The turbulent layer in the water at an air—water interface

Abstract

The velocity fields beneath an air—water interface have been determined in a laboratory facility for the cases of wind-generated waves, with wind speeds ranging from 1.5 to 13.1 m/s, and of wind-ruffled mechanically generated waves of about 22 mm amplitude and 1 Hz frequency, with wind speeds ranging from 1.7 to 6.2 m/s. The velocity was measured in a fixed frame of reference with a two-component, laser-Doppler anemometer. It was possible to determine the lengthscales and evaluate the behaviour of the mean, wave-related and turbulent components of the flows. The waves affect the mean flows, even though the profiles remain essentially logarithmic and the wave field conforms generally with the results of linear theory. In the wind-wave cases the turbulent quantities behave similarly to those in flows over flat plates. They have different trends in the mechanical-wave cases, suggesting a coupling between waves and turbulence. Finally, measured values of the mean wave-induced shear stress were negative, leading to an energy transfer from the waves to the mean flow.

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

“…In contrast, for water wave cases of H12, H13 and H15, any influences of water waves appear significantly in the Reynolds stress near the air/water interface, i.e., the negative values appear near the free surface. It is known that the negative values of the Reynolds stress occur under wave-induced water waves, as pointed out by Cheung and Street [2]. In order to reveal turbulence structure in the wave field in detail, the velocity fluctuation, i.e., turbulence component (u, v) should be decomposed to the wave component and background turbulence component.…”

Field Verification of ADCP Surface Gravity Wave Elevation Spectra

“…The proposed linear filtration technique is in some respect analogous to existing linear filtration techniques using cross-spectra of pressure and current velocity to separate orbital wave velocities and turbulence (Kitaigorodskii et al 1983;Terray and Bliven 1985;Cheung and Street 1988;Green 1992). These techniques use the assumption that the coherence between p or and ũ or is unity, which neglects the influence of directional spreading (Kitaigorodskii et al 1983;Herbers et al 1991).…”

Turbulence beneath finite amplitude water waves

“…Given the limitations of their PIV system, careful analysis would be required to apply turbulence theories derived for isotropic, steady flow such as the -5/3 slope Kolmogorov energy cascade to unsteady, anisotropic flows. It has been shown by Magnaudet and Thais (1995) and Cheung and Street (1988) that correctly filtering the non-turbulent components is crucial when attempting to adequately characterise turbulence in wavy flows. Chang and Liu (2000) showed that PIV data could wrongly be interpreted as turbulence, an artefact they named pseudo-turbulence.…”

“…Perhaps, the most advanced techniques were developed by Cheung and Street (1988) and Magnaudet and Thais (1995). They developed a suite of techniques to extract the turbulent velocity fluctuations from near-surface velocity motions.…”