17,741 research outputs found
Dynamical Models of Extreme Rolling of Vessels in Head Waves
Rolling of a ship is a swinging motion around its length axis. In particular vessels transporting containers may show large amplitude roll when sailing in seas with large head waves. The dynamics of the ship is such that rolling interacts with heave being the motion of the mass point of the ship in vertical direction. Due to the shape of the hull of the vessel its heave is influenced considerably by the phase of the wave as it passes the ship. The interaction of heave and roll can be modeled by a mass-spring-pendulum system. The effect of waves is then included in the system by a periodic forcing term. In first instance the damping of the spring can be taken infinitely large making the system a pendulum with an in vertical direction periodically moving suspension. For a small angular deflection the roll motion is then described by the Mathieu equation containing a periodic forcing. If the period of the solution of the equation without forcing is about twice the period of the forcing then the oscillation gets unstable and the amplitude starts to grow. After describing this model we turn to situation that the ship is not anymore statically fixed at the fluctuating water level. It may move up and down showing a motion
modeled by a damped spring. One step further we also allow for pitch, a swinging motion around a horizontal axis perpendicular to the ship. It is recommended to investigate the way waves may directly drive this mode and to determine the amount of energy that flows along this path towards the roll mode. Since at sea waves are a superposition of waves with different wavelengths, we also pay attention to the properties of such a type of forcing containing stochastic elements. It is recommended that as a measure for the
occurrence of large deflections of the roll angle one should take the expected time for which a given large deflection may occur instead of the mean amplitude of the deflection
Wave interactions in localizing media - a coin with many faces
A variety of heterogeneous potentials are capable of localizing linear
non-interacting waves. In this work, we review different examples of
heterogeneous localizing potentials which were realized in experiments. We then
discuss the impact of nonlinearity induced by wave interactions, in particular
its destructive effect on the localizing properties of the heterogeneous
potentials.Comment: Review submitted to Intl. Journal of Bifurcation and Chaos Special
Issue edited by G. Nicolis, M. Robnik, V. Rothos and Ch. Skokos 21 Pages, 8
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Parametric pitch instability investigation of Deep Draft Semi-submersible platform in irregular waves
Parametric pitch instability of a Deep Draft Semi-submersible platform (DDS) is investigated in irregular waves. Parametric pitch is a form of parametric instability, which occurs when parameters of a system vary with time and the variation satisfies a certain condition. In previous studies, analyzing of parametric instability is mainly limited to regular waves, whereas the realistic sea conditions are irregular waves. Besides, parametric instability also occurs in irregular waves in some experiments. This study predicts parametric pitch of a Deep Draft Semi-submersible platform in irregular waves. Heave motion of DDS is simulated by wave spectrum and response amplitude operator (RAO). Then Hill equation for DDS pitch motion in irregular waves is derived based on linear-wave theory. By using Bubnov-Galerkin approach to solve Hill equation, the corresponding stability chart is obtained. The differences between regular-waves stability chart and irregular-waves stability chart are compared. Then the sensitivity of wave parameters on DDS parametric pitch in irregular waves is discussed. Based on the discussion, some suggestions for the DDS design are proposed to avoid parametric pitch by choosing appropriate parameters. The results indicate that it's important and necessary to predict DDS parametric pitch in irregular waves during design process
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Stochastic stability of viscoelastic systems under Gaussian and Poisson white noise excitations
As the use of viscoelastic materials becomes increasingly popular, stability of viscoelastic structures under random loads becomes increasingly important. This paper aims at studying the asymptotic stability of viscoelastic systems under Gaussian and Poisson white noise excitations with Lyapunov functions. The viscoelastic force is approximated as equivalent stiffness and damping terms. A stochastic differential equation is set up to represent randomly excited viscoelastic systems, from which a Lyapunov function is determined by intuition. The time derivative of this Lyapunov function is then obtained by stochastic averaging. Approximate conditions are derived for asymptotic Lyapunov stability with probability one of the viscoelastic system. Validity and utility of this approach are illustrated by a Duffing-type oscillator possessing viscoelastic forces, and the influence of different parameters on the stability region is delineated
Linear and Nonlinear Optical Properties of Graphene Quantum Dots: A Computational Study
Due to the advantage of tunability via size, shape, doping and relatively low
level of loss and high extent of spatial confinement, graphene quantum dots
(GQDs) are emerging as an effective way to control light by molecular
engineering. The collective excitation in GQDs shows both high energy plasmon
frequency along with frequencies in the terahertz (THz) region making these
systems powerful materials for photonic technologies. Here, we report a
systematic study of the linear and nonlinear optical properties of large
varieties of GQDs (400 systems) in size and topology utilizing the strengths of
both semiempirical and first-principles methods. Our detailed study shows how
the spectral shift and trends in the optical nonlinearity of GQDs depends on
their structure, size and shape. Among the circular, triangular, stripe, and
random shaped GQDs, we find that GQDs with inequivalent sublattice atoms always
possess lower HOMO-LUMO gap, broadband absorption and high nonlinear optical
coefficients. Also, we find that for majority of the GQDs with interesting
linear and nonlinear optical properties have zigzag edges, although reverse is
not always true. We strongly believe that our findings can act as guidelines to
design GQDs in optical parametric oscillators, higher harmonic generators and
optical modulators.Comment: 21 pages, 11 figures, 4 table
Recent activities within the Aeroservoelasticity Branch at the NASA Langley Research Center
The objective of research in aeroservoelasticity at the NASA Langley Research Center is to enhance the modeling, analysis, and multidisciplinary design methodologies for obtaining multifunction digital control systems for application to flexible flight vehicles. Recent accomplishments are discussed, and a status report on current activities within the Aeroservoelasticity Branch is presented. In the area of modeling, improvements to the Minimum-State Method of approximating unsteady aerodynamics are shown to provide precise, low-order aeroservoelastic models for design and simulation activities. Analytical methods based on Matched Filter Theory and Random Process Theory to provide efficient and direct predictions of the critical gust profile and the time-correlated gust loads for linear structural design considerations are also discussed. Two research projects leading towards improved design methodology are summarized. The first program is developing an integrated structure/control design capability based on hierarchical problem decomposition, multilevel optimization and analytical sensitivities. The second program provides procedures for obtaining low-order, robust digital control laws for aeroelastic applications. In terms of methodology validation and application the current activities associated with the Active Flexible Wing project are reviewed
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