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.