36 research outputs found
Active and passive pitch-controlled flapping wing propulsors; usage of the wake structure as a performance qualifier
Economic and ecological needs dictate for an ever growing need for increased
efficiency, both in marine propulsion and energy saving systems. Biomimetic (flapping wing)
systems, have already shown a serious potential as propulsors [1] and an even greater as a
mechanism that converts energy from ship motions to thrust [2], [3]. In this paper,
the problem of passively (spring loaded) or actively pitched controlled wing is formulated
and solved using a free wake 3D Boundary Element Method [4]. For the spring loaded case, the
unsteady BEM code is used to calculate the instantaneous forcing (i.e. pitching moment)
entered in the nonlinear second order PDE in time, expressing equilibrium of moments
including damping and inertia, around the pitch axis. Systematic simulations were conducted for a
series of harmonically heaving wings of different aspect ratios, with the instantaneous pitch
selected either passively via a spring-damper system or actively using a proper control algorithm.
The results regarding developed mean thrust coefficient are presented in the form of systematic
diagrams compatible with the design diagrams introduced in [1], allowing comparison of the
different flapping wing propulsors. Results are also presented for the wake patterns of the
different configurations, at similar propulsive conditions, revealing the
connection between the propulsive effectiveness and 3D wake structure
Hydroelastic analysis of ice shelves under long wave excitation
Abstract. The transient hydroelastic response of an ice shelf under long wave excitation is analysed by means of the finite element method. The simple model, presented in this work, is used for the simulation of the generated kinematic and stress fields in an ice shelf, when the latter interacts with a tsunami wave. The ice shelf, being of large length compared to its thickness, is modelled as an elastic Euler-Bernoulli beam, constrained at the grounding line. The hydrodynamic field is represented by the linearised shallow water equations. The numerical solution is based on the development of a special hydroelastic finite element for the system of governing of equations. Motivated by the 2011 Sulzberger Ice Shelf (SIS) calving event and its correlation with the Honshu Tsunami, the SIS stable configuration is studied. The extreme values of the bending moment distribution in both space and time are examined. Finally, the location of these extrema is investigated for different values of ice shelf thickness and tsunami wave length
Propagation of acoustic-gravity waves in inhomogeneous ocean environment based on modal expansions and HP-FEM
A coupled mode model is presented for the propagation of acoustic-gravity waves
in layered ocean waveguides. The analysis extends previous work for acoustic waves in
inhomogeneous environment. The coupled mode system is derived by means of a variational
principle in conjunction with local mode series expansion, obtained by utilizing eigenfunction
systems defined in the vertical section. These are obtained through the solution of vertical
eigenvalue problems formulated along the waveguide. A crucial factor is the inclusion of
additional modes accounting for the effects of spatialy varying boundaries and interfaces. This
enhancement provides an implicit summation for the slowly convergent part of the localmode
series, rendering the series rapidly convergent, increasing substantialy the efficiency of
the method. Particular aspects of the method include high order Lagrange Finite Element
Methods for the solution of local vertical eigenvalue problems in the case of multilayered
waveguides, and Gauss-type quadrature for the computation of the coupled-mode system
coefficients. The above aspects make the present method quite efficient for long range
propagation in extended waveguides, such as the ones found in geophysical applications, e.g.
ocean basins, as only few modes are needed for the accurate representation of the wave field
Finite element simulation of long wave impact on floating breakwaters with variable stiffness
The hydroelastic response of flexible, floating breakwaters is a subject of interest for coastal engineering applications. In this study, a higher order hydroelastic finite element is applied to the simulation of floating breakwaters of variable stiffness undergoing long wave impact. The main aim is the evaluation of breakwater efficiency in terms of transmitted and reflected wave characteristics. It is established that, for the wave-lengths examined, the maximum amplitude and wave-length of the transmitted pulse are strongly dependent on the breakwater stiffness. Finally it is shown that for case of a periodic stiffness profile the transmitted energy is minimised when the modulation wavelength is comparable to the wavelength of the incoming excitation
Higher-order fem for nonlinear hydroelastic analysis of a floating elastic strip in shallow-water conditions
The hydroelastic response of a thin, nonlinear, elastic strip floating in shalow-water
environment is studied by means of a special higher order finite element scheme. Considering
non-negligible stress variation in lateral direction, the nonlinear beam model, developed by
Gao, is used for the simulation of large flexural displacement. Full hydroelastic coupling
between the floating strip and incident waves is assumed. The derived set of equations is
intended to serve as a simplified model for tsunami impact on Very Large Floating Structures
(VLFS) or ice floes. The proposed finite element method incorporates Hermite polynomials of
fifth degree for the approximation of the beam deflection/upper surface elevation in the
hydroelastic coupling region and 5-node Lagrange finite elements for the simulation of the
velocity potential in the water region. The resulting second order ordinary differential
equation system is converted into a first order one and integrated with respect to time with the
Crank-Nicolson method. Two distinct cases of long wave forcing, namely an elevation pulse
and an N-wave pulse, are considered. Comparisons against the respective results of the
standard, linear Euler-Bernoulli floating beam model are performed and the effect of large
displacement in the beam response is studied
Hydroelastic analysis of flapping-foil thrusters using a partitioned BEM-FEM
Understanding the mechanics of aquatic locomotion has been an active field of research for decades and continues to inspire technological solutions ranging from small-scale propulsion systems for autonomous underwater vehicles (AUVs) to larger-scale energy saving devices (ESDs) for ships. The bio-inspired thrust-producing kinematics are shared among most flapping-foil systems, however joint experimental and numerical research suggests that incorporating additional biomimetic features, such as hydrodynamic shape and elasticity, in new designs can enhance the efficiency. Focusing on the latter, the response of passively deforming wings is implicitly non-linear, since deformations affect the hydrodynamic load excitation and vice-versa. Therefore, fluid-structure interaction simulations are essential for accurate predictions of the wings’ response. In the present work, a cost-effective computational tool is proposed for the hydroelastic analysis of flexible flapping-foil thrusters, which consists of a 3-D unsteady boundary element method (BEM) weakly coupled with a finite element solver (FEM) based on plate elements. The verification of the present method is accomplished by means of comparison against experimental data from the literature. The prediction capabilities and the limitations of the weakly coupled BEM-FEM are discussed. Finally, the proposed numerical tools serve as the building blocks for the fully coupled BEM-FEM scheme that is currently under development
Evolution of surface gravity waves over a submarine canyon
The effects of a submarine canyon on the propagation of ocean surface waves
are examined with a three-dimensional coupled-mode model for wave propagation
over steep topography. Whereas the classical geometrical optics approximation
predicts an abrupt transition from complete transmission at small incidence
angles to no transmission at large angles, the full model predicts a more
gradual transition with partial reflection/transmission that is sensitive to
the canyon geometry and controlled by evanescent modes for small incidence
angles and relatively short waves. Model results for large incidence angles are
compared with data from directional wave buoys deployed around the rim and over
Scripps Canyon, near San Diego, California, during the Nearshore Canyon
Experiment (NCEX). Wave heights are observed to decay across the canyon by
about a factor 5 over a distance shorter than a wavelength. Yet, a spectral
refraction model predicts an even larger reduction by about a factor 10,
because low frequency components cannot cross the canyon in the geometrical
optics approximation. The coupled-mode model yields accurate results over and
behind the canyon. These results show that although most of the wave energy is
refractively trapped on the offshore rim of the canyon, a small fraction of the
wave energy 'tunnels' across the canyon. Simplifications of the model that
reduce it to the standard and modified mild slope equations also yield good
results, indicating that evanescent modes and high order bottom slope effects
are of minor importance for the energy transformation of waves propagating
across depth contours at large oblique angles
Wave modelling - the state of the art
This paper is the product of the wave modelling community and it tries to make a picture of the present situation in this branch of science, exploring the previous and the most recent results and looking ahead towards the solution of the problems we presently face. Both theory and applications are considered.
The many faces of the subject imply separate discussions. This is reflected into the single sections, seven of them, each dealing with a specific topic, the whole providing a broad and solid overview of the present state of the art. After an introduction framing the problem and the approach we followed, we deal in sequence with the following subjects: (Section) 2, generation by wind; 3, nonlinear interactions in deep water; 4, white-capping dissipation; 5, nonlinear interactions in shallow water; 6, dissipation at the sea bottom; 7, wave propagation; 8, numerics. The two final sections, 9 and 10, summarize the present situation from a general point of view and try to look at the future developments
HYDRODYNAMIC ANALYSIS OF FLOATING BODIES IN GENERAL BATHYMETRY
ABSTRACT A hybrid technique, based on the coupled-mode theory developed b