P. Naaijen

Real Time Wave Forecasting for Real Time Ship Motion Predictions

This paper presents results of a validation study into a linear short term wave and ship motion prediction model for long crested waves. Model experiments have been carried out during which wave elevations were measured at various distances down stream of the wave maker simultaneously. Comparison between predicted and measured wave elevation are presented for 6 different wave conditions. The theoretical relation between spectral content of an irregular long crested wave system and optimal prediction distance for a desired prediction time is explained and validated. It appears that predictions can be extended further into the future than expected based on this theoretical relation.© 2008 ASME

Phase Resolved Wave Prediction From Synthetic Radar Images

A method is presented for the inversion of images of the sea surface taken by nautical radar into wave elevation that is specifically suitable for the prediction of the wave elevation outside the observation domain covered by the radar. By means of a beam-wise analysis of the image obtained by a scanning radar, the image information is translated into wave elevation. Subsequently a 2D FFT is applied in order to obtain the directional wave components required for a linear propagation of the wave field. Assuming knowledge of the significant wave height, a method to obtain the correct scaling of the wave prediction is proposed. The proposed method is verified using synthetic radar images which are modelled by applying shadowing and tilt effect to synthesised short crested linear waves.Copyright

Limits to the extent of the spatio-temporal domain for deterministic wave prediction

The authors discuss the spatio-temporal domain, here referred to as the predictable zone, in which waves can be predicted deterministically based on an observation in a limited spatial or temporal domain. A key issue is whether the group or phase speed of the observed waves governs the extent of the predictable zone. The authors have addressed this issue again using linear wave theory on both computer-generated synthetic wave fields and laboratory experimental observations. The authors find that the group speed adequately indicates the predictable zone for forecasting horizons relevant for offshore and maritime applications.

Simulations for design and reconstruction of breaking waves in a wavetank

To determine forces on fixed and flexible structures such as wind mills and oil platforms, experiments in wave tanks are useful to investigate the impacts in various types of environmental waves. In this paper we show that the use of an efficient simulation code can optimize the experiments by designing the influx such that waves will break at a predefined position of the structure. The consecutive actual measurements agree well with the numerical design of the experiments. Using the measured elevation close by the wave maker as input, the software recovers the experimental data in great detail, even for rather short (up to L/D=1) and very steep breaking waves with steepness parameter (ak) till 0.4. n nThe experiments were carried out in the TUD-wavetank and the simulation is done by HaWaSSI-AB, a spatial-spectral implementation of a Hamiltonian Boussinesq model with an eddy-viscosity breaking mechanism that is initiated by a kinematic breaking condition.

Propagation of a Radar Modulated Ocean Wave Field

In this article a study has been carried out to explore the feasibility of a wave propagation model that is able to predict the wave field in a deterministic sense, based on remote observations of the sea surface. The surface is modulated in order to simulate images created by a marine radar operating at grazing incidence. The developed model uses an integral equation method, utilizing the frequency domain Green’s function which fulfills the linear free surface boundary condition. Synthesized observations of either the wave elevation or surface tilt at the source points are used to initialize the wave model. At each of the locations of the added remote free surface panels, time traces of the observed wave elevation or surface tilt can be recorded. A Fourier Transform (FFT) of these time traces yields the frequency domain description of the boundary condition that has to be satisfied by the wave potential. The derived Green’s function for the free surface source panels is then used to solve the source of strength at these panels. Once values have been found for the sources, the potential, and thus the surface elevation, may be calculated at the ship’s location.Copyright