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Lin R.-Q.,David Taylor Model Basin | Kuang W.,NASA
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences

In this paper, we describe the details of our numerical model for simulating ship solid-body motion in a given environment. In this model, the fully nonlinear dynamical equations governing the time-varying solid-body ship motion under the forces arising from ship-wave interactions are solved with given initial conditions. The net force and moment (torque) on the ship body are directly calculated via integration of the hydrodynamic pressure over the wetted surface and the buoyancy effect from the underwater volume of the actual ship hull with a hybrid finite-difference/finite-element method. Neither empirical nor free parametrization is introduced in this model, i.e. no a priori experimental data are needed for modelling. This model is benchmarked with many experiments of various ship hulls for heave, roll and pitch motion. In addition to the benchmark cases, numerical experiments are also carried out for strongly nonlinear ship motion with a fixed heading. These new cases demonstrate clearly the importance of nonlinearities in ship motion modelling. Source

Silber G.K.,National Oceanic and Atmospheric Administration | Slutsky J.,David Taylor Model Basin | Bettridge S.,National Oceanic and Atmospheric Administration
Journal of Experimental Marine Biology and Ecology

All endangered large whale species are vulnerable to collisions with large ships; and "ship strikes" are the greatest known threat to one of the world's rarest whales, the North Atlantic right whale (Eubalaena glacialis). The magnitude of this threat is likely to increase as maritime commerce expands. Factors influencing the incidence and severity of ship strikes are not well understood, although vessel speed appears to be a strong contributor. The purpose of this study was to characterize the hydrodynamic effects near a moving hull that may cause a whale to be drawn to or repelled from the hull, and to assess the accelerations exerted on a whale at the time of impact. Using scale models of a container ship and a right whale in experimental flow tanks, we assessed hydrodynamic effects and measured accelerations experienced by the whale model in the presence of a moving vessel. Accelerations at impact were measured while the whale was at the surface, for various vessel speeds, orientations of the whale relative to the vessel path, and distances off the direct path of the vessel. Accelerations experienced by the whale model in a collision: increased in magnitude with increasing ship speed; were not dependent on whale orientation to the vessel path; and decreased exponentially with increasing separation distances from the ship track. Subsequent experiments with the whale model submerged at one to two times the ship's draft indicated a pronounced propeller suction effect, a drawing of the whale toward the hull, and increased probability of propeller strikes resulting from this class of encounter. Measured accelerations are a proxy for impact severity, but do not constitute a detailed study of injury mechanism in a living animal, though they may help inform future work. We present a heuristic map of the hydrodynamic field around a transiting hull likely involved in close whale/vessel encounters. These results may have bearing on policy decisions, particularly those involving vessel speed, aimed at protecting large whales from ship strikes worldwide. © 2010. Source

Noblesse F.,David Taylor Model Basin | Huang F.,George Mason University | Yang C.,George Mason University
Journal of Engineering Mathematics

The classical Neumann-Kelvin (NK) linear potential flow model of 3D flow about a ship hull steadily advancing in calm water is reconsidered, and a modified theory-called Neumann-Michell (NM) theory-is given. The main difference between the two theories is that the line integral around the ship waterline that occurs in the classical NK boundary-integral flow representation is eliminated in the NM theory. Specifically, the integrand of the waterline integral in the NK theory is Gφx-φGx where x is the coordinate along the ship length, φ is the flow potential, and G is the Green function associated with the Kelvin-Michell linear free-surface boundary condition. It is shown that the term Gφx does not appear in a consistent linear flow model. Furthermore, the term φGx can be eliminated using a mathematical transformation, which amounts to an integration by parts. © 2012 US Government. Source

Noblesse F.,David Taylor Model Basin | Huang F.,George Mason University | Yang C.,George Mason University
European Journal of Mechanics, B/Fluids

The dual basic tasks of evaluating ship waves at the free surface and of removing unwanted short waves are considered within the framework of the 'free-surface Green function potential flow theory', based on a Green function that satisfies the radiation condition and the Kelvin-Michell linearized boundary condition at the free surface. A practical approach based on parabolic extrapolation within an extrapolation layer bordering the free surface is used. The height of the extrapolation layer is defined explicitly via simple analytical relations in terms of the Froude number and the slenderness of the ship hull, and varies from the bow to the stern. The bow-to-stern variation is an important ingredient that accounts for the fact that waves along the ship hull aft of the bow wave differ from the bow wave. Indeed, a ship bow wave is significantly higher and shorter than waves aft of the bow wave, is affected by nearfield effects related to the rapid variation of the hull geometry at a ship bow, and consequently contains more short wave components. Illustrative calculations demonstrate the need for removing short ship waves and the effectiveness of the approach based on parabolic extrapolation. © 2012 Elsevier Masson SAS. All rights reserved. Source

Noblesse F.,David Taylor Model Basin | Delhommeau G.,Ecole Centrale Nantes | Huang F.,George Mason University | Yang C.,George Mason University
Journal of Engineering Mathematics

A practical mathematical representation of the flow velocity due to a distribution of sources on the mean wetted hull surface and the mean waterline of a ship that steadily advances along a straight path in calm water, of large depth and lateral extent, is presented. A main feature of this flow representation is a simple analytical approximation-valid within the entire flow region-to the local flow component in the expression for the gradient of the Green function associated with the classical Kelvin-Michell linearized free-surface boundary condition. Another notable feature of the flow representation is that the singularity associated with the gradient of the Green function is removed, using a straightforward regularization technique. The flow representation only involves elementary continuous functions (algebraic, exponential and trigonometric) of real arguments. These functions can then be integrated using ordinary Gaussian quadrature rules. Thus, the flow representation is particularly simple and well suited for practical flow calculations. The specific case of a low-order panel method-in which the hull geometry, the source density, and the flow velocity are consistently represented via piecewise linear approximations within flat triangular hull panels or straight waterline segments-is considered. © 2011 US Government. Source

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