37,286 research outputs found
Computations of fully nonlinear three-dimensional wave-body interactions
Thesis (Ph. D. in Ocean Engineering)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references.Nonlinear effects in hydrodynamics of wave-body interaction problems become critically important when large-amplitude body motions and/or extreme surface waves are involved. Accurate prediction and understanding of these fully nonlinear effects are still challenges in the design of surface ships and marine structures, owing to the complexity of the hydrodynamic problem itself and limited computational facilities. This research is focused on: (i) development of a highly efficient numerical scheme for the computation of fully-nonlinear three-dimensional wave-body interactions; and (ii) investigation of several highly nonlinear wave-body interaction problems for understanding associated key nonlinear effects. A highly efficient high-order boundary element method is developed based on the framework of the quadratic boundary element method (QBEM) for the boundary integral equation and using the pre-corrected fast Fourier transform (PFFT) algorithm to accelerate the evaluation of far-field influences of source and/or normal dipole distributions on boundary elements. The resulting numerical scheme reduces the computational effort of solving the boundary-value problem from O(N 2 ~3) (with the traditional boundary element methods) to O(N ln N) where N represents the total number of boundary unknowns. Combining with the mixed-Eulerian-Lagrangian (MEL) approach for nonlinear free surface tracking, we develop an efficient and accurate initial boundary value problem (IBVP) solver, PFFT-QBEM, which allows for practical simulations of fully nonlinear three-dimensional wave-body interaction problems. Three nonlinear wave-body interaction problems, which are of scientific interest and practical importance, are investigated in detail: water surface impact of threedimensional objects, cavity dynamics in water entries, and coupled unstable motions of floating structures in waves. For the water impact problem, with the development of an adaptive jet flow treatment and an effective approach for accurately tracking water-body separation point/line, we obtain a thorough understanding of the gravity effect on the characteristics of slamming pressure/load on the object and free-surface profiles. For the cavity problem, we investigate the formation and evolution of an air cavity behind an object dropped into water (from air) at relatively low Froude numbers where the inertia and gravity effects are comparable. A theoretical solution is newly derived based on a matched asymptotic approach and a fully nonlinear numerical simulation is carried out, for the description of the kinematics and dynamics of the air cavity. Satisfactory quantitative comparisons are obtained among the theoretical predictions, numerical simulations, and existing experimental measurements for the dependence of cavity shape and closure time/height on Froude number and body geometry. For floating structures in waves, our focus is on the understanding of the fundamental mechanism and basic characteristics for coupled unstable heave-pitch motions of floating platforms/vessels. Through stability analyses, we identify that the second-order difference-frequency interaction between surface waves and body motions is the key mechanism for the excitation of unstable resonant motions. Fully nonlinear simulations are conducted to study the development of large-amplitude body motions and investigate quantitatively the dependence of the instability on related physical parameters, such as incident wave amplitude and phase, frequency detuning, body geometry, and system damping. Theoretical analyses and numerical simulations are verified by comparison to available experiments for the coupled unstable motions of a deep draft caisson vessel (DDCV).by Hongmei Yan.Ph.D.in Ocean Engineerin
Spectral/hp element methods: recent developments, applications, and perspectives
The spectral/hp element method combines the geometric flexibility of the
classical h-type finite element technique with the desirable numerical
properties of spectral methods, employing high-degree piecewise polynomial
basis functions on coarse finite element-type meshes. The spatial approximation
is based upon orthogonal polynomials, such as Legendre or Chebychev
polynomials, modified to accommodate C0-continuous expansions. Computationally
and theoretically, by increasing the polynomial order p, high-precision
solutions and fast convergence can be obtained and, in particular, under
certain regularity assumptions an exponential reduction in approximation error
between numerical and exact solutions can be achieved. This method has now been
applied in many simulation studies of both fundamental and practical
engineering flows. This paper briefly describes the formulation of the
spectral/hp element method and provides an overview of its application to
computational fluid dynamics. In particular, it focuses on the use the
spectral/hp element method in transitional flows and ocean engineering.
Finally, some of the major challenges to be overcome in order to use the
spectral/hp element method in more complex science and engineering applications
are discussed
Generalized thick strip modelling for vortex-induced vibration of long flexible cylinders
We propose a generalized strip modelling method that is computationally efficient for the VIV prediction of long flexible cylinders in three-dimensional incompressible flow. In order to overcome the shortcomings of conventional strip-theory-based 2D models, the fluid domain is divided into “thick” strips, which are sufficiently thick to locally resolve the small scale turbulence effects and three dimensionality of the flow around the cylinder. An attractive feature of the model is that we independently construct a three-dimensional scale resolving model for individual strips, which have local spanwise scale along the cylinder's axial direction and are only coupled through the structural model of the cylinder. Therefore, this approach is able to cover the full spectrum for fully resolved 3D modelling to 2D strip theory. The connection between these strips is achieved through the calculation of a tensioned beam equation, which is used to represent the dynamics of the flexible body. In the limit, however, a single “thick” strip would fill the full 3D domain. A parallel Fourier spectral/hp element method is employed to solve the 3D flow dynamics in the strip-domain, and then the VIV response prediction is achieved through the strip-structure interactions. Numerical tests on both laminar and turbulent flows as well as the comparison against the fully resolved DNS are presented to demonstrate the applicability of this approach
Time domain prediction of first- and second-order wave forces on rigid and elastic floating bodies
The application and development of a transient three-dimensional numerical code ITU-WAVE which is
based on panel method, potential theory and Neumann-Kelvin linearization is presented for the prediction
of hydrodynamics characteristics of mono-hull and multi-hull floating bodies. The time histories of unsteady
motions in ambient incident waves are directly presented with regards to impulse response functions (IRFs)
in time. The first order steady forces of wave-resistance, sinkage force and trim moment are solved as the
steady state limit of surge radiation IRFs. The numerical prediction of the second order mean force which
can be computed from quadratic product of first-order quantities is presented using near-field method
based on the direct pressure integration over floating body in time domain. The hydrodynamic and
structural parts are fully coupled through modal analysis for the solution of hydroelastic problem in which
Euler-Bernoulli beam is used for the structural analysis. A stiff structure is then studied assuming that
contributions of rigid body modes are much bigger than elastic modes. A discrete control of latching is used
to increase the bandwidth of the efficiency of Wave Energy Converters (WEC). ITU-WAVE numerical results
for different floating
Institute for Computational Mechanics in Propulsion (ICOMP) fourth annual review, 1989
The Institute for Computational Mechanics in Propulsion (ICOMP) is operated jointly by Case Western Reserve University and the NASA Lewis Research Center. The purpose of ICOMP is to develop techniques to improve problem solving capabilities in all aspects of computational mechanics related to propulsion. The activities at ICOMP during 1989 are described
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