Time-domain Higher-order Boundary Element Research Articles

Wave energy capturing of an offshore stationary platform equipped with three oscillating wave column devices

This paper deals with the hydrodynamic properties of an offshore stationary multi-functional platform consisting of triple oscillating water column (OWC) wave energy converters (WECs). The three OWCs are arranged on the vertices of an equilateral triangle, and one of them is forced to face the incident wave, and is labeled as the seaward OWC. Based on the potential flow theory, a second-order time-domain higher order boundary element method (HOBEM) model was established to investigate the wave energy capturing of this triple-OWCs-platform. It is found that there exists a quarter-cycle phase lag between the surface elevation and the air pressure measured in the chamber of the OWCs. In addition, the wave energy capturing capability of the seaward OWC is found to be improved significantly when the incident wave approaches the platform with an angle of 0 and π/3. Its maximum efficiency can reach up to 97.4% with the optimized device spacing. The averaged efficiency of the three OWCs at the resonant frequency is found to be enhanced when the device spacing is four times larger than the diameter of the OWCs.

Wave–current entry of an asymmetric wedge in 3DOF free motions

A time domain higher-order boundary element method (HOBEM) is developed to simulate the coupling process of a two-dimensional freely falling wedge entering obliquely waves in the presence of uniform currents. The wave–current-wedge slamming model is established in the stretched coordinate system, in which the velocity potential is decomposed as the potential induced by the current speed, the scattering potential and the incident potential at the initial stage. Some auxiliary functions are used to decouple the mutual dependence of structure motions with three degrees of freedom (3DOF) and fluid flow. The presented model is verified against the published first-order boundary element method (FOBEM) solutions for wedge entering calm water in free fall motion and wedge entering waves with the prescribed velocity in the absence of currents. Simulations and comparisons are then made for a wedge entering into waves in the following current, zero current and opposing current, respectively. Various parametric studies in the opposing current are performed for a wedge hitting wave crest vertically, obliquely and with initial rotational motion. Detailed results are provided through free surface elevation, pressure distribution, accelerations and velocities, and their physical implications are discussed.

Higher Harmonics Induced by Dual-Submerged Structures

ABSTRACT Ning, D.-z.; Li, Q.-x.; Chen, L.-f.; Zhao, M., and Teng, B., 2017. Higher harmonics induced by dual-submerged structures. Various types of submerged multibody systems are being inserted into and being devised in the real sea. Nonlinear diffraction of regular waves attributable to dual-submerged cylinders and boxes is investigated both experimentally and numerically in this study. The ultimate objective of the study is to provide best practice guidelines for the engineers in the field leading to the better design of the submerged structures with better hydrodynamic performance and higher survival characteristics. A two-dimensional fully nonlinear numerical wave flume based on a time-domain higher order boundary element method is used, and the comparisons between the numerical and experimental results are carried out for cross-checking both the numerical model and physical experiments. The influence of the submergence, gap distance, size, and geometric shape of the two submerged structures on the h...

A numerical study of the second-order wave excitation of ship springing by a higher-order boundary element method

ABSTRACT This paper presents some of the efforts by the authors towards numerical prediction of springing of ships. A time-domain Higher Order Boundary Element Method (HOBEM) based on cubic shape function is first presented to solve a complete second-order problem in terms of wave steepness and ship motions in a consistent manner. In order to avoid high order derivatives on the body surfaces, e.g. mj-terms, a new formulation of the Boundary Value Problem in a body-fixed coordinate system has been proposed instead of traditional formulation in inertial coordinate system. The local steady flow effects on the unsteady waves are taken into account. Double-body flow is used as the basis flow which is an appropriate approximation for ships with moderate forward speed. This numerical model was used to estimate the complete second order wave excitation of springing of a displacement ship at constant forward speeds.