Turbulence Induced by Storm Waves on Deep Water

Abstract

Results of laboratory and field experiments performed in November-December, 2014 to study turbulence induced by wave motion are represented. Elevations of free sea surface and fluctuations of three components of water particle velocity are synchronously measured at calm conditions when wave breaking is absent. The data is obtained in the ranges of wave and turbulence frequencies, and in the depth range from the surface to half a length of surface waves. A method for separating the measured velocity fluctuations into at a wave and two turbulence components (the one is induced by wave motions and another is a background hydrodynamic turbulence) is developed. In field experiments the bound frequency of ~0.8 Hz had been determined. The coherence of the turbulent fluctuations and sea surface elevations sharply decreases above this frequency. Direct instrumental measurements confirmed stratification of a wave flow into a near-surface turbulent layer induced by wave motion and an underlying layer with the background hydrodynamic turbulence. The characteristics of turbulence in a near-surface layer are connected linearly with magnitude of envelope of vertical component velocity. This fact enables to attribute the turbulent fluctuations in this layer to wave-induced turbulence. Based on analysis of experiments the conclusion about existence of wave-induced turbulence in absentia other sources of turbulence such as wave breaking, wind stresses and Langmuir circulations is drawn. Introducton. According to the definition given by A.S. Monin and R. V. Ozmidov, turbulence is a phenomenon observed in many liquid and gas flows, and is that thermodynamical and hydrodynamical characteristics of such flows (velocity, temperature, density, pressure etc.) undergo chaotic fluctuations (caused by numerous vortices of different sizes in these flows) and owing to this change from point to point and in the course of time irregularly [1]. The occurrence of the turbulent velocity pulsations of water particles in a wave flow was experimentally registered for the first time with the light-polarization method by A. A. Dmitriev and T. V. Bonchkovskaya. In their paper are represented three consecutive photos of wave propagation at the still water, showing the leading wavefront to move farther from the survey point and the greater (by height) surficial area of flow to be involved in turbulent motion [2].The existence of laminar and turbulent mode areas in laboratory wave flow at the deep water was registered by S. V. Dobroklonsky and N. V. Kontoboytseva by means of Reynolds color jet method. Grains of paint were dropped into the water (during the disposition they left thin vertical traces), then wave generator was turned on and the observations, fixing the colored traces to keep their individuality, were performed [3]. As a result, the following facts were ascertained: turbulence is initiated in the upper flow layer with δ > 0.021 wave steepness; 2) subsurface turbulent layer thickness l T is linearly linked with wave height and length: 0.75 0.0...

Differing Aspects of Free and Bound Waves in Obtaining Orbital Velocities from Surface Wave Records

“…The methods of spectral and cross-spectral analysis were utilized in this study. Preprocessing of the laboratory experiment's time series involved linear filtering of frequencies above 3 Hz using the direct and inverse Fourier transform method to eliminate the influence of turbulence [35]. Additionally, linear filtering in the frequency and space domains was applied to the model data to extract free and bound wave chronograms.…”

Model and Experimental Estimates of Vertical Mixing Intensity in the Sea Upper Homogeneous Layer

“…The third turbulence generation mechanism is "hydrodynamic instability of wave motions in the upper mixed layer, induced by surface waves" [1, p. 40]. It is not taken into account in many models, although there is strong evidence of the influence of wave motions on turbulence [28][29][30][31][32][33]. The influence of surface waves on the turbulent regime (without analyzing the actual breakdowns) was discussed in detail in [34].…”