Procedia IUTAM最新文献

In this paper, we consider the problem of estimating a directional wave spectrum from 3-dimensional displacement data recorded by a wave buoy. We look at some of the limitations of existing methods to extend the "first five" directional moments directly obtainable from such data. With a view to providing the most detailed possible comparisons with directional spectra obtained from numerical models, we propose the use of a "diagnostic" directional spectrum, defined to be the closest possible spectrum to a given model spectrum that satisfies all measured directional moments. This method allows us to quantify the minimum error in a modelled directional spectrum consistent with a buoy record.

The new method is tested on a range of artificial test cases, and applied to data obtained from a wave buoy deployment off the New Zealand coast, in conjunction with outputs from a numerical spectral wave model simulation. It is shown that the method can provide satisfactory results in a wide range of conditions. Unlike existing approaches, the proposed method can accommodate sea states with more than two directional peaks, and can assist in removing spurious spectral energy arising from existing methods for estimating directional spectra from buoy data.

We compare two recently developed sets of source terms, based on different assumptions of wave energy input and dissipation, for Hasselmann equation. The numerical simulation, performed for limited fetch conditions with the constant wind speed shows the difference in total energy and mean frequency distributions along the fetch as well as in wave energy spectra. Possible reasons of such deviations are offered.

Oceanic rogue waves are a subject of great interest and can cause devastating consequences. Rogue waves are abnormal in that they stand out from the waves that surround them. Rogue waves are often observed accompanied by high wind in reality, and some earlier studies have demonstrated that the energy input due to the wind can enhance the dynamics of the rogue waves, which further causes huge concern about the safety of the human’s oceanic activities. Thus it is important, to better understand the mechanisms between the wind-wave interactions and to study the rogue waves with the presence of wind, especially on a three-dimensional large scale. In this study, numerical simulations are performed by using the Enhanced Spectral Boundary Integral (ESBI) method based on the fully nonlinear potential theory, in order to investigate the effects of wind on the rogue waves. The wind effects are introduced by imposing a wind-driven pressure on the free surface, which is empirically formulated based on intensive numerical investigation using multiple-phase Navier-Stokes solver. The results of the simulation confirm that the presented ESBI can produce satisfactory results on the formation of rogue waves under the action of wind. It provides a foresight of modelling rogue waves with presence of wind on a large scale in a phase-resolved fashion, which may motivate relevant studies in the future.

Progress in operational sea state forecasting is discussed in the context of the energy balance equation. This fundamental law describes the evolution of the wave spectrum due to adiabatic processes such as advection and refraction and due to physical processes such as generation of waves by wind, nonlinear interactions and wave dissipation. Progress in wave prediction is illustrated by means of a verification of operational wave height forecasts against wave height observations from buoys over the last 25 years of operational practice. Verification of modelled spectra against observed spectra by buoys is shown as well.

At the moment a number of weather forecasting centres spend a considerable amount of effort in the development of a fully comprehensive coupled atmosphere, ocean-wave, ocean circulation, sea-ice model. Central in the coupling of atmosphere and ocean in the ECMWF earth system model (see e.g. [1]) are the ocean waves that determine the momentum and energy transfer across the sea surface. In this paper we therefore concentrate of the sea-state dependence of the momentum (and heat) fluxes by studying in some detail the wind input source function of the energy balance equation. The importance of the strong coupling between wind and waves is illustrated by means of impact studies.

Stratified air-water flow in a horizontal pipe is investigated experimentally using particle image velocimetry and conductance probes. This flow regime is characterized by a complex interplay between a turbulent airflow and propagating waves at the interface. The waves are generated by interfacial shear and pressure forces exerted by the faster flowing airflow. The goal of this study is to characterize the waves by means of statistical and spectral methods, and to explore the influence of different wave regimes on the airflow.

Two cases in which the air bulk velocity increases from 2.4 m/s (case A) to 3.5 m/s (case B), while the liquid velocity remains constant at 0.26 m/s, are assessed in detail. Case A belongs to a region of flow conditions in which wave amplitudes grow as a consequence of increasing gas flow rates, i.e., wave growth regime. Meanwhile, case B is in a regime of saturated wave amplitudes. In the first case, the interface was populated by small amplitude 2D waves of relatively small steepness (ak ≈ 0.07). These waves obey Gaussian statistics and are thus considered to be linear. In the second case, the waves are larger, steeper (ak ≈ 0.13) and considerably more irregular. They display non-linear behaviour (steep crests and long troughs) and their exceedance probability distribution deviates significantly from Gaussian statistics. Bicoherence maps show evidence of both overtone and sub-harmonic interactions.

Airflow velocity fields acquired by PIV were subjected to a conditional phase-averaging method based on a steepness criterion. The phase-averaged vorticity field shows evidence of shear-layer separation above the steeper waves of case B. Hence, in addition to non-linear mode interactions and micro-breaking, shear-layer separation may contribute to the transition from the growth regime to the saturation regime.

The general characteristics of wind waves and drag coefficient are studied using the data from the buoy observation near shore in Taiwan Strait. An algorithm of the bulk aerodynamic method using 10-minute mean wind speed and the temperature difference between the air and sea surface was developed to calculate the equivalent wind speed at 10 m in height and the surface friction velocity. The observation shows that the sea states contains a wide range of wave ages driven by the synoptic wind systems, i.e. the strong northeast monsoon in winter and southwest monsoon in summer, mixed with the thermally diurnal variation across the sea-land boundaries. The large-scale winds generate a number of swells with long fetch and the mesoscale circulation perpendicular to the main streams causing wind and wave misalignments. The drag coefficients display considerable scattering around the linear growth formulation along with the increase of neutral wind speed. This is attributed to the dependence of surface roughness on wave ages. The present observation confirms that the drag coefficients are sensitive to the sea state described using the Charnock constant and hence the wave ages.

A self-sustained analytic theory of a wind-driven sea is presented. It is shown that the wave field can be separated into two ensembles: the Hasselmann sea that consists of long waves with frequency ω < ω_H, ω_H ~ 4 − 5ω_p (ω_p is the frequency of the spectral peak), and the Phillips sea with shorter waves. In the Hasselmann sea, which contains up to 95 % of wave energy, a resonant nonlinear interaction dominates over generation of wave energy by wind. White-cap dissipation in the Hasselmann sea in negligibly small. The resonant interaction forms a flux of energy into the Phillips sea, which plays a role of a universal sink of energy. This theory is supported by massive numerical experiments and explains the majority of pertinent experimental facts accumulated in physical oceanography.

We use our previous numerical and measuring experience and the evidence from a rather unique episode at sea to summarise our doubts on the present physical approach in wave modelling. The evidence strongly suggests that generation by wind and dissipation by white-capping have a different physics than presently considered. Most of all they should be viewed as part of a single physical process.

Recent results concerning transient effects of variation of short sea-surface wave roughness on near-surface turbulent wind are briefly outlined. This variation can be caused by oil, surfactants, inhomogeneous currents, internal waves, ship wakes, etc. To describe the wind parameters including surface stress and turbulent energy density, we use a direct solution of the Reynolds-type equations in the boundary-layer approximation. The solutions include a sharp and smooth roughness variation, 2-D surface variation, and a moving slick. The applicability of the theory was verified by comparison with laboratory data. Further on, the theory was applied to a problem related to the devastating tsunami near Fukushima Daichi in 2011.

Evolution of waves excited by wind that varies in time is not yet understood sufficiently well. In the present study, waves generated from rest by an effectively impulsive wind forcing are studied in a small laboratory wind-wave tank. Multiple parameters characterizing evolution of the wave field in time as well as in space are presented. Measurements of the variation with time of the instantaneous surface elevation were performed simultaneously with determination of two components of the instantaneous surface slope at a number of fetches along the test section. For each wind forcing conditions, numerous independent realizations were recorded. Thus, sufficient data were collected for computation of statistically reliable ensemble-averaged values of parameters characterizing the evolving random wind-wave field as a function of time elapsed since the initiation of wind. In each realization, data acquisition started when the water surface was calm, and lasted until statistically steady random wave field conditions were attained. The analysis of the ensemble-averaged wind-wave characteristics indicated that distinct stages in the wind-waves evolution could be identified. These stages were compared with the predictions based on the viscous instability theory and on the random resonant wind-waves generation model.