Understanding breaking waves
Lead Research Organisation:
Imperial College London
Department Name: Civil & Environmental Engineering
Abstract
From an engineering perspective, wave breaking is of particular interest in the design and re-assessment of offshore and coastal structures. As this is an area of active research, the process of wave breaking is probably one of the least understood topics in wave mechanics. Although it has significant effects on the magnitude of wave loads, the understanding of the process itself is rendered difficult due to the complexity given by the different length scales of turbulent eddies forming and the interaction between two fluid phases, amongst others.
The overarching aim of this research project is to further the understanding of wave breaking. The objectives of the project include:
- Examining whether a geometric criterion be successfully applied to detect wave breaking if more advanced transforms are employed to accurately calculate the wave number.
- Investigate the coupled effects of changes in crest speed and increase in particle kinematics in describing the formation of breaking waves and the role of nonlinearity, spectral type and bandwidth.
- Develop a new wave breaking identification method that only uses a single time history of the surface elevation, as typically recorded in field measurements.
- Develop a new numerical model that is capable of accurately incorporating nonlinearity, directional-spreading and wave breaking. This model should be computationally efficient such that it can run hundreds of simulations of random irregular waves to provide key statistics such as distributions for the wave height, crest elevation and underlying kinematics.
In order to achieve these objectives, the project will examine existing experimental investigations of breaking waves to determine the best breaking criteria, test the breaking detection methodology and quantify the energy dissipation caused by breaking waves. Computational fluid dynamics simulations will be run to complement the experimental measurements. A high-order spectral method will also be improved to include the effect of wave breaking.
The novel part of this research includes improving our understanding of wave breaking, providing an accurate method to detect breaking waves in the ocean from single-point time histories, quantifying the degree of energy dissipation that breaking causes in realistic sea states and providing a computationally-efficient numerical model that can accurately simulate random sea states that incorporates nonlinearities, directional spreading and wave breaking. This project is relevant to the "Fluid dynamics and aerodynamics" EPSRC research area.
The overarching aim of this research project is to further the understanding of wave breaking. The objectives of the project include:
- Examining whether a geometric criterion be successfully applied to detect wave breaking if more advanced transforms are employed to accurately calculate the wave number.
- Investigate the coupled effects of changes in crest speed and increase in particle kinematics in describing the formation of breaking waves and the role of nonlinearity, spectral type and bandwidth.
- Develop a new wave breaking identification method that only uses a single time history of the surface elevation, as typically recorded in field measurements.
- Develop a new numerical model that is capable of accurately incorporating nonlinearity, directional-spreading and wave breaking. This model should be computationally efficient such that it can run hundreds of simulations of random irregular waves to provide key statistics such as distributions for the wave height, crest elevation and underlying kinematics.
In order to achieve these objectives, the project will examine existing experimental investigations of breaking waves to determine the best breaking criteria, test the breaking detection methodology and quantify the energy dissipation caused by breaking waves. Computational fluid dynamics simulations will be run to complement the experimental measurements. A high-order spectral method will also be improved to include the effect of wave breaking.
The novel part of this research includes improving our understanding of wave breaking, providing an accurate method to detect breaking waves in the ocean from single-point time histories, quantifying the degree of energy dissipation that breaking causes in realistic sea states and providing a computationally-efficient numerical model that can accurately simulate random sea states that incorporates nonlinearities, directional spreading and wave breaking. This project is relevant to the "Fluid dynamics and aerodynamics" EPSRC research area.
Organisations
People |
ORCID iD |
| Marios Christou (Primary Supervisor) | |
| Constantin Craciunescu (Student) |
Publications
Craciunescu C
(2019)
The effect of wave crest speed in breaking waves
in IOP Conference Series: Materials Science and Engineering
Craciunescu C
(2020)
Wave breaking energy dissipation in long-crested focused wave groups based on JONSWAP spectra
in Applied Ocean Research
Craciunescu C
(2020)
On the calculation of wavenumber from measured time traces
in Applied Ocean Research
Craciunescu C C
(2019)
Identifying Breaking Waves from Measured Time Traces
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509486/1 | 01/10/2016 | 31/03/2022 | |||
| 1855727 | Studentship | EP/N509486/1 | 01/10/2016 | 31/03/2020 | Constantin Craciunescu |
| Description | This award has funded work into understanding waves breaking in the ocean, which are very complex and there are many open research questions. Several findings have resulted from this project, which has focused on studying the initiation, detection and experimental modelling of realistic breaking waves and ultimately on their inclusion within efficient numerical models. Historically, breaking thresholds have been quantified using geometric parameters, such as the wave steepness that depends on the wavelength, which therefore needs to be accurately estimated. The majority of field measurements record a time history of the water surface at a single location in the ocean. This directly provides the wave periods, but the user needs to assume a relationship between the wave period and wavelength: this is referred to as the dispersion equation. The present work has demonstrated that the linear dispersion equation, which is commonly used in design calculations, can be up to 30\% inaccurate; the nonlinear version of the dispersion equation performs only slightly better. An alternative proposed within the literature uses the Hilbert Transform, however, the present work found that this incorrectly estimates the wavelength by up to 20\% for broad-banded sea states. This project demonstrated that the best estimate of the wavelength (within 2\%) is provided by a highly-nonlinear numerical model, however it is computationally expensive requiring several hours to run. Therefore, the present work also provided simple empirical corrections to the linear dispersion equation for the largest waves in the ocean, which can easily be employed in design calculations. Given that field measurements only provide a time history of the water surface, it is not possible to directly ascertain whether waves are breaking; this requires a profile in space rather than in time. Therefore, this project developed a novel methodology using a numerical model to detect wave breaking within measured time-histories. The numerical model was shown to be in excellent agreement with experimental measurements and successfully identified breaking events. With respect to physical understanding, results show that increased water particle velocity coupled with reduced wave crest speed explains the formation of deep water breaking waves. The present study has provided new physical insight that explains why large waves see a reduction in their crest speeds. The results also confirm the validity of a recently proposed kinematic wave breaking criteria, which the present study has employed to include the effects of wave breaking into a numerical model. Futhermore, the present work involved new experimental investigations that quantified the energy dissipated by realistic breaking waves. These experiments demonstrated that a wave breaking dissipation coefficient needs to be increased by 85\% for realistic sea states. Finally, all these findings were implemented into an efficient numerical model allowing it to successfully incorporate breaking waves. Using this new numerical model, the present work was the first to replicate experimental measurements of wave crest elevations including nonlinear amplification and wave breaking in realistic sea states. The numerical model was compared with three large experimental datasets and it demonstrated excellent agreement across all cases. |
| Exploitation Route | There are several ways that the outcomes can be taken forward in academia and industry. First, the empirical corrections to the linear dispersion equation can be used to accurately calculate the wavelength from experimental investigations and field measurements, both of which typically only measure time-histories. This will improve academic research and is easily applicable within industry for design calculations. Second, the novel wave breaking detection methodology can be employed to determine the occurrence of wave breaking within measured time traces, whether recorded in the laboratory or in the field. This will aid with the quality control of field measurements, as well as enabling the frequency of wave breaking to be ascertained. Third, the new numerical model that includes wave breaking can be used in both academia and industry to determine wave statistics of realistic sea states. This could previously only be achieved by running expensive model test campaigns, the cost of which limits both the range as well as the duration of sea states that can be examined. Finally, the recommendations proposed within the present work can be put to use towards better and safer design practices and methodologies in the marine and offshore industries. |
| Sectors | Aerospace, Defence and Marine,Construction,Energy,Environment,Transport |