The spatial and temporal structure of the flow, vorticity and stress over equilibrium orbital-scale sand ripples are investigated at turbulence-resolving scales with a wide-band coherent Doppler profiler (MFDop) in an oscillating tray apparatus. The oscillation period and horizontal excursion were 10 s and 0.5 m. Velocity profiles were acquired with 3 mm vertical resolution and at a 42 Hz sampling rate. Ripple wavelength and amplitude were 25 cm and 2.2 cm. The MFDop measurements are used to investigate the development of the lee vortex as a function of phase, and the co-evolution of turbulent kinetic energy, Reynolds stress and turbulence production. Shear stress is determined from the vertically-integrated vorticity equation and using the double-averaging approach. Friction factors obtained from the two methods are comparable and range from 0.1 to 0.2.

Orientador: Geraldo de Freitas Maciel / Resumo: No contexto de Desastres "Naturais", as corridas de lama têm sido objeto de estudo devido ao seu poder erosivo e, muitas vezes, destrutivo, acarretando perdas materiais vultosas e ceifando vidas. Nestes escoamentos, quando em condições favoráveis de vazão, inclinação e reologia do fluido, podem surgir instabilidades que se propagam em forma de trem de ondas na superfície, denominadas roll waves. A literatura acerca do assunto trata, geralmente, dos critérios de geração, estabilidade, e determinação das características principais do fenômeno, como amplitude, comprimento e celeridade de onda. Nesta dissertação buscou-se estudar a tensão de cisalhamento no fundo na presença de roll waves, em duas vertentes: a primeira apresenta as roll waves em água limpa, baseando-se no trabalho clássico de Dressler (1949); a segunda, dando continuidade aos trabalhos do Grupo de Pesquisa de Reologia de Materiais Viscosos e Viscoplástcos (Grupo RMVP), focou no estudo de roll waves desenvolvendo-se em fluidos do tipo Herschel-Bulkley, em duas situações - canal de fundo impermeável e fundo com condição de permeabilidade, grande contribuição desta dissertação. Os modelos matemáticos foram desenvolvidos com base nas equações de águas rasas, cuja implementação numérica permitiu confrontar resultados experimentais e numéricos, que apresentaram boa aderência. Para o modelo com condição de permeabilidade no fundo, verificou-se a influência do fator de porosidade nas características das roll waves (ampli... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Mudflows have been the focus of studies because of their erosive ability and often destructive power, causing material losses and taking away lives. Under favorable conditions of discharge, slope, disturbance, and rheology, these flows can develop a specific type of instability that is propagated downstream as shock waves, called roll waves. Most of the roll waves literature provides information on generation criteria, stability, and information of amplitudes, wavelengths, and celerity. This works brings a study of bottom shear stress in pulsating flows (roll waves) in 2 parts. In the first part, we present roll waves in clean water, based on Dressler’s work. In the second part, we continue the works of research team about roll waves developing in Herschel-Bulkley fluid under 2 conditions: impermeable bottom and porous bed. The mathematical models developed were based on shallow water equations. The results of these models were compared to Fluent and experimental results, showing a good agreement. For the mathematical model with porous bed condition, we evaluated the effect of porosity factor in properties of roll waves, and we observed that the amplitude of the roll wave usually increases, whereas the length and the celerity decrease with the presence of this factor. Finally, considering an impermeable bottom, the bottom shear stress presented an increase of 12% to 27%, depending on nature (laminar-turbulent) and dynamics (Froude number) of flow. Taking in account a porous b... (Complete abstract click electronic access below) / Mestre