Étude numérique d'écoulements de fluides par une méthode vortex la marche descendante et les cavités sur les ailes de papillons

Dans cette thèse sont considérés des écoulements isothermes de fluides visqueux (newtoniens) et incompressibles sans forces de volume. Une méthode numérique bidimensionnelle basée sur l’équation du transport de la vorticité est utilisée pour simuler les écoulements en considérant trois modèles géométriques. Plus spécifiquement, la méthode des vortex aléatoires est combinée à l’algorithme Vortex-in-cell où des transformations algébriques permettent de raffiner le maillage aux endroits critiques des écoulements. Dans cette méthode mixte eulérienne-lagrangienne, la discrétisation principale se fait au niveau du champ de vorticité, celui-ci étant représenté par un ensemble d’éléments lagrangiens (singuliers) qui transportent chacun une quantité de circulation prédéterminée. Ces éléments de vorticité sont générés sur les parois solides pour vérifier la condition d’adhérence et évoluent dans le temps selon l’équation du transport de la vorticité. Le nombre d’éléments à être générés à chaque pas de temps est déterminé par l’entremise d’un nouvel algorithme de génération qui est construit à partir d’algorithmes existants. Étant bien étudié et bien connu, l’écoulement débutant (starting flow) au-dessus d’une marche descendante s’avère un excellent modèle de validation pour des méthodes numériques bidimensionnelles à nature instationnaire : des simulations numériques ont par conséquent été effectuées dans le cadre de cette thèse en considérant un tel modèle géométrique. Les résultats de simulations sont comparés avec succès à des résultats expérimentaux où une analyse plus générale démontre les aptitudes de la méthode à simuler ce type d’écoulements. En particulier, les résultats montrent que pour des écoulements à faibles valeurs du nombre de Reynolds (Re = 97), la zone de recirculation est composée d’une seule région de vorticité pour la durée totale des simulations. Pour des écoulements à valeurs plus élevées du nombre de Reynolds (Re = 153 et 303), la zone de recirculation est respectivement composée de trois et de quatre régions de vorticité durant les stades intermédiaires de l’écoulement, tandis que pour des temps plus élevés, les structures à l’intérieur de la zone de recirculation ne sont pas très bien définies. Les résultats dévoilent aussi que, pour la gamme complète des valeurs du nombre de Reynolds étudiée, la distance qui sépare la paroi verticale et le point de recollement de la région principale de vorticité augmente quasi linéairement avec le temps durant les stades intermédiaires de l’écoulement.Abstract: Isothermal flows of viscous (Newtonian) fluids without body forces are considered in this thesis. A two-dimensional numerical method based on the vorticity transport equation is used to simulate flows for three different model geometries. Specifically, the Random Vortex method is combined with the Vortex-In-Cell algorithm. In this mixed Eulerian-Lagrangian method, the main discretization is performed on the vorticity field, which is represented by a number of singular Lagrangian vortex elements. These vortex elements are generated on solid walls to verify the no-slip condition and are subsequently transported in time according to the vorticity transport equation. The laminar starting flow down a step is a well defined flow. Considering this flow model, instantaneous numerical simulations have been carried out and the results are compared to experimental results in the literature. Good agreement is obtained. Analysis of the numerical results and comparisons to experimental results show that for low Reynolds number flows (Re = 97), the recirculation zone is composed of one vorticity region throughout the simulations. For higher Reynolds number flows (Re = 153 and 303), the recirculation zone is respectively composed of three and four distinct vorticity regions at intermediate stages of its development, while for later times the structures inside the recirculation zone are not clearly defined. During intermediate stages of the flow development, and for the range of Reynolds numbers investigated, the distance from the step to the reattachment point of the main vorticity region increases quasi-linearly with the dimensionless time. Again, good agreement is obtained between experimental data and computed solutions. Using the same flow model, we present a numerical convergence proof by successively refining the numerical parameters. It is shown that the refinement of the vorticity generation parameters seems to primarily affect the smoothness of the solution rather than the overall structure of the flow. On the other hand, it is shown that for a given time step, higher order convection schemes improve the overall flow structure. Steady state numerical simulations were performed. Numerical streamwise velocity profiles are compared to their experimental counterparts for four Reynolds number flows (Re = 73, 125, 191 and 229). The computed length of the recirculation zone is also compared to various results. In all steady step flows, excellent agreement was obtained. Finally, both steady and unsteady flow simulations were performed on a model representing the cavities that result from the shingle-like arrangement of the scales on the upper surface of a butterfly wing. In gliding flight, the lifting force was experimentally proven to be stronger on a wing with scales. Excellent agreement is obtained in comparing results of our numerical simulations to experimental results in the literature. In particular, for low and very low Reynolds number flows (Re = 0.62, 1.00, 3.30 and 100), the recirculation zone is composed of one primary vorticity region. For higher Reynolds number flows (Re = 624), the recirculation zone area exhibits strong dynamics where coherent structures are shed at regular intervals from the vertical wall and eventually coalesce during intermediate stages of the developing flow"--Résumé abrégé par UM

Invasion of the Red Seaweed \u3cem\u3eHeterosiphonia japonica\u3c/em\u3e Spans Biogeographic Provinces in the Western North Atlantic Ocean

The recent invasion of the red alga Heterosiphonia japonica in the western North Atlantic Ocean has provided a unique opportunity to study invasion dynamics across a biogeographical barrier. Native to the western North Pacific Ocean, initial collections in 2007 and 2009 restricted the western North Atlantic range of this invader to Rhode Island, USA. However, through subtidal community surveys, we document the presence of Heterosiphonia in coastal waters from Maine to New York, USA, a distance of more than 700 km. This geographical distribution spans a well-known biogeographical barrier at Cape Cod, Massachusetts. Despite significant differences in subtidal community structure north and south of Cape Cod, Heterosiphonia was found at all but two sites surveyed in both biogeographic provinces, suggesting that this invader is capable of rapid expansion over broad geographic ranges. Across all sites surveyed, Heterosiphonia comprised 14% of the subtidal benthic community. However, average abundances of nearly 80% were found at some locations. As a drifting macrophyte, Heterosiphonia was found as intertidal wrack in abundances of up to 65% of the biomass washed up along beaches surveyed. Our surveys suggest that the high abundance of Heterosiphonia has already led to marked changes in subtidal community structure; we found significantly lower species richness in recipient communities with higher Heterosiphona abundances. Based on temperature and salinity tolerances of the European populations, we believe Heterosiphonia has the potential to invade and alter subtidal communities from Florida to Newfoundland in the western North Atlantic

Étude numérique d'écoulements de fluides par une méthode vortex la marche descendante et les cavités sur les ailes de papillons

Isothermal flows of viscous (Newtonian) fluids without body forces are considered in this thesis. A two-dimensional numerical method based on the vorticity transport equation is used to simulate flows for three different model geometries. Specifically, the Random Vortex method is combined with the Vortex-In-Cell algorithm. In this mixed Eulerian-Lagrangian method, the main discretization is performed on the vorticity field, which is represented by a number of singular Lagrangian vortex elements. These vortex elements are generated on solid walls to verify the no-slip condition and are subsequently transported in time according to the vorticity transport equation. The laminar starting flow down a step is a well defined flow. Considering this flow model, instantaneous numerical simulations have been carried out and the results are compared to experimental results in the literature. Good agreement is obtained. Analysis of the numerical results and comparisons to experimental results show that for low Reynolds number flows (Re = 97), the recirculation zone is composed of one vorticity region throughout the simulations. For higher Reynolds number flows (Re = 153 and 303), the recirculation zone is respectively composed of three and four distinct vorticity regions at intermediate stages of its development, while for later times the structures inside the recirculation zone are not clearly defined. During intermediate stages of the flow development, and for the range of Reynolds numbers investigated, the distance from the step to the reattachment point of the main vorticity region increases quasi-linearly with the dimensionless time. Again, good agreement is obtained between experimental data and computed solutions. Using the same flow model, we present a numerical convergence proof by successively refining the numerical parameters. It is shown that the refinement of the vorticity generation parameters seems to primarily affect the smoothness of the solution rather than the overall structure of the flow. On the other hand, it is shown that for a given time step, higher order convection schemes improve the overall flow structure. Steady state numerical simulations were performed. Numerical streamwise velocity profiles are compared to their experimental counterparts for four Reynolds number flows (Re = 73, 125, 191 and 229). The computed length of the recirculation zone is also compared to various results. In all steady step flows, excellent agreement was obtained. Finally, both steady and unsteady flow simulations were performed on a model representing the cavities that result from the shingle-like arrangement of the scales on the upper surface of a butterfly wing. In gliding flight, the lifting force was experimentally proven to be stronger on a wing with scales. Excellent agreement is obtained in comparing results of our numerical simulations to experimental results in the literature. In particular, for low and very low Reynolds number flows (Re = 0.62, 1.00, 3.30 and 100), the recirculation zone is composed of one primary vorticity region. For higher Reynolds number flows (Re = 624), the recirculation zone area exhibits strong dynamics where coherent structures are shed at regular intervals from the vertical wall and eventually coalesce during intermediate stages of the developing flow"--Résumé abrégé par UM

Learning Objects: Research and Development (R&D) Implications for the Australian Vocational Education and Training (VET) Sector: Rodrigue Savoie Critiques Technology for Sharing

NRC publication: Ye

Numerical Simulation of the Starting Flow Down a Step

The starting flow down a step in an open channel is simulated using a vortex method. A mixed Eulerian-Lagrangian scheme based on the vortex-in-cell algorithm and random walk diffusion is used with a non-uniform mesh. The comparison of numerical and experimental instantaneaous streamline plots show good agreement, especially for early stages of the flow development. It is also shown that, during intermediate stages of the flow development and for the range of Reynolds numbers investigated, the distance from the step to the reattachment point of the main vorticity domain increases linearly with the dimensionless time. Again, good agreement is obtained between experimental data and computed solutions. 1 Introduction The random vortex method (RVM), proposed by Chorin [4], was initially developed as a grid-free, full-Lagrangian method. In the RVM, the vorticity field is discretized among N vortex elements in which the vorticity is distributed according to a radially symmetric core function..

Numerical simulation of the starting flow down a step

The starting flow down a step in an open channel is simulated using a vortex method. A mixed Eulerian-Lagrangian scheme based on the vortex-in-cell algorithm and random walk diffusion is used with a non-uniform mesh. The comparison of numerical and experimental instantaneaous streamline plots show good agreement, especially for early stages of the flow development. It is also shown that, during intermediate stages of the flow development and for the range of Reynolds numbers investigated, the distance from the step to the reattachment point of the main vorticity domain increases linearly with the dimensionless time. Again, good agreement is obtained between experimental data and computed solutions