsunamis impacting low-lying coastal areas. NFA simulations of wave breaking events are compared to the NSWCCD cube impact experiments and the Oregon State University, O.H. Hinsdale Wave Research Laboratories Tsunami experiments, and it is shown that NFA is able to accurately simulate the propagation of waves over long distances after which it also accurately predicts highly-energetic impact events.

Modeling Breaking Ship Waves for Design and Analysis of Naval Vessels

One of the remaining challenges involved in modern naval ship design and analysis is to account for the effects of breaking waves, spray and air entrainment on the performance and non-acoustical signature of a surface ship. The near field flow about a surface ship is characterized by complex physical processes such as: (i) spray sheet and jet formation; (ii) strong free-surface turbulence interactions with (large-amplitude) breaking waves; (iii) air entrainment and bubble generation; and (iv) post-breaking turbulence and dissipation. The challenges associated with this task are twofold. The first is robustly simulating the large-scale problem which involves the flow about an entire surface ship. The second is the development of physics-based closure models for steep breaking waves in the presence of turbulence. To wit, a two-pronged approach consisting of developing an understanding for closure model development and applying cutting-edge computational capabilities has been developed to accurately simulate the free-surface flow around naval combatants. Using high-resolution direct numerical simulation of the Navier-Stokes equations employing the level set method, we have successfully simulated an ensemble of unsteady breaking waves at Reynolds numbers O(103-4 ). This includes steady and unsteady as well as spilling and plunging events. This dataset is continually being improved upon in terms of depth and breadth as a direct result of this Challenge Project. The goal of this core research area is to develop understanding of the physics of breaking waves to help guide the development of physics-based breaking wave modes. The dataset is being used for the evaluation of closure models for inclusion in current larger scale simulations such as large eddy simulation and Reynolds-Averaged Navier-Stokes. Robustly simulating the near-field flow of a surface ship requires the development of new models and numerical techniques suitable for use in large scale applications. We have performed more moderate-scale simulations to design, verify, and validate these capabilities before their implementation on the large- scale simulations. Using Numerical Flow Analysis (NFA), simulations of several naval combatants were performed at a range of speeds. The numerical results show wave overturning at the bow and flow separation at the transom. Air is entrained along the side of the hull and in the rooster-tail region behind the stern. In both regions, numerical predictions agree well with experimental measurements. This work marks the first time that NFA has been used to simulate an entire ship hull. The numerical simulations were performed on the Engineer Research and Development Center (ERDC) Cray XT3 using 128-256 processors. Approximately, 90 million grid points were used in the simulations.