Thomas T. O'Shea

A detailed assessment of numerical flow analysis (NFA) to predict the hydrodynamics of a deep-V planing hull

Over the past few years much progress has been made in Computational Fluid Dynamics (CFD) in its ability to accurately simulate the hydrodynamics associated with a deep-V monohull planing craft. This work has focused on not only predicting the hydrodynamic forces and moments, but also the complex multiphase free-surface flow field generated by a deep-V monohull planing boat at high Froude numbers. One of these state of the art CFD codes is Numerical Flow Analysis (NFA). NFA provides turnkey capabilities to model breaking waves around a ship, including both plunging and spilling breaking waves, the formation of spray and the entrainment of air. NFA uses a Cartesian-grid formulation with immersed body and volume-of-fluid methods. The focus of this paper is to describe and document a recent effort to assess NFA for the prediction of deep-V planing craft hydrodynamic forces and moments and evaluate how well it models the complex multiphase flows associated with high Froude number flows, specifically the formation of the spray sheet. This detailed validation effort was composed of three parts. The first part focused on assessing NFAs ability to predict pressures on the surface of a 10 degree deadrise wedge during impact with an undisturbed free surface. Detailed comparisons to pressure gauges are presented here for two different drop heights, 15.24 cm (6 in) and 25.4 cm (10 in). Results show NFA accurately predicted pressures during the slamming event. The second part examines NFAs ability to match sinkage, trim and resistance from Fridsmas experiments performed on constant deadrise planing hulls. Simulations were performed on two 20 degree deadrise hullforms of varying length to beam ratios (4 and 5) over a range of speed-length ratios (2, 3, 4, 5 and 6). Results show good agreement with experimentally measured values, as well as values calculated using Savitskys parametric equations. The final part of the validation study focused on assessing how well NFA was able to accurately model the complex multiphase flow associated with high Froude number flows, specifically the formation of the spray sheet. NFA simulations of a planing hull fixed at various angles of roll (0, 10, 20 and 30 degrees) were compared to experiments. Comparisons to underwater photographs illustrate NFAs ability to model the formation of the spray sheet and the free surface turbulence associated with planing boat hydrodynamics. Overall these three validation studies provide a detailed assessment on the current capabilities of NFA to predict the hydrodynamics of a deep-V planing hull.

A numerical simulation of a plunging breaking wave

ONR Program Manager: Dr. Patrick Purtell. ONR Contract Number: N00014-07-C-0184. Computer resources provided by the DoD High Performance Computing Modernization Program at the ERDC DoD Supercomputing Research Center, Vicksburg MS.

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.