Navier-Stokes simulations of steep breaking water waves with a coupled air-water interface

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2005.Includes bibliographical references (p. 367-378).Wave breaking on the ocean surface significantly facilitates the transfer of mass, momentum, heat and energy across the air-sea interface. In the context of the near field flow about a surface ship, the breaking bow wave is a key element to the bubbly signature and an appreciable portion of the wave drag of the ship. Yet, despite its direct effect on many aspects of ocean engineering, this phenomenon is not well understood even at a basic level. Most of the knowledge has been contributed by experiments in the laboratory and the field although results are often limited due to the difficulty in taking measurements of local quantities during the breaking event. Numerical solution of the breaking wave problem has generally been limited to the pre-breaking phase as it avoids complex mechanisms such as surface re-entry, spray formation, air entrainment and strong turbulence. Additionally, relatively few experimental or numerical studies exist which dynamically couple the air-water interface. The objective of this thesis is to contribute to the knowledge of steep breaking waves in the context of the coupled air-water interface. Of central importance are basic kinematics and dynamics, the rate of energy dissipation and energy flux at the interface during the breaking event.(cont.) To this end, a systematic study of a range of breaking waves is performed by direct numerical simulation (DNS) of the Navier- Stokes equations using an Eulerian interface capturing method. The advantage of the DNS approach is that all physical scales are resolved and no turbulence closure models are necessary. However, because of this, DNS is limited to the study to moderate Reynolds numbers with a relatively high computational cost for each simulation. For this reason, this study is limited to two-dimensional flows at Reynolds number 0(10³). The interface capturing method used is a modified form of the level set method which is better suited for simulating coupled air-water flows. The level set method provides a natural numerical treatment of the coupled air-water interface through complex surface topology changes. Thus, no ad-hoc treatment of the air-water interface during the breaking event is necessary. The key findings of this thesis represent new contributions to the study of breaking waves in three distinct areas. The first is the kinematics and dynamics of deep water breaking waves for both spilling and plunging types. For the waves in this study, there was no indication of flow reversal or separation in the water while the air flow showed separation on the front face of the wave and over the crest.(cont. ) Localized shear regions are found in spilling breaking waves and curvature effects are identified as the dominant mechanism of vorticity generation in both types of breaking waves. The second area is the energy dissipated by breaking waves. The volumetric dissipation rates as well as its spatial variation for both air and water are presented for the range of waves in this study. While the water volume experienced an increase in dissipation rate during the breaking event, the increase is more pronounced in the air volume to the point that it becomes the same order of magnitude as that in the water for some waves. The amount of energy in the wave lost due to breaking is quantified as a function of the energy in the wave prior to breaking. A threshold below which waves do not break is identified and qualitative comparisons to experiment are made when applicable. The third area is the transfer of energy at the air-water interface during breaking which is an aspect of the breaking process that has not received much attention in the literature. In this thesis, the formulation of a term in the energy equation which accounts for the energy flux rate at the air-water interface is presented. The waves in this numerical study give evidence that this quantity is appreciable.(cont.) Although the calculation of this term is sensitive to errors associated with the conservation of energy, values as high as 25% of the energy lost to breaking are found. At the Reynolds numbers in this study, the dominant mechanism for each type of wave is identified as inviscid for spilling breaking waves and viscous for plunging breaking waves. This numerical effort has contributed to the basic knowledge of wave breaking at moderate Reynolds numbers. Through the inclusion of the coupled air-water interface, unique insight to the kinematics, dynamics, dissipation and energy fluxes of breaking waves was obtained. The information gained in this study provides an initial step towards physics-based turbulence models for the study of wave breaking at larger scales.by Kelli L. Hendrickson.Sc.D

Introduction to Asian American Studies: Final Zine Project (4)

ABOUT US… We are a group of USD students studying Asian American history & politics. Over the course of the semester, we have studied histories of labor, migration, war, incarceration, and displacement. This course has allowed us to better understand the ways in which Asian American identity has emerged. This course has given us the tools necessary to explore Asian American history and to contextualize what we have learned so we can apply it to contemporary issues both within and outside of the United States. ABOUT OUR ZINE… In each of our weekly zines, we really focused on the significance of each text while intertwining it with the idea of racial inequality. One of our main focuses throughout all weeks was how Asian Americans made their way into the United States and assimilated. Being able to read and reflect on the personal stories of Asian American refugees gave us the opportunity to think outside of the box. It really opened our eyes to some of the unwritten challenges that a lot of individuals face. In each individual zine, there are approximately three main sections. The first is the “Key Points of the Text.” In this section, we highlight what exactly the author is talking about and how they go about making these points. The next section is “Why Does It Matter?” This section assesses the text’s social, political and intellectual significance and why it is important. Finally, the last section is the “Connection to Today.” This section relates what we read and analyzed to the happenings in our communities and societies now. In this section, we analyze political contexts that help us better understand the texts. Lastly, in all of the Zines we emphasize the importance of visuals in helping us understand the deeper meaning. We included images and other important types of visuals to analyze all of the texts and give our readers a better understanding of what we learned! Overall, the goal of this Zine is to portray the inequalities and struggles that Asian Americans faced in the process of migrating to the United States. Their norms and ways of life were disrupted in the process, but individuals were forced to face the challenges and adapt in order to give themselves better opportunity. The similarities seen in some of today’s societal issues compared to those of the past were eye-opening. Our hope is that you are able to make those connections as well! Enjoy!https://digital.sandiego.edu/ethn-zines/1015/thumbnail.jp

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. © 2011 IEEE.United States. Office of Naval Research (N00014-07-C-0184)United States. Office of Naval Research (N00014-01-1-0124

Scale separation and dependence of entrainment bubble-size distribution in free-surface turbulence

© The Author(s) 2019. We consider the size spectrum of entrained bubbles under strong free-surface turbulence (SFST). We investigate the entrainment bubble-size spectrum per unit (mean) interface area, , with dimension length, and develop a physical/mechanistic model for through energy arguments. The model obtains two distinct regimes of , separated by bubble-size scale. For bubble radius r-{0}]>, the effects of gravity dominate those of the surface tension force , and , where is the turbulence dissipation rate. For and <![CDATA[r; the value ; the scaling ; and the predictions of the-entrainment regime map

Informed component label algorithm for robust identification of connected components with volume-of-fluid method computers and fluids

The connected component labeling technique (CCL), which labels regions of connected Eulerian field data, will inaccurately identify closely spaced components when applied to the volume-of-fluid function. We present two modifications to the CCL that improve its robustness and accuracy. This Informed Component Labeling algorithm (ICL) incorporates the normal and uses multilevel thresholding to improve and refine connectivity decisions for components with spacing just larger than the grid size. Through detailed verification and validation using synthetic volume fraction data, we show that the ICL algorithm removes the bias to larger components, provide guidelines for its use, and estimate its error bounds for the smallest components. The ICL produces zero standard deviation in the number of components identified for those with radius larger than twice the grid size and can reduce it by ~38% for smaller components. The modifications that comprise the ICL can be applied to any existing CCL algorithm with a known increase in computational cost. It enables robust identification of connected components for accurate transfer of information in mixed Eulerian-Lagrangian methods and statistical analysis that use the volume-of-fluid function

Wake behind a three-dimensional dry transom stern. Part 1: Flow structure and large-scale air entrainment

We present high-resolution implicit large eddy simulation (iLES) of the turbulent air entraining flow in the wake of three-dimensional rectangular dry transom sterns with varying speeds and half-beam-to-draft ratios B/D. We employ two-phase (air/water), time-dependent simulations utilizing conservative Volume-of-Fluid (cVOF) and Boundary Data Immersion (BDIM) methods to obtain the flow structure and large-scale air entrainment in the wake. We confirm that the convergent corner wave region that forms immediately aft of the stern wake is ballistic, thus predictable only by the speed and (rectangular) geometry of the ship. We show that the flow structure in the air-water mixed region contains a shear layer with a streamwise jet and secondary vortex structures due to the presence of the quasi-steady, three-dimensional breaking waves. We apply a Lagrangian cavity identification technique to quantify the air entrainment in the wake and show that the strongest entrainment is where wave breaking occurs. We identify an inverse dependence of the maximum average void fraction and total volume entrained with B/D. We determine that the average surface entrainment rate initially peaks at a location that scales with draft-Froude number and that the normalized average air cavity density spectrum has a consistent value providing there is active air entrainment. A small parametric study of the rectangular geometry and stern speed establishes and confirms the scaling of the interface characteristics with draft-Froude number and geometry. In part 2 we examine the incompressible highly-variable density turbulence characteristics and turbulence closure modeling