Aifeng Tao

The Most Unstable Conditions of Modulation Instability

Modulation instability is one of the most ubiquitous types of instabilities in nature. As one of the key characteristics of modulation instability, the most unstable condition attracts lots of attention. The most unstable condition is investigated here with two kinds of initial wave systems via a numerical high-order spectral method (HOS) for surface water wave field. Classically, one carrier wave and a pair of sidebands are implied as the first kind of initial wave system: “seeded” wave system. In the second kind of initial wave system: “un-seeded” wave system, only one carrier wave is implied. Two impressive new results are present. One result shows that the grow rates of lower and upper sideband are different within the “seeded” wave system. It means that, for a given wave steepness, the most unstable lower sideband is not in pair with the most unstable upper sideband. Another result shows the fastest growing sidebands are exactly in pair from “unseeded” wave system. And the most unstable conditions of “unseeded” wave system are more or less the mean value of those derived from the lower sidebands and upper sidebands within the “seeded” wave system.

New Concept for Assessment of Tidal Current Energy in Jiangsu Coast, China

Tidal current energy has attracted more and more attentions of coastal engineers in recent years, mainly due to its advantages of low environmental impact, long-term predictability, and large energy potential. In this study, a two-dimensional hydrodynamic model is applied to predict the distribution of mean density of tidal current energy and to determine a suitable site for energy exploitation in Jiangsu Coast. The simulation results including water elevation and tidal current (speed and direction) were validated with measured data, showing a reasonable agreement. Then, the model was used to evaluate the distribution of mean density of tidal current energy during springtide and neap tide in Jiangsu Coast. Considering the discontinuous performance of tidal current turbine, a new concept for assessing tidal current energy is introduced with three parameters: total operating time, dispersion of operating time, and mean operating time of tidal current turbine. The operating efficiency of tidal current turbine at three locations around radial submarine sand ridges was taken as examples for comparison, determining suitable sites for development of tidal current farm.

Evaluation of Tidal Stream Energy around Radial Sand Ridge System in the Southern Yellow Sea

Tidal stream energy as a new type of marine renewable energy attracts more and more attention of coastal engineers due to its advantages of great power density, long-term predictability and low environment impact. The radial sand ridge (RSR) system is located in the southern Yellow Sea off the Jiangsu coast, China, and it is characterized by a radial current field. The exploitation of tidal stream energy in the RSR system means great significance to the local industry and ecosystem of this region. In this study, a two-dimensional depthaveraged hydrodynamic model was established to predict one-year tidal hydrodynamics in the RSR system. The numerical results including water elevation and tidal current (speed and direction) were validated by the measured data, showing a reasonable agreement between simulation and measurement. Then, the validated model was applied to evaluate the tidal stream energy resources of the whole region with a concept consisting of total operating time (TOT), dispersion of operating time (DOT) and mean operating time (MOT), aiming to reveal the periodic interrupted working condition of tidal stream turbine. The distributions of TOT, DOT and MOT around the RSR system in one-year period are investigated, providing useful guide maps for the exploitation of tidal stream energy in this coastal area.

Swells of the East China Sea

Over the past few decades, an increasing number of marine activities have been conducted in the East China Sea, including the construction of various marine structures and the passage of large ships. Marine safety issues are paramount and are becoming more important with respect to the likely increase in size of ocean waves in relation to global climate change and associated typhoons. In addition, swells also can be very dangerous because they induce the resonance of floating structures, including ships. This study focuses on an investigation of swells in the East China Sea and uses hindcast data for waves over the past 5 years in a numerical model, WAVEWATCH III (WW3), together with historical climate data. The numerical calculation domain covers the entire North West Pacific. Next, swells are separated and analyzed using simulated wave fields, and both the characteristics and generation mechanisms of swells are investigated.

Experimental Study on Upstream-Advancing Waves Induced by Currents

ABSTRACT Fan, J.; Zheng, J.; Tao, A.; Yu, H, and Wang, Y, 2016. Experimental study on upstream-advancing waves induced by currents. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 846–850. Coconut Creek (Florida), ISSN 0749-0208. Due to the widespread existence of large-scale continuous submarine sandbars the coasts and estuaries, the interaction between the ambient current and this kind of topography is important and complicated. It has been known that regular free surface waves can be generated and propagate in the upstream direction, when a steady current with specific depth and velocity flows past the fixed periodic wavy bottoms with large steepness. This particular free surface response, called the upstream-advancing wave, has drawn attention in the past few decades but still lacks detailed quantitative study identifying the critical conditions and the generation mechanism. A series of experiments in a large-sized flume were designed and performed to obtain the critical flow condition corresponding to the most intensive upstream-advancing waves. The wavy bottoms were formed by eight continuous fixed sinusoidal corrugated wood surfaces with wave steepness of 0.33. Wave height distributions were measured by adjusting the relative water depth (the ratio of the water depth to the bottom wavelength, 0.5–1.1, seven groups) and Froude number (0.15–0.35, 21 cases in each group). These distributions presented a unimodal pattern generally. It indicates that the critical relative water depth was 0.8 and the corresponding critical Froude number was 0.26. Once the waves were triggered, the wave periods concentrated on a small range between 1.2 s and 1.4 s. Wavelengths kept basically invariable along the propagating path.

DISCUSSIONS ON THE OCCURRENCE PROBABILITIES OF OBSERVED FREAK WAVES

Field observational is the most direct and reliable research approach to investigate the freak waves, particularly for the occurrence probability. In order to present the latest research state, the results related to the observed freak waves from six sea areas are summarized and analyzed with comparisons. Three key conclusions are addressed. Firstly, both the occurrence probability and the strength of freak waves vary much for different sea areas. It is shown that there is still no answer to the big question of whether freak waves are rare events. Secondly, there is no single accepted, unified definition of freak waves. It means the scientists still do not exactly know what a freak wave is. Lastly, it is suggested better to study the combination of potential disaster risk and freak wave occurrence together instead of focusing only on the characteristics of freak waves. Because the results induced by even a same freak wave will change a lot for different structures and victims.