An interaction by exchange with the conditional mean (IECM) micromixing model is coupled to a three-dimensional single-particle Lagrangian stochastic (LS) model to estimate concentration fluctuations in plumes of a passive (i.e., non-buoyant), non-reactive (i.e., no chemistry) tracer dispersing from a variety of source configurations in four neutrally stratified flows: a horizontally-homogeneous wall shear layer flow; a horizontally-homogeneous representation of the Tombstone canopy flow; a three-dimensional inhomogeneous representation of the Tombstone canopy flow; and a three-dimensional inhomogeneous representation of the Mock Urban Setting Trials (MUST) canopy flow. The IECM micromixing model incorporates the combined effects of turbulent and molecular mixing on particle concentration. This allows the numerical estimation of all moments of the scalar concentration field, which is a significant advance over traditional LS models given that concentration fluctuations are a ubiquitous feature of a dispersing plume. The single-particle implementation of the LS-IECM model is based upon a previously reported implementation that used simultaneously computed particle trajectories to estimate the conditional mean concentration field [Cassiani, M. A., Franzese, P. A. and Giostra, U. A.: 2005, A PDF micromixing model of dispersion for atmospheric flow. Part I: development of model, application to homogeneous turbulence and to a neutral boundary layer, Atmospheric Environment 39, 1457-1469]. The model used in this thesis pre-calculates the conditional mean concentration field with an LS model for use with the IECM model, which runs as a separate simulation. The principal advantage of this single-particle approach is the performance increase on parallel computer architecture, which scales directly with the number of processors. The simulations presented in this thesis go beyond those performed with the previous model by considering three-dimensional inhomogeneous flows, as well as one-dimensional horizontally-homogeneous flows. The accuracy of the LS-IECM model was good for the flows with horizontal-homogeneity, and comparable to the results of previous simulations from older models. Rogue velocities in the simulations utilising inhomogeneous flow statistics resulted in acceptable to poor accuracy in these simulations. Suggestions for improvements to the model are made.

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[pt] Neste trabalho três métodos foram utilizados para estudar o escoamento turbulento em regiões de florestas. No primeiro método, a influência da vegetação no escoamento foi modelada através da adição de termos fontes nas equações de quantidade de movimento, energia cinética turbulenta e sua taxa de dissipação. No segundo, a vegetação foi considerada um meio poroso homogêneo. Finalmente, a camada do dossel foi representada por modelos 3-D de árvores, consideradas como obstáculos individuais. As equações foram resolvidas através do modelo de turbulência k −E padrão com o programa comercial FLUENT 6.2.16. As previsões dos perfis verticais da velocidade do vento médio, da intensidade da turbulência e dos tensores de Reynolds, foram comparadas com dados de experimentos de túnel de vento. Os resultados preditos dos perfis verticais da velocidade média e intensidade da turbulência, na primeira e na segunda metodologias, apresentaram boa concordância com os valores experimentais, porém, foram observadas discrepâncias nos perfis modelados do tensor de Reynolds. Entretanto, qualitativamente, a modelagem consegue capturar o comportamento físico do tensor de Reynolds no interior de florestas. Uma possível explicação para este fato, é que o modelo considera a isotropia para a viscosidade turbulenta, implicando na incapacidade de prever qualquer forte anisotropia do campo turbulento. Na terceira metodologia, as previsões dos perfis verticais de velocidade média e intensidade da turbulência apresentaram discrepâncias em relação às medições. Porém, os perfis verticais do tensor de Reynolds apresentaram boa concordância. Todos os perfis verticais da velocidade média apresentaram um ponto de inflexão na interface vegetação-atmosfera, característico de uma camada de mistura. Nas duas primeiras metodologias, este padrão foi confirmado nos perfis de tangente hiperbólica de uma camada de mistura. / [en] This work investigates different procedures in order to study the turbulent flow over the scale model of a forest region. Initially, the canopy flow was modeled by using source terms in the momentum, turbulent kinetic energy and its dissipation rate equations. After that, the forest canopy was considered a homogeneous porous medium. In the last step, the canopy boundary layer was modeled by artificial 3-D tree models. This was done by using the standard k−E turbulence model with the FLUENT commercial program. The modeled profiles of mean velocity, turbulence intensity and Reynolds stress were compared against data from wind tunnel experiments. In the two first methodologies, the model predictions of the vertical profiles of the wind speed and turbulence intensity showed good agreement with the experimental data. It was found that predictions of the Reynolds tensor were sensitive to the parameterization scheme of the standard k −E model. However, qualitatively, the model was capable of predicting the physical behavior of the Reynolds stress tensor in the canopy flow. A possible explanation for this behavior is the omission of any anisotropic eddy-viscosity effects within the k - E modelling approach. When it was considered the tree array, the model predictions for the wind speed and turbulence intensity were less satisfactory. However, it was found that the predicted results of the Reynolds stress tensor agreed well with the measured data. All the vertical profiles of the mean velocity contained an inflection point, something which is a necessary criterion for the mixing layer flow. In the tree array, the modeled results failed to the capture this behavior of the canopy flow. In the 2-D numerical simulations, it was found the characteristic hyperbolic tangent profile of a mixing layer.

This thesis presents a numerical method for the simulation of fluid-structure interaction (FSI) problems on high-performance computers. The proposed method is specifically tailored to interactions between Newtonian fluids and a large number of slender viscoelastic structures, the latter being modeled as Cosserat rods. From a numerical point of view, such kind of FSI requires special techniques to reach numerical stability. When using a partitioned fluid-structure coupling approach this is usually achieved by an iterative procedure, which drastically increases the computational effort. In the present work, an alternative coupling approach is developed based on an immersed boundary method (IBM). It is unconditionally stable and exempt from any global iteration between the fluid part and the structure part. The proposed FSI solver is employed to simulate the flow over a dense layer of vegetation elements, usually designated as canopy flow. The abstracted canopy model used in the simulation consists of 800 strip-shaped blades, which is the largest canopy-resolving simulation of this type done so far. To gain a deeper understanding of the physics of aquatic canopy flows the simulation data obtained are analyzed, e.g., concerning the existence and shape of coherent structures.