Modeling Breaking Ship Waves for Design and Analysis of Naval Vessels

Abstract

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(10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-4</sup> ). 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.