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Written by: Frederic Dias

Category: Blog (Science)

Published: 29 June 2024

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The Morison’s equation continues to be an important approximation of the load effect of waves on structures. However, such forces can evolve with time due to marine growth and the evolution of long term growth within the Morison equation is challenging. Incorporation of long term marine growth gives rise to two timescales associated with the Morison equation. A short time scale is associated with the period of a wave, while a long time scale is associated with the change in geometry of the structure due to marine growth. This paper proposes a new modification of the Morison’s equation where these multiple time scales of change are addressed. The proposed method allows for a better understanding of how the force predicted by the Morison equation changes over time. The approach allows considering marine growth as an integral part of the Morison equation, expanding the capability to handle a significantly wider range of conditions related to lifetime performance of marine infrastructure, like wind turbines. The proposed approach is compatible with ecological processes and consequently the sampling of randomness over time is physically and biologically viable. The proposed approach allows incorporation of changes in the geometry of these structures through simple biological sampling and subsequently provides a way of updating Morison’s equation to provide more accurate force estimates. This ecologically compatible geometry allows for forces estimates closer to reality, departing from the existing and simplistic smooth and rough regimes which are typically used as binary parameters (https://doi.org/10.1016/j.mechrescom.2024.104293).

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Written by: Frederic Dias

Category: Blog (Science)

Published: 29 June 2024

Hits: 176

Knowledge of the directional distribution of a wave field is crucial for a better understanding of complex air-sea interactions. However, the dynamic and unpredictable nature of ocean waves, combined with the limitations of existing measurement technologies and analysis techniques, make it difficult to obtain precise directional information, leading to a poor understanding of this important quantity. This study investigates the potential use of a wavelet-based method applied to GPS buoy observations as an alternative approach to the conventional methods for estimating the directional distribution of ocean waves. The results indicate that the wavelet-based estimations are consistently good when compared in the framework of widely used parameterisations for the directional distribution. The wavelet-based method presents advantages in comparison to the conventional methods, including being purely data-driven and not requiring any assumptions about the shape of the distribution. In addition, it was found that the wave directional distribution is narrower at the spectral peak and broadens asymmetrically at higher and lower scales, particularly sharply for frequencies below the peak. The directional spreading appears to be independent of the wave age across the entire range of frequencies, implying that the angular width of the directional spectrum is primarily controlled by nonlinear wave-wave interactions rather than by the wind forcing. These results support the use of the wavelet-based method as a practical alternative for the estimation of the wave directional distribution. In addition, this study highlights the need for continued innovation in the field of ocean wave measuring technologies and analysis techniques to improve our understanding of air-sea interactions (https://doi.org/10.1175/JTECH-D-23-0058.1).

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Written by: Frederic Dias

Category: Blog (Science)

Published: 25 May 2024

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This study focuses on improving the accuracy of the energy distribution in directional wave spectra from temporal sequences of marine radar images. Wave spectra are obtained by converting the image spectrum to a wave spectrum by employing a modulation transfer function (MTF). While the imaging mechanism in the plane of the radar beam has received much attention, the effect of the combination of different beam positions has been largely neglected. Herein, we demonstrate that any MTF can only be valid for a limited azimuthal coverage. However, the proposed directional MTF is well suited for small windows for which it corrects the relative projection between the average radar beam position and the wavenumber vector. Spectra of different sea states are compared, using the derived MTF and the established MTF. The analysis shows a significant improvement in the energy distribution in the directional wave spectra when the directional spreading is high. It is suggested that future work should aim to remove the necessary restriction to small windows by synthesizing the full directional spectrum from multiple small windows in different directions. In this way, marine radars are suitable to better reproduce sea states with extreme directional spreading, including multi-modal seas (https://doi.org/10.1016/j.oceaneng.2024.118126).

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Written by: Frederic Dias

Category: Blog (Science)

Published: 31 March 2024

Hits: 308

We analyze the microwave imaging of shoaling nearshore surface gravity waves during the process of steepening and breaking over two beaches for a wide range of environmental conditions. The extracted backscatter intensity and Doppler speed form distinct patterns that reveal a nonnegligible dependence on environmental conditions. The backscatter signature exhibited by breaking waves remained consistent and similar throughout all datasets. Before consolidating the findings, it is imperative to conduct a further investigation of radar imaging of nonlinear wave dynamics in shallow water to eliminate any possible influence of imaging mechanisms. The details are presented in a paper that just came out in IEEE Transactions on Geoscience and Remote Sensing (https://ieeexplore.ieee.org/document/10460567)