Experimental Analysis of Waves in Environmental Flows

Abstract

This thesis presents three projects; two projects were conducted to study through laboratory experiments the applicability of linear wave theory on the prediction of interfacial internal waves in quasi-two-layer stratified water, and an additional project to study the complementarity of wind and solar energy resources for the electricity production in the north western of Africa.Interfacial internal wave excitation in the wake of towed ships is studied experimentally in a quasi-two-layer fluid. At a critical 'resonant' towing velocity, whose value depends on the structure of the vertical density profile, the amplitude of the internal wave train following the ship reaches a maximum, in unison with the development of a drag force acting on the vessel, known in the maritime literature as 'dead water'. The amplitudes and wavelengths of the emerging internal waves are evaluated for various ship speeds, ship lengths and stratification profiles. The results are compared to linear twoand three-layer theories of freely propagating waves and lee waves. We find that despite the fact that the observed internal waves can have considerable amplitudes, linear theories can still provide a surprisingly adequate description of subcritical-to-supercritical transition and the associated amplification of internal waves. We argue that the latter can be interpreted as a coalescence of frequencies of two fundamental stable wave motions, namely lee waves and propagating interfacial wave modes.The damping of water surface standing waves (seiche modes) and the associated excitation of baroclinic internal waves are studied experimentally in a quasi-two-layer laboratory setting with a topographic obstacle at the bottom representing a seabed sill.We find that topography-induced baroclinic wave drag contributes markedly to seiche damping in such systems. Two major pathways of barotropic-baroclinic energy conversions were observed: the stronger one -involving short-wavelength internal modes of large amplitudes -may occur when the node of the surface seiche is situated above the close vicinity of the sill. The weaker, less significant other pathway is the excitation of long waves or internal seiches along the pycnocline that may resonate with the low-frequency components of the decaying surface forcing. i We analyzed wind speed and solar irradiation data of high spatial and temporal resolution for an extended area of north-western Africa including the Mediterranean Sea.We ex-ploit the ERA5 data bank compiled and maintained by the European Centre for Medium Range Weather Forecast (ECMWF). One of the new products they provide is horizontal wind speed components at a height of 100 m (modern wind turbines have a hub height between 80 and 120 m). We demonstrate that the desert area is an optimal location for wind-and solar electricity production for two peculiar aspects. Firstly, the wind speeds at 100 m over the Sahara are al-most as large as wind speeds over the open sea. Wind speed differences between the standard 10 m altitude and 100 m level are...