Wind-induced Water Waves Research Articles

Experimental study on characteristics of turbulence and sediment transport produced by wind-induced water waves

Many researchers and engineers have shown great interest in mass transfer processes produced by wind-induced waves. Such waves contribute significantly to the transfer of environmental materials, such as sediment and marine debris, and the turbulence occurring beneath the waves further complicates wave-induced mass transport. The phase cycle of wave motion is generally considered to be a key determinant of mean flow and turbulence. In aqueous environmental engineering, this relationship is a crucial one to investigate, since turbulence is closely related to mass transport. To address this question, we measured the time-series of instantaneous velocity vectors by means of particle image velocimetry in a laboratory flume to reveal the turbulence structure induced by wave motion. By using a wavelet analysis free of specific assumptions, we were able to decompose the instantaneous velocity data into mean current, wave motion, and turbulence components. This analysis allowed for the objective evaluation of the shear stresses related to wave energy and turbulence energy production. Furthermore, we found significant phase characteristics of energy transfer among mean velocity, wave, and turbulence components. In order to examine the diffusion and convection properties induced by wind waves, we also conducted tracking analysis of imaginary sediment markers. Our results support the conclusion that mass transfer induced by wind waves impacts the entire range of water depths, at least in relatively shallow aqueous environments.

Secondary current properties generated by wind-induced water waves in experimental conditions

Secondary currents such as the Langmuir circulation are of high interest in natural rivers and the ocean because they have striking impacts on scour, sedimentation, and mass transport. Basic characteristics have been well-studied in straight open-channel flows. However, little is known regarding secondary circulation induced by wind waves. The presented study describes the generation properties of wind waves observed in the laboratory tank. Wind-induced water waves are known to produce large scale circulations. The phenomenon is observed together with high-speed and low-speed streaks, convergence and divergence zones, respectively. Therefore, it is important to determine the hydrodynamic properties of secondary currents for wind-induced water waves within rivers and lakes. In this study, using two high-speed CMOS cameras, stereoscopic particle image velocimetry (PIV) measurements were conducted in order to reveal the distribution of all three components of velocity vectors. The experiments allowed us to investigate the three-dimensional turbulent structure under water waves and the generation mechanism of large-scale circulations. Additionally, a third CMOS camera was used to measure the spanwise profile of thefree-surface elevation. The time-series of velocity components and the free-surface were obtained simultaneously. From our experiments, free-surface variations were found to influence the instantaneous velocity distributions of the cross-sectional plane. We also considered thegeneration process by the phase analysis related to gravity waves and compared the contribution of the apparent stress.

Secondary current properties generated by wind-induced water waves in experimental conditions

Secondary current properties generated by wind-induced water waves in experimental conditions

風波流れの発達過程と３次元構造に関する実験的研究

Wind-induced water waves play a vital role in the mass and momentum exchange across interface between air and water. They are closely related to various scaled problems such as global warming and water environment, and it is thus important to know the flow structure of them. So, many researchers have conducted intensively theoretical, experimental and numerical studies on this flow. In particular, Langmuir circulation is well highlighted, which is a large-scale secondary current with longitudinal axis.-wave. However, most of previous studies using laboratory flume could not cover the longitudinal variations of current structure which has fetch-depending properties. Therefore, in the present study, stereoscopic PIV measurements were conducted at several fetches in order to reveal the fetch dependency of three dimensional current property and related turbulence structure.

PIV and PTV measurements in hydro-sciences with focus on turbulent open-channel flows

PIV is one of the most popular measurement techniques in hydraulic engineering as well as in fluid sciences. It has been applied to study various turbulent phenomena in laboratory experiments related to natural rivers, e.g., bursting phenomena near the bed, mixing layers observed at confluences, wake turbulence around dikes and piers, and so on. In these studies, PIV plays important roles in revealing the space-time structure of velocity fluctuations and coherent vortices. This review article focuses particularly on the applications of PIV to turbulent open-channel flows, which have been conducted for the past decade in Hydraulics Laboratory of Kyoto University. In Section 2, we introduce our experimental setup and PIV/PTV algorithm. In Section 3, we apply the PIV measurements to reveal turbulence characteristics and coherent structures in open-channel flows as well as in vegetated canopy flows. For complex flow situations, various applications of PIV to compound open-channel flows and wind-induced water waves are considered to reveal coherent vortices. In Section 4, we discuss some advanced PIV measurements in open-channel flows. The free-surface-elevation fluctuations and velocity components were measured simultaneously with two sets of cameras to examine phase-averaged parameters of turbulence. A multi-layer scanning PIV was developed to reveal 3D turbulence structure in compound open-channel flows. Our discriminator PIV/PTV was applied successfully to sediment-laden open-channel flows and revealed the fluid/particle interaction and the relationship between coherent structures and sediment concentration. Finally, we conducted simultaneous measurements of velocity and dye concentration with a combination of PIV and LIF in vegetated open-channel flow, which enables us to examine turbulent scalar flux of a passive contaminant.

Turbulence structure and coherent vortices in open-channel flows with wind-induced water waves

When wind-induced water waves appear over the free-surface flows such as natural rivers and artificial channels, large amounts of oxygen gas and heat are transported toward the river bed through the interface between water and wind layers. In contrast, a bed region is a kind of turbulent boundary layer, in which turbulence generation and its transport is promoted by the production of bed shear stress. In particular, coherent hairpin vortices, together with strong ejection events toward the outer part of the layer, promote mass and momentum exchanges between the inner and outer layers. It is inferred that such a near-bed turbulence may be influenced significantly by these air–water interfacial fluctuations accompanied with free-surface velocity shear and wind-induced water waves. However, these wind effects on the wall-turbulence structure are less understood. To address these exciting and challenging topics, we conducted particle imagery velocimetry (PIV) measurements in open-channel flows combined with air flows, and furthermore the present measured data allows us to investigate the effects of air–water interactions on turbulence structure through the whole depth region.

PIV studies on turbulence structure in air/water interface with wind-induced water waves

When wind appears over the free surface, water waves and turbulence are generated by an interfacial shear stress. In particular, turbulent diffusion promotes significantly mass and momentum transport beneath the interface between the water and air significantly in ocean and lakes, and thus it is very important for global environment problems to reveal such turbulence property and coherent structure. Simultaneous measurements of velocities and free-surface elevation allow us to conduct reasonably the phase analysis of the coherent structure in interfacial shear layer. Furthermore, multi-point measurements such as PIV are very powerful to detect the space-time structure of coherent motions. Therefore, in the present study, we developed a specially designed PIV system which can measure the velocity components and surface-elevation fluctuation simultaneously by using two sets of high-speed cameras to reveal the coherent structure in the interfacial shear layer.

Sediment Quality and Resuspension Rate in Mangueira Bay, Brazil

Mangueira Bay is located in southern Brazil (between 30° and 32° south latitude). It is a significant regional resource that lends itself to activities such as tourism, fishing, and fish and shellfish breeding. Studies performed by Rio Grande University (FURG) have demonstrated that, due to its natural conditions, Mangueira Bay is an area of shelter and feeding for the local biota. As a result, the possibility of implementing intensive shrimp breeding has been analyzed. It is therefore important to study the hydrodynamics, water quality and sediments of this water body. This paper describes the spatial and temporal distribution of heavy metals in sediments during monitoring campaigns carried out in October 2002 and June 2003 as a part of the “Estuaries and Coastal Areas. Basis and Tools for a More Sustainable Development” (ECOSUD) project. The distribution of metals in the bed was estimated as well as the potential for resuspension due to the waves caused by typical storm conditions similar to those measured in the May 2003 campaign. This sediment resuspension in the seabed can repollute the water column. Classic expressions were used to estimate the wind-induced water waves, the resuspension rate and the concentration of suspended material. The results show differences in spatial distribution between the two campaigns, but in both cases the south of Mangueira Bay was found to be the proper area for aquaculture. Furthermore, in all of the sediment samples analyzed, the different metals were present in concentrations under the limits stipulated by the Canadian Environmental Quality Guidelines of the Canadian Environmental Protection Agency (CEPA). The calculation of sediment resuspension demonstrated that conditions favor shrimp breeding along the southwest coast under typical winds from the west, especially in the southern part of Mangueira Bay.

風波・植生開水路流れの組織乱流構造に関する実験的研究

Enviromental problems about aquatic plants are now increasing in actual rivers. Coherent turbulence events such as ejections and sweeps are generated by the shear instability near the boundary between above-canopy layer and within-canopy layer. When wind-induced water-waves appear on the free-surface flows, a large amount of oxygen gas may be transported toward the vegetation layer, and it is infer red that the mass and momentum exchanges are promoted significantly by the coherent motions. It is thus very important in river environment to investigate these turbulence structure and coherent motions in such flows. In this study, we conducted turbulence measurements by using a high-speed PIV technique. It was found from time-averaged velocity profiles that some wave effects appear even in the canopy layer. The correlation analysis was also conducted in order to examine turbulence transport process promoted by the wind-induced air/water interface. Furthermore, relation between the coherent event and waves were revealed on the basis of the present PIV data.

風波発生下における植生キャノピー開水路流れの乱流構造について

It is very important in liver environment to reveal the hydrodynamic properties in open-channel flows with vegetated canopies, in which velocity pmfiles are largely changed in the vertical direction. In paiticular, windinduced water-waves generate strong down-bursting phenomenon which promotes mass, momentum and gas transfer toward the vegetation region across the air-water interfere. Therefore, it is reeded to investigate these turbulence mechanism and coherent motions in onzler to maintain the aquatic eco-system in actual rivers. In this study, we conducted turbulence measurements by using two-component laser Doppler anemometer (LDA), and revealed the turbulence structure in the vegetated open-channel flows with wind-induced water waves.

FUNDAMENTAL STUDY ON AIR-WATER INTERFACIAL COHERENT STRUCTURES IN WIND-INDUCED WATER WAVES FLOWS BY PIV MEASUREMENTS

This paper describes an experimental study on turbulent bursting phenomena across air-water interface in wind-induced water waves by using Particle-Image Velocimetry (pry). The purpose of this study is to clarify coherent structures near the air-water interface in a wind-driven shear layer, and to evaluate an effect on momentum transport due to the busting events from the interfacial water to water-body flow. As the results of experiments, it is shown that the water flow is a combined flow with a wall shear flow and orbital motions, and that the wave-induced fluctuations have major power in the whole velocity fluctuations. In particular, when the bursting events are detected near the air-water interface by human eyes in the time-series of PIV raw images, only the low-frequency fluctuations are enhanced obviously at that time in wavelet-filter analysis. Therefore, it is inferred that the mechanism of water momentum transport is organized by the wave-induced fluctuations from the air-water interface to the water bulk, not by the bursting events with small fluctuations. It is also recognized by PIV measurements that the busting events are closely related to the unsteady wave-induced velocity fluctuations near the interface.

EXPERIMENTAL STUDY ON MUTUAL TURBULENCE STRUCTURES ACROSS AIR-WATER INTERFACE IN WIND-INDUCED WATER WAVES BY SIMULTANEOUS LDA MEASUREMENTS

This paper describes an experimental study on mutual turbulence interactions across air-water interface in wind-induced water waves by using two sets of laser Doppler anemometers (LDA) at the air-water layers sim ultaneously. The purpose of this study is to clarify turbulence structures at a wind-driven shear layer of each phase (air and water), and to evaluate the turbulent momentum transport phenomena across air-water interface and from water near the interface to water-body flow. As the results of experiments, it is shown that the turbulent structure in an air layer is like a stream over a rough bed and that the water flow is a combined flow with a wall shear flow and orbital motions. In particular, as the wind blows stronger, it is well observed that the shape of an air-water interface changes dynamically and that the mechanism of water momentum transport is different from each case. It is clearly recognized by simultaneous LDA measurements that the time-series of raw air and water velocities and filter-conducted water velocities show some coherent busting phenomena with low frequent fluctuations

Wind-generated gravity-capillary waves: laboratory measurements of temporal growth rates using microwave backscatter

The growth rates of wind-induced water waves at fixed fetch were measured in a laboratory wave tank using microwave backscatter. The technique strongly filters out all wavenumber component pairs except for a narrow window at the resonant Bragg scattering conditions. For these waves the spectral amplitude was measured as a function of the time after a fixed wind was abruptly started. The radars were aligned to respond to waves travelling in the downwind direction at wavelengths of 0·7-7 cm. Wind speeds ranged from 0·5 to 15 m/s. Fetches of 1·0, 3·0 and 8·4 m were used. In every case, the spectral amplitude initially grew at a single exponential rate β over several orders of magnitude, and then abruptly ceased growing. No dependence of the growth rate on fetch was observed. For all wavelengths and wind speeds the data can be fitted by \[ \beta (k,u_{*},{\rm fetch})=f(k)\,u^n_{*}, \] with n = 1·484 ± 0·027. Here u* is the friction velocity obtained from vertical profiles of mean horizontal velocity. For each wind speed, f(k) had a relative maximum near k = kn ≃ 3·6 cm−1. Rough estimates of β/2ω, where ω is the water wave frequency, and of the wind stress supported by short waves indicate that the observed growth rates are qualitatively very large. These waves are tightly coupled to the wind, and play a significant role in the transfer of momentum from wind to water.