Experimental Investigation On Infragravity Waves

Surf beat is a low-frequency wave motion with a period of0.5-5.0min outside the surf zone. The research of surf beat has found that this kind of low-frequency wave motion plays an important role in the harbor tranquility, motions of the moored ships, sediment transport as well as tdesign coastal structures.In this thesis, a brief review on the studies of infragravity waves is present first, mainly focusing on the generation mechanisms of infragravity waves. By both experimental and numerical ways, some new aspects of infragravity waves are achieved:1. Physical experiments on low-frequency waves are succefully designed, bichromatic wave groups are used to excitate long waves. To supress the generation of spurious long waves in the wave flume, the second-order wave generation theory is adopted. In addtion, the shorelines of the slopes are replaced by horizontal reaches of small depths, which can reduce the reflection of waves near the shoreline significantly. Since free long waves are generated when the primary waves break near this abrupt change in slope, the reflection of long waves is reduced significantly. Therefore, the experiments for the first time make it is possible to directly observe the outgoing breakpoint forced long waves. The results indicate that the mechanics of breakpoint forced long waves dominate over the low-frequency wave field on the steep slope, while the released bound long waves are found to be significant on the mild slope, which are excited by bichromatic wave groups on a steep (1:10) slope and a mild (1:40) slope, are carried out.2. Physiccal experiment of bichromatic wave groups propagating on a submerged curvilinear sill is also conducted. In this experiment, the effects of the amplitude modulation, mean frequency(group main frequencies) and difference frequencies(low frequency) on the generation of low-frequency waves and higher harmonics are examined. It is found that the all three factors can signifiantly influence the generation of low-frequency waves.3. Nonlinear transformation of random waves over a steep slope (1:10) and a mild (1:40) slope are studied. Two experimental cases of random waves mechanically generated by JONSWAP spectra are used for this purpose. The wavelet-based bispectrum is adopted to study the nonlinear wave-wave interactions. The bispectrum reveals that low frequency waves on the mild slope can gain more energy that that on the steep slope, further verifing that the release mechanics of long waves on mild slops is significant. 4. A set of Boussinesq equations with improved dispersion relationship is used to simulate waves propagating on a mild slope. The source funtion is adopt to generate waves. The resluts show that the numerical model can correctly simulate long wave generation out the surf zone, but fails after wave breaking. The numerical results indicate that free long waves also can excite its higher harmonics.