Coherent structures in open channel flows with bed load transport over an hydra...

• Author(s): Santos, Bruno José Oliveira
• Date: 2013
• Persistent ID: http://hdl.handle.net/10362/11204
• Origin: Repositório Institucional da UNL
• Subject(s): Bursting phenomena; Coherent structures; Quadrant threshold; PIV; Sediment transport
• Document Type: Master Thesis
• Language: English
• Advisor(s): Franca, Mário; Ferreira, Rui

Description

Dissertação para obtenção do Grau de Mestre em Engenharia Civil – Perfil de Estruturas The degradation of forests through the impacts of devastating wildfires increase unprotected soils area which consequently favours soil erosion processes. The sediment production is continuously reaching water courses in these areas which may result in important impacts in the flow morphodynamics and hydrodynamics. Sediment overfeeding induces important changes in the turbulent structure of the flow, mainly in momentum fluxes and exchange of momentum and mass between different layers in the flow structure, consequently affecting its ecological features. Coherent structures play an important role on sediment transport and mixing processes which are important in the fluxes that govern the turbulent structure. This study is aimed at evaluating the impacts of sediment transport on flow hydrodynamics, namely on the statistics characterizing coherent movements. In order to accomplish the purposed objective, experimental tests were undertook in laboratorial environment where two-dimensional instantaneous flow velocity fields in both directions, streamwise and vertical, were measured through means of Particle Image Velocimetry (PIV) technique. Two laboratory tests were simulated, consisting on a framework gravel bed with sand matrix and a framework gravel bed with sediment transport imposed at near capacity conditions. For both tests, the quadrant threshold analysis technique was employed and shear stress distribution statistics were analysed and discussed in what concerns their contribution and persistence. The results show that, in the near bed region, mobile bed conditions make sweep events assume a major role in the shear stress production processes. Also, larger events become less frequent in the pythmenic region, comparing with the immobile bed results. The impacts of mobile sediment in the near bed region over the flow structure are analysed and discussed in detail through probability density function distributions, in dimensional and non-dimensional data.