Offshore Technology Research Center Research Articles

Wave radiation of a cycloidal wave energy converter

Numerical results from a three-dimensional radiation model are presented where a cycloidal wave energy converter (WEC) is interacting with an incoming straight crested airy wave. The radiation model was developed in response to experimental observations from 1:10 scale experiments which were conducted in the Texas A&M Offshore Technology Research center wave basin. These experiments were the first investigations involving a WEC where three dimensional wave radiation effects were present due to the fact that the span of the WEC was much smaller than the width of the basin. The radiation model predicted the observed surface wave patterns in the experiment well, and showed that radiation induced wave focusing increased the recoverable wave power beyond the two-dimensional predictions for small WEC spans, while approaching the two-dimensional limit for very large spans. The numerical model was subsequently used to investigate the sensitivity of the WEC to misalignment between the incoming waves and the WEC shaft as well as the impact of a gap in the blade setup of a double WEC. For misalignment, the loss in efficiency was found to be strongly dependent on the ratio between WEC span and incoming wavelength, where short spans (on the order of one wavelength or less) which are realistic for actual ocean deployment showed only minor reductions in efficiency, while very long spans were found to be more sensitive to misalignment. The blade gap in a double WEC setup was found to have a relatively minor effect (up to 30%) on efficiency. Efficiency was found to either increase or decrease depending on the size of the gap.

Fully Nonlinear Multidirectional Waves by a 3-D Viscous Numerical Wave Tank

A finite-difference scheme and a modified marker-and-cell (MAC) method are used for numerical wave tank (NWT) simulations to investigate the characteristics of nonlinear multidirectional waves. The Navier-Stokes (NS) equations are solved for two fluid layers and the boundary values updated at each time step by a finite-difference time-marching scheme in the frame of rectangular coordinate system. The fully nonlinear kinematic free-surface condition is satisfied by the density-function technique developed for two fluid layers. The directional incident waves are generated from the inflow boundary by prescribing a snakelike motion along the wavemaker direction. The outgoing waves are numerically dissipated inside an artificial damping zone located at the end of the tank. Using the NS-MAC NWT with both solid and transparent side-wall conditions, the effects of side-wall reflections are studied. Bull’s-eye waves are also numerically generated by the phase control of neighboring wavemaker segments or the reverse process of cylindrical wavemakers. The simulation results are compared with the computations by an independently developed potential-based NWT and the experiments conducted in the Offshore Technology Research Center’s 3-D wave basin.

Quantification of nonlinear phenomena in laboratory-generated random sea waves

In this paper higher-order statistical signal processing and Volterra series models of nonlinear phenomena are utilized to quantify the role of nonlinear second-order effects in laboratory-generated random seas. It is indicated how higher-order moment spectra may be utilized to determine a frequency-domain Volterra model of the linear and second-order wave physics occurring between two closely spaced points at which the random wave field is sampled. It will be shown that although the amount of ‘‘energy’’ associated with second-order effects is very small, such second-order phenomena play an important role in the generation of large amplitude waves. By inverse Fourier transforming the outputs of the linear and quadratic Volterra filters, it is demonstrated that the generation of large amplitude waves is due to momentary phase locking of the first- and second-order components. All of the phenomena mentioned above will be demonstrated with the aid of experimental data collected at the Offshore Technology Research Center’s Model Basin. [This study was supported by the National Science Foundation Engineering Research Centers Program Grant Number CDR-8721512 through the Offshore Technology Research Center (OTRC). The Volterra modeling techniques were developed under ONR Grant N00014-92-J-1046 and the Joint Services Electronics Program AFOSR F-49620-92-C-0027.]

The Offshore Technology Research Center

The Offshore Technology Research Center was founded in 1988 as a joint venture of Texas AM R. Peter Johnson is the facilities manager.