Experimental Investigation Of Weakly Three-Dimensional Wave Breaking

The breaking of ocean waves is a common occurrence and plays an important role in ocean engineering and nonlinear wave dynamics.Actually,one should not lose sight of the fact that the waves on the ocean surface are multi-directional,which consequently could significantly affect the kinematics and kinetics of breaking waves.Therefore,it is very important to study the breaking waves considered the influence of directionality.However,the relating experimental research is still scarce.In order to improve the understanding of the three-dimensional wave breaking,a reasonable experimental model will seek to be established for the study of nonlinear interactions of two wave trains with different approach angles by applying a configuration with two waterways in the wave basin where the two wave trains propagate in reversed directions.After they meet in the specific region the nonlinear interactions occur,thus the wave breaking occurs.Since this is the simplest three-dimensional wave interaction experiment,it is called weakly three-dimensional wave interaction.For this thesis,the method of weakly three-dimensional wave interactions and breaking was firstly stated by author,which illustrates the important scientific significance of this research.At the same time,two wave trains with different approach angles are successfully simulated in the wave basin.Then the characteristics of weakly three-dimensional wave breaking are studied in detail through physical experiments.During the experiment,the influence of approach angle with 8° and 12° are considered for the studying of weakly three-dimensional breaking.Regular and focusing wave signals were used during the experiments and the wave surface characteristics,spectrum changes as well as energy loss by breaking are studied.In addition,a study of the two-dimensional wave focusing experiment was performed which reveals the phenomenon of wave focusing detuning.The major contributions of this dissertation are obtained as follows:1.The high-speed camera was applied to record the process of weakly three-dimensional interaction of two wave trains.This phenomenon was not observable in previous two-dimensional investigations that two wave trains enter the interaction region at the same time and interact together merging into a smooth,crescent-shaped crest.Compared with the previous study,the process of weakly three-dimensional interactions and breaking is more complicated,which means the direction angle plays a significant role in this process.2.The effects of different frequencies and initial wave steepness on weakly three-dimensional wave interactions and breaking are considered in the experiments.The wave surface profiles and the variations of wave height during the process of weakly three-dimensional interactions with downstream of interaction region for the regular wave case are investigated.The results show that the highest wave height is just related to the approach angle and the frequency of the regular wave as well as the initial wave steepness will play a significant role in the weakly three-dimensional interactions and breaking,that is,the higher of frequency,the larger of initial wave steepness the regular wave have,the more furious nonlinear interaction is.Additionally,under the same conditions,the larger approach angle contributes more severe interactions,which results in 3D wave surface profiles downstream of interaction region.3.In the meanwhile,the focusing wave signals with different initial wave steepness was also applied to research the weakly three-dimensional interactions and breaking.The wave surface profiles and spectra are studied,which finds that severe interactions and breaking results from larger initial wave steepness causing huge change in the wave spectra.At the same time,it can be seen that the energy loss due to breaking mainly from the high end of the first harmonic band.This is consistent with the results of the two-dimensional experiment.In addition,the approach angle of the waves has a significant influence on the energy transfer of the main frequency band,but the effect of the energy transfer of the low frequency and the high frequency is not prominent.4.According to the energy loss estimated from surface elevations,it is found that the energy dissipation depends strongly on the initial steepness.More interestingly,the energy loss was greater for the two-dimensional experiment than that of the weakly three-dimensional case.At the same time,the energy loss increases with the increase of approach angle.5.In addition,using an energy focusing technique,several transient wave trains were generated in a two-dimensional wave flume to investigate frequency detuning.By increasing the initial wave steepness,non-breaking through breaking conditions were achieved with prescribed breaking locations.It is found that the frequency detuning is an important factor that affect the energy dissipation.