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Noblesse F.,David Taylor Model Basin | Huang F.,George Mason University | Yang C.,George Mason University
Journal of Engineering Mathematics | Year: 2013

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.

Noblesse F.,David Taylor Model Basin | Delhommeau G.,École Centrale Nantes | Huang F.,George Mason University | Yang C.,George Mason University
Journal of Engineering Mathematics | Year: 2011

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.

Noblesse F.,David Taylor Model Basin | Delhommeau G.,École Centrale Nantes | Yang C.,George Mason University | Kim H.Y.,George Mason University | Queutey P.,École Centrale Nantes
Journal of Ship Research | Year: 2011

The bow wave generated by a steadily advancing ship is considered for a family of fine ruled ship bows with rake and flare. This family of ship bows is defined in terms of four parameters: the ship draft D, the entrance angles α and α′ at the top and bottom waterlines, and the rake angle δ. The corresponding bow wave similarly depends on four parameters: the draft-based Froude number F and the three angles α, α′, and δ. An extensive parametric study, based on thin-ship theory, is performed to explore the variations of the water height Z0 at the ship stem X = 0, the location X0 (measured from the ship stem) of the intersection of the bow-wave profile with the mean freesurface plane Z = 0, and the bow-wave profile, with respect to the four parameters F, α, α′, and δ. This parametric study extends the previously reported similar study of the height Zb of the bow wave and the location Xb of the bow-wave crest. These two complementary parametric studies yield simple analytical relations, which extend relations given previously for wedge-shaped ship bows without rake or flare. In spite of their remarkable simplicity, the analytical relations given here yield bow waves that are comparable to computational fluid dynamics (CFD) waves given by Euler-flow calculations. The analytical relations, which explicitly account for the influence of the four primary parameters F, α, α′, and δ, can be used immediately-without hydrodynamic calculations-for ship design, notably at early design stages when the precise hull geometry is not yet known. The study also provides insight for ship bow design. Specifically, it suggests that a bow with positive rake and negative flare may be beneficial, and that a bulb located aft of the stem and integrated with the hull may be an advantageous alternative to a traditional bulb protruding ahead of the bow, in agreement with the results of a hull-form optimization analysis.

Noblesse F.,David Taylor Model Basin | Huang F.,George Mason University | Yang C.,George Mason University
European Journal of Mechanics, B/Fluids | Year: 2013

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.

Noblesse F.,David Taylor Model Basin | Wang L.,George Mason University | Yang C.,George Mason University
Journal of Ship Research | Year: 2012

Simple analytical relations that can readily be applied to verify a critical aspect of numerical predictions of fully nonlinear free-surface flows around ship hulls steadily advancing in calm water are given. The relations do not involve the flow field equations; that is, they are only based on the boundary conditions at the ship hull surface and at the free surface. These boundary conditions have a predominant influence on free-surface flows around advancing ship hulls. The analytical relations are exact for inviscid flows, and can be applied to numerical methods that solve either the Laplace equation (potential-flow methods) or the Euler flow equations (CFD Euler-flow methods). They provide a simple test to verify if numerical predictions given by nonlinear potential-flow or Euler-flow methods correctly satisfy the hull-surface and free-surface boundary conditions along the contact curve between the hull surface and the free surface. The relations might also be used to verify CFD methods that solve the RANS equations if they are applied at the edge of the viscous boundary layer. The analytical test can identify an inconsistency, which might point to a "method issue" related to a feature of a numerical method (e.g., a numerical-differentiation scheme) or an "implementation issue" in the implementation of the method (e.g., a poor discretization). For purposes of illustration, the test is applied to predictions of flows around the Wigley parabolic hull given by two CFD methods that solve the Euler equations with fully nonlinear boundary conditions at the free surface. This illustrative example demonstrates that the test can indeed be useful to identify numerical inaccuracies. The analytical relations can also be used to determine experimental values of the flow velocity at a ship wave profile that correspond to measurements of the wave profile.

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 | Year: 2010

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.

Lin R.-Q.,David Taylor Model Basin | Hughes M.,David Taylor Model Basin | Smith T.,David Taylor Model Basin
Journal of Marine Science and Technology | Year: 2010

This paper introduces a new method for the prediction of ship maneuvering capabilities. The new method is added to a nonlinear six-degrees-of-freedom ship motion model named the digital, self-consistent ship experimental laboratory (DiSSEL). Based on the first principles of physics, when the ship is steered, the additional surge and sway forces and the yaw moment from the deflected rudder are computed. The rudder forces and moments are computed using rudder parameters such as the rudder area and the local flow velocity at the rudder, which includes contributions from the ship velocity and the propeller slipstream. The rudder forces and moments are added to the forces and moments on the hull, which are used to predict the motion of the ship in DiSSEL. The resulting motions of the ship influence the inflow into the rudder and thereby influence the force and moment on the rudder at each time step. The roll moment and resulting heel angle on the ship as it maneuvers are also predicted. Calm water turning circle predictions are presented and correlated with model test data for NSWCCD model 5514, a pre-contract DDG-51 hull form. Good correlations are shown for both the turning circle track and the heel angle of the model during the turn. The prediction for a ship maneuvering in incident waves will be presented in Part 2. DiSSEL can be applied for any arbitrary hull geometry. No empirical parameterization is used, except for the influence of the propeller slipstream on the rudder, which is included using a flow acceleration factor. © 2010 JASNAOE.

Lin R.-Q.,David Taylor Model Basin | Kuang W.,NASA
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2011

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.

Belenky V.,David Taylor Model Basin | Weems K.,David Taylor Model Basin | Lin W.-M.,N-of-One
Ocean Engineering | Year: 2016

The paper reviews a multi-year research effort for using the split-time method to calculate the probability of ship capsizing due to pure loss of stability in irregular waves. The idea of the split-time method is to separate the complex problem into two less complex problems: a non-rare problem that involves the upcrossing of an intermediate level of roll and a rare problem that focuses on capsizing after an upcrossing. An initial implementation using a dynamic model with piecewise linear stiffness, which can be considered to be the simplest model of capsizing in beam seas, led to the concept of critical roll rate as the smallest roll rate at the instant of upcrossing that inevitably leads to capsizing. The extension of the split-time method to pure loss of stability required the consideration of the change of roll stiffness in waves and led to calculating the critical roll rate at each upcrossing. A metric of the likelihood of capsizing has been defined as the difference between the observed and critical roll rate at the instances of upcrossing. The probability of capsizing after upcrossing is found by approximating the tail with the Generalized Pareto Distribution. © 2016.

Lin R.-Q.,David Taylor Model Basin | Kuang W.,NASA
Modelling and Simulation in Engineering | Year: 2011

This is the continuation of our research on development of a fully nonlinear, dynamically consistent, numerical ship motion model (DiSSEL). In this paper we report our results on modeling ship maneuvering in arbitrary seaway that is one of the most challenging and important problems in seakeeping. In our modeling, we developed an adaptive algorithm to maintain dynamical balances numerically as the encounter frequencies (the wave frequencies as measured on the ship) varying with the ship maneuvering state. The key of this new algorithm is to evaluate the encounter frequency variation differently in the physical domain and in the frequency domain, thus effectively eliminating possible numerical dynamical imbalances. We have tested this algorithm with several well-documented maneuvering experiments, and our results agree very well with experimental data. In particular, the numerical time series of roll and pitch motions and the numerical ship tracks (i.e., surge, sway, and yaw) are nearly identical to those of experiments. Copyright © 2011 Ray-Qing Lin and Weijia Kuang.

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