Edith Beatriz Camano Schettini

The influence of stable stratification on the transition to turbulence in a temporal mixing layer

The transition to turbulence in a stably stratified flow is a problem of considerable interest in fluid dynamics with applications in both geophysical sciences and engineering. This transition is controlled by competition between the vertical shear of the base flow and the buoyancy forces due to the density stratification. The present work investigates numerically the effect of stable stratification on the development of a Kelvin-Helmholtz (KH) instability and the formation of streamwise vortices, which are developed after the saturation of the primary billows of KH. The Direct Numerical Simulation (DNS) technique was used to solve the complete Navier-Stokes equations, in the Boussinesq approximation. Numerical tests were done with different Richardson numbers and forced initial conditions for velocity fluctuations. The results showed that high stratification inhibits the pairing process, reduces the buoyancy flux, weakens the vertical motions, decreases the thickness of the mixing layer and affects the formation of streamwise vortices. The three-dimensional results demonstrated that the streamwise vortices are clearly formed in non-stratified cases. In the stratified cases, on the other hand, the streamwise vortices are weakened, due to the streamwise density gradient, which decrease the levels of vorticity in the billows of KH, while increase in the braid zone.

Eclusas de Navegação: Caracterização das Pressões a Jusante das Válvulas de Enchimento e Esvaziamento para Operação a 60% e 70% de Abertura

Hydraulic problems in the filling and emptying systems, especially close to the valves, are among the major limitations to increasing the lock lift. This study provided a better understanding of the flow dynamics downstream from lock valves. This was achieved by conducting an experimental investigation in the Laboratório de Obras Hidráulicas of the Instituto de Pesquisas Hidráulicas at the Universidade Federal do Rio Grande do Sul. The behavior of the data showed that, for an equal ∆ , the conditions of 60% and 70% of valve opening would result in the lowest mean pressures for the culvert base and roof, respectively. For Reynolds greater than3 10 , it was possible to characterize the behavior of the mean and extreme pressures along the culvert base and roof by the relationship between dimensionless coefficients of position and pressure. Key-words: Navigation lock, reverse radial gate, reverse tainter gate, extreme pressures, experimental hydraulics.

DISTRIBUIÇÕES DE PROBABILIDADE DAS FLUTUAÇÕES DE VELOCIDADE E CONCENTRAÇÃO NOS PROCESSOS DE DISPERSÃO DE POLUENTES

Testes na atmosfera e num tunel de camada limite foram realizadospara avaliar o comportamento probabilistico das flutuacoes de velocidadee concentracao associadas a um processo de dispersao. Distribuicoesproximas a Gaussiana foram obtidas na analise das velocidades.Para as flutuacoes de concentracao, diferentes comportamentos podemser determinados, conforme a regiao do campo das concentracoes analisada.

Numerical Simulation Flow Analyses around a Cylinder under Transversal and Eight Trajectories

This work aims to study a two-dimensional incompressible flow around a cylinder in forced movement in order to understand the phenomena that occur in cylindrical structures under periodic oscillation using the Direct Numerical Simulation technique. The simulations were taken using the computational code named Incompact3d. It was used the Reynolds Number as 400 in all simulations. The trajectories transversal and eight shapes were simulated for a range of displacement amplitude. The Navier-Stokes and continuity equations were used to discretize the flow in a Cartesian mesh. It was used the third-order Runge-Kutta scheme with low-storage for the pass-time and the body was represented by the virtual boundary method. The temporal mean of the drag and lift coefficients and vortex shedding mode were computed. The results show large variations of the mean lift coefficient with the amplitude displacement for all trajectories, and so altering the vortex shedding mode. The 2S mode was observed for simulations with transversal trajectories, except for amplitude of 0.55D, which presented the 2P mode. Simulations with eight trajectory presented 2P mode in most cases, and for certain amplitudes the periodic shedding modes were not observed.