Transport and mixing processes in stratified flow

The processes of transport and mixing in stratified open channel flows are investigated in this thesis. Detailed measurements of velocity and salinity were conducted, through the use of Laser-Induced Fluorescence (LIP) technique together with Laser Doppler anemometry, so that the effects of secondary current and stratification on the flow behaviour could be analysed. Two configurations were investigated: a rectangular open channel, and a compound open channel. For each configuration, four different stratification levels were analysed. The main flow characteristics, such as corner flow and velocity dip in a rectangular channel, and the twin vortices formed in compound channels, were found to be affected by stratification. In order to understand the mechanisms involved in secondary flow generation, the vorticity balance was carried out. Through the vorticity balance, the contribution of each term in the longitudinal vorticity equation could be evaluated. The mechanisms involved in the turbulence generation were also verified through the turbulent kinetic energy (TKE) budget. One of the contributions of this work refers to the understanding of the effects of stratification on turbulence and secondary flow generation. The exchange coefficients of momentum and solute were also investigated. These coefficients were found to depend not only on stratification level but also on other flow parameters, like for instance the aspect ratio. A new formulation is proposed for narrow channels, but more research is necessary in order to evaluate the effect of other parameters on the exchange coefficients.