Numerical study on the Wave Boundary Layer, its interaction with turbulence and consequences on the wind energy resource in the offshore environment

Abstract

The wind energy farming in the offshore environment is characterized by ever-increasing structures and costs, for which reducing structural damage and maximizing production become an imperative. Such challenge is faced by site planning, intelligent design and active control systems that ultimately require a fine Atmospheric Boundary Layer (ABL) description. Contributing to refine the wind flow description in the offshore environment for engineering purposes, this work considers: (i) The interaction between turbulent and Wave Induced fluctuations, pronounced in the lower portion of the Marine ABL (MABL): Region so-called Wave Boundary Layer (WBL); (ii) The impact of the WBL in the flow above it. Focusing in the MABL sustained by non-equilibrium old-seas in neutral atmospheric conditions, the free-surface position and velocities are here prescribed into a Large Eddy Simulation (LES) according to a fifth order Stokes solution. The swell disturbances on the WBL are explored through mean profiles and spectral analyses. An original definition of the Wave Induced flow is presented, considering correlated turbulent and Wave Induced motions thus accessing the coupled dynamics between those fields and allowing the evaluation of the WBL height. Employing the proposed decomposition, the turbulent flow characteristics are recovered as expected in a flat bottom ABL, though some of its scales change considerably, forced by the WBL existent below.

A Dynamic Large-Scale Driving-Force to Control the Targeted Wind Speed in Large Eddy Simulations above Ocean Waves

“…Combined with the case-dependent WI stresses, and with the free-slip condition (zerovertical-gradient of horizontal velocities) imposed in the upper boundary, both of these strategies allow a varying wind speed profile across the vertical extension of the MABL. In old seas and neutral atmospheric conditions, we reported the speed-up of the longitudinal velocity up to the upper boundary of the numerical domain [52]. The WI disturbance of the freestream is unlikely to occur in the physical MABL but is intrinsic to the numerical formulations employed in the literature.…”

“…[46]). A pioneer LES over wavy surfaces was proposed in [47], and was followed by the developments of [48][49][50][51][52][53][54][55][56][57][58][59], with a recent review of the state-of-the-art methodology presented in [60]. The DNS model presented in Sullivan et al [38] was adapted in [48][49][50], into the LES model (from the NCAR, USA) employed herein.…”

“…To study large-scale dynamics, atmospheric LES approaches use these physics to drive the flow in the numerical domain [48,60]. To avoid having to model the entire ABL and above, a homogeneous longitudinal pressure gradient (∂P /∂x) is widely used in micro-scale studies to drive the flow instead [47,[49][50][51][52][53][54][55][56][57]59]. In this case, unlike in reality, the flow travels along the pressure gradient direction as in a wind tunnel.…”

“…In previous studies of wind-wave interactions by LES, the large-scale driving force (here ∂P /∂x) was usually prescribed as a constant [47][48][49][50][51][52][53][54][55][56][57]60]. As an exception, an arbitrary time-varying force was recently employed to simulate a wind gust event [59].…”

Evidence of Ocean Waves Signature in the Space–Time Turbulent Spectra of the Lower Marine Atmosphere Measured by a Scanning LiDAR

“…The discussion that follows assumes that the total fluctuations u R = ûR + ũR are decomposed into atmospheric turbulence ( ûR ) and WI motions ( ũR ). This WI decomposition is usually achieved by a wave-coherent (WC) filter [7,21,76]. Due to their importance and for the sake of brevity, WC and WI decompositions will be properly investigated in future works.…”