Young Wind Waves Research Articles

Laboratory study of the effect of mean water current on the evolution of young wind waves

Laboratory study of the effect of mean water current on the evolution of young wind waves

Turbulence over young wind waves dominated by capillaries and micro-breakers

Turbulence over young wind waves dominated by capillaries and micro-breakers

Turbulent boundary layer profiles in airflow over young wind waves in co- and counter-wind water current

Understanding the air velocity profile over random and three-dimensional wind-waves is crucial for evaluating momentum and energy transfer between air and water. Unfortunately, determining accurate air velocity profiles in field conditions is nearly impossible. It is well accepted, though, that the airflow above water waves has a logarithmic profile usually expressed in terms of the effective roughness parameter. However, this parameter cannot be evaluated directly from the wave measurements at the water surface. For young wind waves, it was recently shown that the airflow over the waves maintains wall similarity. In this case, the airflow vertical velocity distribution can be described by the Nikuradse fully rough logarithmic profile, using the root mean square value of the water surface elevation as the equivalent sand grain roughness height. The existence of mean current in water in either co- or counter-wind direction may significantly modify the wave field, the vertical wind-velocity profile and thus the momentum and energy transfer from air to water. The effect of the water current on the spatially developing boundary layer over young wind waves and the wall similarity is examined. Combined laboratory measurements at several fetches of finely resolved mean air velocity profile above the water surface and of the characteristics of the wind-wave field are performed at multiple wind-forcing and mean water current conditions. The shear stress at the air–water interface estimated using two independent approaches is weakly dependent on current, while the resultant wave field differs significantly.

Wall similarity in turbulent boundary layers over wind waves

The structure of the steady boundary layer in the airflow over young wind waves is studied in detail using extensive data accumulated in our laboratory as well as in additional laboratory facilities and in field measurements. Following the approach adopted in studies of turbulent flow over solid rough surfaces, the coupling between the spatial evolution of wind waves and wind velocity profiles over the water is analysed. The roughness of the moving water surface under wind is not constant; wind waves that constitute the roughness elements are unsteady, random and three-dimensional. The effective water surface roughness increases with airflow velocity as well as with downstream distance. Nevertheless, the existence of wall similarity as observed in a flow over solid surfaces is demonstrated; a fully rough boundary layer is obtained for young wind waves at diverse operational conditions. This approach enables quantitative study of the coupling between the airflow and local wind wave characteristics.

Spatial evolution of young wind waves: numerical modelling verified by experiments

Abstract

On Evolution of Young Wind Waves in Time and Space

The mechanisms governing the evolution of the wind-wave field in time and in space are not yet fully understood. Various theoretical approaches have been offered to model wind-wave generation. To examine their validity, detailed and accurate experiments under controlled conditions have to be carried out. Since it is next to impossible to get the required control of the governing parameters and to accumulate detailed data in field experiments, laboratory studies are needed. Extensive previously unavailable results on the spatial and temporal variation of wind waves accumulated in our laboratory under a variety of wind-forcing conditions and using diverse measuring techniques are reviewed. The spatial characteristics of the wind-wave field were determined using stereo video imaging. The turbulent airflow above wind waves was investigated using an X-hot film. The wave field under steady wind forcing as well as evolving from rest under impulsive loading was studied. An extensive discussion of the various aspects of wind waves is presented from a single consistent viewpoint. The advantages of the stochastic approach suggested by Phillips over the deterministic theory of wind-wave generation introduced by Miles are demonstrated. Essential differences between the spatial and the temporal analyses of wind waves’ evolution are discussed, leading to examination of the applicability of possible approaches to wind-wave modeling.

Structure of the Airflow above Surface Waves

AbstractIn recent years, much progress has been made to quantify the momentum exchange between the atmosphere and the oceans. The role of surface waves on the airflow dynamics is known to be significant, but our physical understanding remains incomplete. The authors present detailed airflow measurements taken in the laboratory for 17 different wind wave conditions with wave ages [determined by the ratio of the speed of the peak waves Cp to the air friction velocity u* (Cp/u*)] ranging from 1.4 to 66.7. For these experiments, a combined particle image velocimetry (PIV) and laser-induced fluorescence (LIF) technique was developed. Two-dimensional airflow velocity fields were obtained as low as 100 μm above the air–water interface. Temporal and spatial wave field characteristics were also obtained. When the wind stress is too weak to generate surface waves, the mean velocity profile follows the law of the wall. With waves present, turbulent structures are directly observed in the airflow, whereby low-horizontal-velocity air is ejected away from the surface and high-velocity fluid is swept downward. Quadrant analysis shows that such downward turbulent momentum flux events dominate the turbulent boundary layer. Airflow separation is observed above young wind waves (Cp/u*< 3.7), and the resulting spanwise vorticity layers detached from the surface produce intense wave-coherent turbulence. On average, the airflow over young waves (with Cp/u* = 3.7 and 6.5) is sheltered downwind of wave crests, above the height of the critical layer zc [defined by 〈u(zc)〉 = Cp]. Near the surface, the coupling of the airflow with the waves causes a reversed, upwind sheltering effect.

Analysis and Interpretation of Frequency–Wavenumber Spectra of Young Wind Waves

AbstractThe energy level and its directional distribution are key observations for understanding the energy balance in the wind-wave spectrum between wind-wave generation, nonlinear interactions, and dissipation. Here, properties of gravity waves are investigated from a fixed platform in the Black Sea, equipped with a stereo video system that resolves waves with frequency f up to 1.4 Hz and wavelengths from 0.6 to 11 m. One representative record is analyzed, corresponding to young wind waves with a peak frequency fp = 0.33 Hz and a wind speed of 13 m s−1. These measurements allow for a separation of the linear waves from the bound second-order harmonics. These harmonics are negligible for frequencies f up to 3 times fp but account for most of the energy at higher frequencies. The full spectrum is well described by a combination of linear components and the second-order spectrum. In the range 2fp to 4fp, the full frequency spectrum decays like f−5, which means a steeper decay of the linear spectrum. The directional spectrum exhibits a very pronounced bimodal distribution, with two peaks on either side of the wind direction, separated by 150° at 4fp. This large separation is associated with a significant amount of energy traveling in opposite directions and thus sources of underwater acoustic and seismic noise. The magnitude of these sources can be quantified by the overlap integral I(f), which is found to increase sharply from less than 0.01 at f = 2fp to 0.11 at f = 4fp and possibly up to 0.2 at f = 5fp, close to the 0.5π value proposed in previous studies.

Importance of wave age and resonance in storm surges: The case Xynthia, Bay of Biscay

This study aims to hindcast and analyze the storm surge associated with Xynthia, a mid-latitude depression that severely hit the French central part of the Bay of Biscay on the 27–28th of February 2010. The main losses in human lives and damages were caused by the associated storm surge, which locally exceeded 1.5m and peaked at the same time as a high spring tide, causing the flooding of low-lying coasts. A new storm surge modeling system was developed, based on the unstructured-grid circulation model SELFE and the spectral wave model WaveWatchIII. The modeling system was implemented over the North-East Atlantic Ocean and resulted in tidal and wave predictions with errors of the order of 3% and 15%, respectively. The storm surge associated with Xynthia was also well predicted along the Bay of Biscay, with only a slight underestimation of the surge peak by 3–8%. Numerical experiments were then performed to analyze the physical processes controlling the development of the storm surge and revealed firstly that the wind caused most of the water level anomaly through an Ekman setup process. The comparison between a wave-dependant and a quadratic parameterization to compute wind stress showed that the storm surge was strongly amplified by the presence of steep and young wind-waves, related to their rapid development in the restricted fetch of the Bay of Biscay. In the central part of the Bay of Biscay, both observed and predicted water level anomalies at landfall displayed ∼6h oscillations, with amplitudes of up to 0.2m (10–20% of the surge peak). An analytical shelf resonance model and numerical experiments demonstrated that the period of the observed oscillations corresponds to the resonant mode of the continental shelf in the central part of the Bay of Biscay. It is concluded that these oscillations originate from the interactions between the water level perturbation and the continental shelf and this phenomenon is expected to be relevant at other places along the world’s coastlines.

Case study of wave dependent drag coefficient in the Baltic Sea

Proper estimation of air flow over sea waves plays a key role in wave, storm surge, wind field prediction. Most hydrological models assume the drag coefficient C d to be dependent only on the local wind speed. However this is a simplification. As it is known (SMITH et al., 1991; DONELAN, 1982, 1997; JANSSEN, 1991), among other factors, the drag coefficient depends also on the stage of wave development. The aim of the work is the study of wave age dependent drag coefficient and wave induced stress in the Baltic Sea. The wave model WAM4, which is a coupled model, is used for study of wave age, wave stress and drag coefficient in the Baltic Sea during storm. It is shown in the paper that as expected significant variability of wave field is characteristic for the Baltic and young wind waves (with wave age 10-20) dominate. Spatial distribution of wave induced stress shows large variability as well. There exist situations when contribution of wave stress to the total stress is greater then 60% over the whole Baltic. Enhancement of wave age dependent drag coefficient (in the comparison with drag coefficient dependent only on wind speed, calculated using the Charnock formula) exceeds 50%.

Ordered motion in the turbulent boundary layer over wind waves

The turbulent boundary layer over young wind waves (C/u* ∼ 1, where C is the phase speed of wind waves and u* is the friction velocity) has been investigated in a laboratory tank. Ordered motions have been found, and their structures studied in detail. Visualization of the outer boundary layer (0.4δ–1δ, where δ is the boundary-layer thickness) by paraffin mist has demonstrated the existence of a train of large-scale ordered motions having a horizontal lengthscale that corresponds to the wavelength of the underlying wind waves. Hot-wire measurements combined with the visualization have shown that the passage of the outer boundary-layer bulge is related to the occurrence of a low-speed air mass, usually accompanied by an upward velocity to produce large Reynolds stress. In the vicinity of the wave surface (0–0.15δ), flow separation occurs over these wind waves. Instantaneous velocity shear measurements, using two hot wires 0.15 cm apart vertically, have detected a high-shear layer at the edge of the separation bubbles. This high-shear layer, the potential site for generating much turbulence, reattaches on the windward side of the preceding wind waves. A pressure rise and a shear-stress spike, expected near the reattachment region, could be the mechanisms for supplying energy to the wind waves.The bursting phenomena over wind waves have been examined in detail in the logarithmic boundary layer (0.15δ–0.3δ). The bursting phenomena are a major mechanism for producing Reynolds stress and have a specific relationship with the phase of the wind wave. To explain the bursting phenomena, two mechanisms (not present in the boundary layer over a flat plate) are proposed, involving air-flow separation and the large-scale ordered motions, respectively. The two mechanisms are a ‘big burst’ related to the discharge of a whole separation bubble, and a ‘small burst’ which is the upward bursting of a low-speed air mass from the unstable separated shear layer into the ordered motions passing over a separation bubble.