Download files
Abstract
The interaction of ocean waves and turbulence is a fundamental process in the ocean. Understanding and prediction of wind-wave generation, propagation, breaking, non-linear interactions and extreme wave development are directly linked to this complex and not well understood phenomenon (Thorpe 2005). Despite its significance in ocean engineering, there are no complete theories that explain the influence of turbulence in these processes. Further, available laboratory and field observations are limited mainly due to the difficulty in measuring turbulence within a wave field, particularly in the region above the wave trough. Instead most engineering applications rely on parameterizations and formulas based on empirical relations and the linear wave theory, which potential flow assumption does not support turbulence (CEM2008, SPM84). This lack of understanding of the wave-turbulence interaction processes is in part responsible for inaccurate predictions that ultimately translate in higher failure risks, and infrastructure and operational costs for engineering and navigational assets.
In this investigation two new wave tank laboratory experiments have been developed to enhance present understanding on wave-turbulence interactions. The first experiment quantified the attenuation of deep water surface waves caused by rainfall induced subsurface turbulence. Significant challenges in the measurement and extraction of the turbulence statistics were overcome. Observations of near-surface velocity fluctuations revealed that wave attenuation rates induced by the unexpectedly weak rainfall-triggered turbulence were greater than previously anticipated by Teixeira and Belcher (2002). Measured near-surface velocity fluctuations are also in excellent agreement with Braun (2003) but contrast strongly with recent measurements by Zappa et al. (2009) and well-established theories that predict high levels of rainfall induced subsurface turbulence (e.g. Ho et al. 2000).
The second experiment intended to verify a controversial and recently proposed laminar-turbulence transition produced under steep freely propagating deep water waves (Babanin and Haus 2009). This new finding has major implications that could dramatically change present understanding of ocean waves. Dye visualization experiments showed no evidence of turbulence unless wave steepness was large enough to produce breaking. Moreover, unique measurements of the Stokes drift and the Stokes harmonic coefficients showed remarkable agreement with high order irrotational Stokes theories. However, measured higher order Stokes coefficients were larger than the theoretical predictions. Nevertheless, general good agreement of the present observations with the irrotational theories is in contrast with the recent challenges by Monismith et al. (2007) who proposed the cancellation of the Stokes drift and inferred closed orbital particle paths in freely propagating waves.