Catalogue Search | MBRL
Are you sure you want to remove the book from the shelf?
{{itemTitle}}
18,716
result(s) for
"Wind waves."
Sort by:
The interaction of ocean waves and wind
This text describes the two-way interaction between wind and ocean waves, and shows how ocean waves affect weather forecasting on timescales of 5 to 90 days.
Book
Surface viscous stress over wind-driven waves with intermittent airflow separation
The small-scale physics within the first centimetres above the wavy air–sea interface are the gateway for transfers of momentum and scalars between the atmosphere and the ocean. We present an experimental investigation of the surface wind stress over laboratory wind-generated waves. Measurements were performed at the University of Delaware's large wind-wave-current facility using a recently developed state-of-the-art wind-wave imaging system. The system was deployed at a fetch of 22.7 m, with wind speeds from 2.19 to $16.63\\ \\textrm {m}\\ \\textrm {s}^{-1}$. Airflow velocity fields were acquired using particle image velocimetry above the wind waves down to $100\\ \\mathrm {\\mu }\\textrm {m}$ above the surface, and wave profiles were detected using laser-induced fluorescence. The airflow intermittently separates downwind of wave crests, starting at wind speeds as low as $2.19\\ \\textrm {m}\\ \\textrm {s}^{-1}$. Such events are accompanied by a dramatic drop in tangential viscous stress past the wave's crest, and a gradual regeneration of the viscous sublayer upon the following (downwind) crest. This contrasts with non-airflow separating waves, where the surface viscous stress drop is less significant. Airflow separation becomes increasingly dominant with increasing wind speed and wave slope $a k$ (where $a$ and $k$ are peak wave amplitude and wavenumber, respectively). At the highest wind speed ($16.63\\ \\textrm {m}\\ \\textrm {s}^{-1}$), airflow separation occurs over nearly 100 % of the wave crests. The total air–water momentum flux is partitioned between viscous stress and form drag at the interface. Viscous stress (respectively form drag) dominates at low (respectively high) wave slopes. Tangential viscous forcing makes a minor contribution (${\\sim }3\\,\\%$) to wave growth.
Journal Article
Spatial evolution of young wind waves: numerical modelling verified by experiments
A numerical model that allows one to study numerically the evolution of waves along the test section of a wind-wave tank is offered. The simulations are directly related to wind-wave tank experiments carried out for a range of steady wind velocities. At each wind forcing condition, the evolving wind-wave field is strongly non-homogeneous, with wave energy growth along the test section accompanied by frequency downshifting. The wave parameters measured at a short fetch serve as a basis for generating numerous realizations of the initial conditions in the Monte Carlo numerical simulations. The computations are based on a modified unidirectional spatial version of the Zakharov equation that accounts for wind input and dissipation and is applicable for the whole range of wind velocities employed. The model contains two empirical parameters that are selected by comparison of the experimental and numerical results; the same values of those parameters are applied for all wind forcing conditions. The availability of an experimentally verified numerical model allows one to study the contributions of nonlinear wave–wave interactions, dissipation and wind input separately. Special attention is given to accounting for the three-dimensional and random nature of wind waves as observed in experiments. The suggested model combines approaches adopted in the wind-wave growth theories by Miles and Phillips.
Journal Article
Revisiting wind wave growth with fully coupled direct numerical simulations
We investigate wind wave growth by direct numerical simulations solving for the two-phase Navier–Stokes equations. We consider the ratio of the wave speed $c$ to the wind friction velocity $u_*$ from $c/u_*= 2$ to 8, i.e. in the slow to intermediate wave regime; and initial wave steepness $ak$ from 0.1 to 0.3; the two being varied independently. The turbulent wind and the travelling, nearly monochromatic waves are fully coupled without any subgrid-scale models. The wall friction Reynolds number is 720. The novel fully coupled approach captures the simultaneous evolution of the wave amplitude and shape, together with the underwater boundary layer (drift current), up to wave breaking. The wave energy growth computed from the time-dependent surface elevation is in quantitative agreement with that computed from the surface pressure distribution, which confirms the leading role of the pressure forcing for finite amplitude gravity waves. The phase shift and the amplitude of the principal mode of surface pressure distribution are systematically reported, to provide direct evidence for possible wind wave growth theories. Intermittent and localised airflow separation is observed for steep waves with small wave age, but its effect on setting the phase-averaged pressure distribution is not drastically different from that of non-separated sheltering. We find that the wave form drag force is not a strong function of wave age but closely related to wave steepness. In addition, the history of wind wave coupling can affect the wave form drag, due to the wave crest shape and other complex coupling effects. The normalised wave growth rate we obtain agrees with previous studies. We make an effort to clarify various commonly adopted underlying assumptions, and to reconcile the scattering of the data between different previous theoretical, numerical and experimental results, as we revisit this longstanding problem with new numerical evidence.
Journal Article
Extreme wind-wave climate projections for the Indian Ocean under changing climate scenarios
Extreme wind-waves will impact coastal regions along the east and west coast of India and countries bordering the Indian Ocean rim having implications on coastal flooding and shoreline changes. Detailed investigation was carried out in this study on future extreme wave projections and its relationship with wind speed, sea level pressure (SLP), and sea-surface temperature (SST) for the mid- and end-century under RCP4.5 and RCP8.5 emission scenarios. Projections collectively highlight on maximum extreme wind and wave activity for south Indian Ocean (SIO) during June-July–August (JJA) and September–October–November (SON) seasons. Results indicate that end-century projections show extreme wind activity over central Bay of Bengal (BoB) signifying the likelihood for more extreme events over this region. Mid-century projections show that extreme significant wave height (SWH) intensifies by 1 m for the SIO during JJA season. Also, an increase of 0.4 m in maximum SWH (H
max
) is projected for regions in the NIO, northwest AS, northeast BoB, and South China Sea (SCS). Projections for eastern SIO show strengthening of extreme wind speed by 3.0 m/s by the end-century under RCP8.5. The projected changes in H
max
is maximum for SCS region evident in RCP4.5, whereas the maximum rise is about 23% under RCP8.5 in the end-century. Projected decline in winds and waves over western tropical IO (TIO) is consistent with weak SLP variations and warm ocean temperatures over that region. Significant increment in SST is projected over entire AS during DJF and JJA seasons ranging between 1.5 and 2.0 °C, which is 0.5 °C greater than BoB. Projections for the Gulf of Oman and Persian Gulf show higher warming rates exceeding 2 °C under RCP8.5 by the end-century. Warming in these areas further reduces SLP gradient leading to enhanced weakening of low-level atmospheric circulation and declining H
max
by the end-century. A westward alignment in these areas with maximum projected changes in SST is noticed in most seasons. Detailed analysis of high wave activity regions shows widening of generalized extreme value fitted probability density function over the BoB, SCS, and SIO indicating more extreme wave activity in the future.
Journal Article
Wind–wave coupling study using LES of wind and phase-resolved simulation of nonlinear waves
We present a study on the interaction between wind and water waves with a broad-band spectrum using wave-phase-resolved simulation with long-term wave field evolution. The wind turbulence is computed using large-eddy simulation and the wave field is simulated using a high-order spectral method. Numerical experiments are carried out for turbulent wind blowing over a wave field initialised using the Joint North Sea Wave Project spectrum, with various wind speeds considered. The results show that the waves, together with the mean wind flow and large turbulent eddies, have a significant impact on the wavenumber–frequency spectrum of the wind turbulence. It is found that the shear stress contributed by sweep events in turbulent wind is greatly enhanced as a result of the waves. The dependence of the wave growth rate on the wave age is consistent with the results in the literature. The probability density function and high-order statistics of the wave surface elevation deviate from the Gaussian distribution, manifesting the nonlinearity of the wave field. The shape of the change in the spectrum of wind-waves resembles that of the nonlinear wave–wave interactions, indicating the dominant role played by the nonlinear interactions in the evolution of the wave spectrum. The frequency downshift phenomenon is captured in our simulations wherein the wind-forced wave field evolves for$O(3000)$peak wave periods. Using the numerical result, we compute the universal constant in a wave-growth law proposed in the literature, and substantiate the scaling of wind–wave growth based on intrinsic wave properties.
Journal Article
The principal stage in wind-wave generation
The dynamics of wind-generated water waves in the principal stage of the Phillips theory (Phillips, J. Fluid Mech., vol. 2, 1957, pp. 417–445) is investigated by a combined numerical and analytical approach. We perform a number of high-resolution direct numerical simulation (DNS) of turbulent wind over initially calm water to capture the multistage generation of water waves. Detailed analyses are conducted to evaluate the Phillips theory in both physical space and wavenumber space. Numerical evidence is obtained for the existence of a principal stage when the surface elevation variance grows linearly with time. We further propose a random sweeping turbulence pressure–wave interaction model by introducing the random sweeping hypothesis of air pressure fluctuations to the Phillips theory, and obtain an asymptotic solution of the mean square of surface wave elevations over time. This asymptotic analysis captures the temporal oscillations of surface elevation variance in the principal stage, which is also confirmed by our DNS results. The wavenumber spectrum of surface wave elevations is analysed using a time-dependent norm to elucidate the role of the resonance mechanism on wave generation. In physical space, we use the random sweeping turbulence pressure–wave interaction model to obtain a quantitative prediction of the growth rate of surface elevation variance in the principal stage, which is found to agree with the DNS results better than the original Phillips model.
Journal Article
Laboratory study of the effect of mean water current on the evolution of young wind waves
The spatial evolution of various statistical parameters of fetch-limited waves generated by steadily blowing wind over mean water flow in a wind-wave flume is investigated experimentally. Measurements are performed in both along- and against-wind current conditions, and compared with measurements in the absence of current. A rake of capacitance-type wave gauges is used to measure surface elevation for a wide range of wind and water current velocities; additionally, an optical wave gauge is used to measure the directional properties of the wind-wave field in the presence of a mean water current at multiple locations. The variation with fetch of essential wave parameters such as characteristic wave energy, dominant frequency, power spectra and temporal coherence, as well as higher-order statistical moments that characterize wave shape, is presented for co- and counter-wind water currents, and compared with the no-current condition. The findings in the presence of mean water flow are interpreted in the framework of the viscous shear flow instability model of Geva & Shemer (Phys. Rev. Lett., vol. 128, 2022, 124501).
Journal Article
Evolution of water wave packets by wind in shallow water
We use the Korteweg–de Vries (KdV) equation, supplemented with several forcing/friction terms, to describe the evolution of wind-driven water wave packets in shallow water. The forcing/friction terms describe wind-wave growth due to critical level instability in the air, wave decay due to laminar friction in the water at the air–water interface, wave stress in the air near the interface induced by a turbulent wind and wave decay due to a turbulent bottom boundary layer. The outcome is a modified KdV–Burgers equation that can be a stable or unstable model depending on the forcing/friction parameters. To analyse the evolution of water wave packets, we adapt the Whitham modulation theory for a slowly varying periodic wave train with an emphasis on the solitary wave train limit. The main outcome is the predicted growth and decay rates due to the forcing/friction terms. Numerical simulations using a Fourier spectral method are performed to validate the theory for various cases of initial wave amplitudes and growth and/or decay parameter ranges. The results from the modulation theory agree well with these simulations. In most cases we examined, many solitary waves are generated, suggesting the formation of a soliton gas.
Journal Article
Spatial growth rates of young wind waves under steady wind forcing
The growth with fetch of young wind waves under steady wind forcing that is commonly attributed to shear flow instability results in a spatially inhomogeneous wave field with a spectrum evolving along the tank. The present laboratory study accounts for multiple co-existing statistically stationary random frequency harmonics. Single-point synchronous measurements of the instantaneous surface elevation and of its along-wind slope component are performed by optical methods at numerous locations. Assuming exponential spatial growth, the phase shift between the surface elevation and surface slope at each frequency is related to the spatial growth rate of each harmonic. The validity of the assumption that the wave energy varies exponentially with fetch is examined in a separate set of experiments; the instantaneous surface elevation at various wind-forcing conditions is measured at multiple locations along the tank. The spatial variation of the energy of individual frequency harmonics is determined. It is found that, below the local peak frequency, the energy of each harmonic grows exponentially, while the evolution of waves at frequencies approaching and exceeding the local peak is strongly affected by sheltering by the dominant wave, as well as by nonlinear bound waves. The outcomes of two independent methods of determination of spatial growth rates at a range of young wave frequencies are compared. The accumulated data also enable quantitative analysis of the sheltering phenomenon. The essential difference between the spatial and the temporal wind-wave evolution cases is discussed.
Journal Article