Yves Perignon

Swell dissipation by induced atmospheric shear stress

Observations of swell dissipation across oceans reveal a significant loss of energy that can be the result of many of processes. Among these candidate mechanisms, this paper examines the properties of the viscous air-sea boundary layer driven by swells in order to characterize the induced atmospheric flow regime and its associated viscous dissipation over swells. A series of 3-D numerical experiments is carried out with a RANS model and appropriate turbulence closure. These experiments reveal a laminar to turbulent transition in the near free-surface region for a common range of characteristic amplitudes and periods of swells under stationary conditions. At low Reynolds number, laminar conditions prevail and computed decay rates conform to the analytical formulation μν of the Stokes interfacial boundary layer for this problem. The turbulent regimes are characterized as well, and the new decay rates follow a nondimensional relation μ=1.42μν(Re⁡1.5×105)0.41 above Re⁡=1.5×105 (e.g., amplitude larger than 1.1 m for a 14 s monochromatic wave period). Typical decay rates are up to 4 times above the laminar values, which is a factor 10 less than the largest rates estimated for oceanic conditions. A sensitivity analysis is finally conducted to evaluate the influence of the stationary hypothesis. It demonstrates a short setup length and low relative variations of the unsteady decay rates for laminar, transitioning and developed turbulent conditions, which confirms the evaluation of steady decay rates.

Extreme Sea Conditions in Shallow Water: Estimations Based on In-Situ Measurements

This paper focuses on the assessment of the environmental extreme conditions in term of wind and waves at the SEMREV wave energy test site for application in the design of Marine Renewable Energy (MRE) devices and components. The paper will first present the existing in situ wind and wave measurements. A prediction chain from global to regional scales, and based on a regional wave model calibrated at the SEMREV location, is then described. It enables to build a specific 22 year hindcast dataset for the test site. Long term extrapolation is finally achieved at the SEMREV location using existing methodologies for deep water conditions. Long term extrapolations methods are usually very sensitive to the parameterization and configuration of the prediction chain and we demonstrate in this study that the best overall performances are reached by a POT method at this stage of development in the prediction chain.Copyright

A Surface KInematics Buoy (SKIB) for wave-current interactionsstudies

Global navigation satellite systems (GNSSs) and modern motion-sensor packages allow the measurement of ocean surface waves with low-cost drifters. Drifting along or across current gradients provides unique measurements of wave–current interactions. In this study, we investigate the response of several combinations of GNSS receiver, motionsensor package and hull design in order to define a prototype “surface kinematics buoy” (SKIB) that is particularly optimized for measuring wave–current interactions, including relatively short wave components that are important for air–sea interactions and remote-sensing applications. The comparison with existing Datawell Directional Waverider and Surface Wave Instrument Float with Tracking (SWIFT) buoys, as well as stereo-video imagery, demonstrates the performance of SKIB. The use of low-cost accelerometers and a spherical ribbed and skirted hull design provides acceptable heave spectra E(f ) from 0.09 to 1 Hz with an acceleration noise level (2πf )4E(f ) close to 0.023 m2 s−3. Velocity estimates from GNSS receivers yield a mean direction and directional spread. Using a low-power acquisition board allows autonomous deployments over several months with data transmitted by satellite. The capability to measure currentinduced wave variations is illustrated with data acquired in a macro-tidal coastal environment.

Non-Linear Initialization in Three-Dimensional High Order Spectra Deterministic Sea State Modeling

This study deals with the initialization of three-dimensional wave field computations. We carry out such simulations with an HOS model developed at LMF-ECN since 2002 and based on the work of West et al. (1987) and Dommermuth & Yue (1987). In such models, initial conditions for three-dimensional realistic sea state computation are obtained by linearly distributing energy density spectrum. This however implies a relaxation of the non-linear effects as proposed by Dommermuth (2000) for bi-dimensional monochromatic wave train and Tanaka (2001), over several tenths of wave periods. The present work tests those former initialization methods and exposes an alternative initialization based on a non-linear three-dimensional approach. Non-linear interaction processes are both accounted in the spectra of elevation and potential of velocity, in accordance with the formulation of Dalzell (1999) at second order in wave steepness. Non-linear energy calculation is then addressed and the efficiency of the methods as well as their possible impact on properties and statistics of the wave field are investigated.Copyright