A novel computational approach, Two Level Simulation (TLS), was developed based on the explicit reconstruction of the small-scale velocity by solving the small-scale governing equations on the domain with reduced dimension representing a collection of one-dimensional lines embedded in the three-dimensional flow domain. A coupled system of equations, that is not based on an eddy-viscosity hypothesis, was derived based on the decomposition of flow variables into the large-scale and the small-scale components without introducing the concept of filtering. Simplified treatment of the small-scale equations was proposed based on modeling of the small-scale advective derivatives and the small-scale dissipative terms in the directions orthogonal to the lines. TLS approach was tested to simulate benchmark cases of turbulent flows, including forced isotropic turbulence, mixing layers and well-developed channel flow, and demonstrated good capabilities to capture turbulent flow features using relatively coarse grids.

This work addresses the application of non-premixed filtered tabulated chemistry as a turbulent combustion modeling strategy in the LES framework. On the first part of this study the effects of the filtering operation on non-premixed flamelets are carefully appraised, considering an individual flamelet and the entire manifold. Subsequently, a systematic approach is followed where first the numerical implementation is verified. Afterwards validation is done on a coflow laminar diffusion flame, where promising results encourage the further model appraisal on a more complex turbulent configuration. This is finally achieved under turbulent conditions of Flames D and E, where the formalism including a SGS wrinkling modeling function adequately describes the wrinkled flame front features. The formalism assessment on a laminar coflow diffusion flame reveals a considerable sensitivity to the flame dimensionality. A flame sensor based on the mixture fraction gradient, with a tolerance to take into account the numerical grid resolution, is introduced and proves to deliver satisfactory results. The sensor-determined model activation allows to adequately represent the underlying physics behind flame filtering and so it endorses the consistency of the numerical procedure. The evaluation of the non-premixed FTACLES model on turbulent flames D and E demonstrates that the formalism coupling with a SGS wrinkling modeling function can adequately describe the wrinkled flame front condition. The model performs significantly well employing a three-dimensional tabulation strategy, where the numerical grid is coupled with the model by the third parameter, i.e. the computational cell size. The predictions for both the major stable species and the minor ones accurately correspond with the undergoing physics. The obtained results have a deep theoretical implication for the combustion research. First, they confirm the idea that SGS closure in diffusive combustion can be derived based on filtering arguments, and not only based on statistical approaches. Second, they demonstrate the enormous potential of the non-premixed FTACLES formalism once a sound flame sensor and a SGS wrinkling modeling function are included. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

A metodologia LES (Large Eddy Simulation) é uma alternativa viável para a solução numérica de escoamentos de interesse prático em virtude da limitação computacional imposta pela resolução direta de todas as escalas presentes em escoamentos turbulentos. Entretanto, a compreensão detalhada do fenômeno da turbulência é ainda uma tarefa desafiadora em consequência do seu comportamento não linear e alta sensibilidade às condições iniciais e de contorno. Dessa forma, o sucesso de simulações LES está associado à utilização de um código computacional eficiente, com modelagem submalha que represente corretamente a dinâmica do escoamento, juntamente com a especificação de condições iniciais turbulentas fisicamente consistentes. Nesse contexto, o presente trabalho tem como objetivo o desenvolvimento de um código LES de alta ordem aliado a um método de geração de perturbações para o estudo de escoamentos turbulentos em camada limite sobre superfície plana. Foi adotada a formulação vorticidadevelocidade. A metodologia numérica baseia-se no método de diferenças finitas em malhas colocalizadas, onde as derivadas nas direções longitudinal e normal ao escoamento são aproximadas usando diferenças compactas de alta ordem. Esse estudo assume periodicidade na direção transversal do escoamento e então um método espectral é adotado nessa direção. A integração temporal é feita através do método Runge-Kutta de 4a ordem e a solução da equação de Poisson se dá por meio de um método multigrid. Para a modelagem submalha é adotado o modelo WALE (Wall-Adapting Local Eddy-viscosity). O método RFG (Random Flow Generation) foi responsável pela geração das flutuações de velocidade. Os resultados obtidos mostraram-se em boa concordância com os dados DNS (Direct Numerical Simulation) e LES presentes na literatura. / LES methodology is a viable alternative for the numerical solution of practical interest flows due to the computational limitations imposed by the direct resolution of all scales presented in turbulent flow. However, the detailed understanding of the turbulence phenomenon is still a challenging task as a result of its non-linear behavior and high sensitivity to initial and boundary conditions. Thus, the success of LES simulations is associated with the use of an efficient computational code, wherein the subgrid scale modeling accurately represents the flow dynamics, together with the specification of realistic inicial boundary conditions. In this context, this study aims to develop a high-order LES code combined with a method for generating velocity fluctuations to compute turbulent boundary layer flows over a flat plate. The vorticity-velocity formulation was adopted. The numerical scheme is based on the finite difference method in collocated grid, where the derivatives in the streamwise and wall-normal are approximated using high order compact finite difference schemes. We also assume periodicity in spanwise direction therefore it is adopted a spectral method in this direction. The method chosen for the temporal evolution is the 4th order Runge-Kutta method and the solution of Poisson equation solution is accessed via a multigrid algorithm. For subgrid modelling it is adopted the Wall-Adapting Local Eddy-viscosity (WALE) model. The RFG (Random Flow Generation) method was responsible for the generation of unsteady turbulent velocity signal. The results obtained were in good agreement with DNS (Direct Numerical Simulation) and LES from the literature.

A metodologia LES (Large Eddy Simulation) é uma alternativa viável para a solução numérica de escoamentos de interesse prático em virtude da limitação computacional imposta pela resolução direta de todas as escalas presentes em escoamentos turbulentos. Entretanto, a compreensão detalhada do fenômeno da turbulência é ainda uma tarefa desafiadora em consequência do seu comportamento não linear e alta sensibilidade às condições iniciais e de contorno. Dessa forma, o sucesso de simulações LES está associado à utilização de um código computacional eficiente, com modelagem submalha que represente corretamente a dinâmica do escoamento, juntamente com a especificação de condições iniciais turbulentas fisicamente consistentes. Nesse contexto, o presente trabalho tem como objetivo o desenvolvimento de um código LES de alta ordem aliado a um método de geração de perturbações para o estudo de escoamentos turbulentos em camada limite sobre superfície plana. Foi adotada a formulação vorticidadevelocidade. A metodologia numérica baseia-se no método de diferenças finitas em malhas colocalizadas, onde as derivadas nas direções longitudinal e normal ao escoamento são aproximadas usando diferenças compactas de alta ordem. Esse estudo assume periodicidade na direção transversal do escoamento e então um método espectral é adotado nessa direção. A integração temporal é feita através do método Runge-Kutta de 4a ordem e a solução da equação de Poisson se dá por meio de um método multigrid. Para a modelagem submalha é adotado o modelo WALE (Wall-Adapting Local Eddy-viscosity). O método RFG (Random Flow Generation) foi responsável pela geração das flutuações de velocidade. Os resultados obtidos mostraram-se em boa concordância com os dados DNS (Direct Numerical Simulation) e LES presentes na literatura. / LES methodology is a viable alternative for the numerical solution of practical interest flows due to the computational limitations imposed by the direct resolution of all scales presented in turbulent flow. However, the detailed understanding of the turbulence phenomenon is still a challenging task as a result of its non-linear behavior and high sensitivity to initial and boundary conditions. Thus, the success of LES simulations is associated with the use of an efficient computational code, wherein the subgrid scale modeling accurately represents the flow dynamics, together with the specification of realistic inicial boundary conditions. In this context, this study aims to develop a high-order LES code combined with a method for generating velocity fluctuations to compute turbulent boundary layer flows over a flat plate. The vorticity-velocity formulation was adopted. The numerical scheme is based on the finite difference method in collocated grid, where the derivatives in the streamwise and wall-normal are approximated using high order compact finite difference schemes. We also assume periodicity in spanwise direction therefore it is adopted a spectral method in this direction. The method chosen for the temporal evolution is the 4th order Runge-Kutta method and the solution of Poisson equation solution is accessed via a multigrid algorithm. For subgrid modelling it is adopted the Wall-Adapting Local Eddy-viscosity (WALE) model. The RFG (Random Flow Generation) method was responsible for the generation of unsteady turbulent velocity signal. The results obtained were in good agreement with DNS (Direct Numerical Simulation) and LES from the literature.

No description available.

<p>A new large eddy simulation (LES) code is developed and used to investigate non-local features of turbulent planetary boundary layers (PBLs). The LES code is based on filtered Navier-Stokes equations, which describe motions of incompressible, Boussinesq fluid at high Reynolds numbers. The code computes directly large-scale, non-universal turbulence in the PBL whereas small-scale, universal turbulence is parameterized by a dynamic mixed subgrid closure. The LES code is thoroughly tested against high quality laboratory and field data. </p><p>This study addresses non-local properties of turbulence which emphasis on the stable stratification. Its basic results are as follows. The flow stability in PBLs is generally caused by two mechanisms: the negative buoyancy force (in the stable density stratification) and the Coriolis force (in the rotating system). The latter stabilizes the flow if the earth’s vorticity and the turbulent vorticity are anti-parallel. The Coriolis force stability suppresses large-scale turbulence and makes large eddies asymmetric. The density stratification suppresses vertical scales of turbulence. Joint actions of the Coriolis and the buoyancy forces result in a more complex behavior of turbulence. Particularly, the layers of vigorous turbulence may appear in the course of development of low-level jets in baroclinic atmosphere. </p><p>Non-local effects determine integral measures of PBLs, first of all the PBL depth. This study clearly demonstrates its pronounced dependences on the Coriolis parameter, the Kazanski-Monin internal stability parameter, and newly introduced imposed-stability and baroclinicity parameters. An LES database is created and used to validate an advanced PBL-depth formulation. LES support the idea that PBLs interact with the stably stratified free flow through the radiation of gravity waves, excited by large turbulent eddies at the interface. </p>

When an electrically conducting fluid moves through a magnetic field, fluid mechanics and electromagnetism are coupled. This interaction is the object of magnetohydrodynamics, a discipline which covers a wide range of applications, from electromagnetic processing to plasma- and astro-physics. In this dissertation, the attention is restricted to turbulent liquid metal flows, typically encountered in steel and aluminium industries. Velocity measurements in such flows are extremely challenging because liquid metals are opaque, hot and often corrosive. Therefore, non-intrusive measurement devices are essential. One of them is the Lorentz force flowmeter. Its working principle is based on the generation of a force acting on a charge, which moves in a magnetic field. Recent studies have demonstrated that this technique can measure efficiently the mean velocity of a liquid metal. In the existing devices, however, the measurement depends on the electrical conductivity of the fluid. In this work, a novel version of this technique is developed in order to obtain measurements that are independent of the electrical conductivity. This is particularly appealing for metallurgical applications, where the conductivity often fluctuates in time and space. The study is entirely numerical and uses a flexible computational method, suitable for industrial flows. In this framework, the cost of numerical simulations increases drastically with the level of turbulence and the geometry complexity. Therefore, the simulations are commonly unresolved. Large eddy simulations are then very promising, since they introduce a subgrid model to mimic the dynamics of the unresolved turbulent eddies. The first part of this dissertation focuses on the quality and reliability of unresolved numerical simulations. The attention is drawn on the ambiguity that may arise when interpretating the results. Owing to coarse resolutions, numerical errors affect the performances of the discrete model, which in turn looses its physical meaning. In this work, a novel implementation of the turbulent strain rate appearing in the models is proposed. As opposed to its usual discretisation, the present strain rate is in accordance with the discrete equations of motion. Two types of flow are considered: decaying turbulence located far from boundaries, and turbulent flows between two parallel and infinite walls. Particular attention is given to the balance of resolved kinetic energy, in order to assess the role of the model. The second part of this dissertation deals with a novel version of Lorentz force flowmeters, consisting in one or two coils placed around a circular pipe. The forces acting on each coil are recorded in time as the liquid metal flows through the pipe. It is highlighted that the auto- or cross-correlation of these forces can be used to determine the flowrate. The reliability of the flowmeter is first investigated with a synthetic velocity profile associated to a single vortex ring, which is convected at a constant speed. This configuration is similar to the movement of a solid rod and enables a simple analysis of the flowmeter. Then, the flowmeter is applied to a realistic three-dimensional turbulent flow. In both cases, the influence of the geometrical parameters of the coils is systematically assessed.

A new large eddy simulation (LES) code is developed and used to investigate non-local features of turbulent planetary boundary layers (PBLs). The LES code is based on filtered Navier-Stokes equations, which describe motions of incompressible, Boussinesq fluid at high Reynolds numbers. The code computes directly large-scale, non-universal turbulence in the PBL whereas small-scale, universal turbulence is parameterized by a dynamic mixed subgrid closure. The LES code is thoroughly tested against high quality laboratory and field data. This study addresses non-local properties of turbulence which emphasis on the stable stratification. Its basic results are as follows. The flow stability in PBLs is generally caused by two mechanisms: the negative buoyancy force (in the stable density stratification) and the Coriolis force (in the rotating system). The latter stabilizes the flow if the earth’s vorticity and the turbulent vorticity are anti-parallel. The Coriolis force stability suppresses large-scale turbulence and makes large eddies asymmetric. The density stratification suppresses vertical scales of turbulence. Joint actions of the Coriolis and the buoyancy forces result in a more complex behavior of turbulence. Particularly, the layers of vigorous turbulence may appear in the course of development of low-level jets in baroclinic atmosphere. Non-local effects determine integral measures of PBLs, first of all the PBL depth. This study clearly demonstrates its pronounced dependences on the Coriolis parameter, the Kazanski-Monin internal stability parameter, and newly introduced imposed-stability and baroclinicity parameters. An LES database is created and used to validate an advanced PBL-depth formulation. LES support the idea that PBLs interact with the stably stratified free flow through the radiation of gravity waves, excited by large turbulent eddies at the interface.

Soot prediction in realistic systems is one of the most challenging problems in theoretical and applied combustion. Soot formation as a chemical process is very complicated and not fully understood up to the moment. The major difficulty stems from the chemical complexity of the soot formation processes as well as its strong coupling with the other thermochemical and fluid processes that occur simultaneously. Soot is a major byproduct of incomplete combustion, having a strong impact on the environment, as well as the combustion efficiency. Therefore, it needs to be predicted in realistic configurations in an accurate and yet computationally efficient way. In the current study, a new soot formation subgrid model is developed and reported here. The new model is designed to be used within the context of the Large Eddy Simulation (LES) framework, combined with Linear Eddy Mixing (LEM) as a subgrid combustion model. The final model can be applied equally to premixed and non-premixed flames over any required geometry and flow conditions in the free, the transition, and the continuum regimes. The soot dynamics is predicted using a Method of Moments approach with Lagrangian Interpolative Closure (MOMIC) for the fractional moments. Since, no prior knowledge of the particles distribution is required, the model is generally applicable. The effect of radiation is introduced as an optically thin model. As a validation the model is first applied to a non-premixed non-sooting flame, then a set of canonically premixed flames. Finally, the model is validated against a non-premixed jet sooting flame. Good results are predicted with reasonable accuracy.

The research focusses on computational analysis of vortex generation behind A-Pillar of simplified model derived from Jaguar XF that excludes air from the underside of vehicle. This vortex formation contributes in generating wall pressure fluctuations especially at speeds higher than 100km/hr. It is a collaborative work between Cranfield University and Jaguar Land Rover. Three dimensional pressure based incompressible flow using Large Eddy Simulation turbulence model is selected for computational analysis in FLUENT v14. This used high parallel computing systems available in Cranfield University. In the initial phase, three grid resolutions (coarse, medium and fine) were prepared in ICEM CFD with fine case consisting of 10 million cells. Qualitative analysis includes extraction of slices, 3-D and surface streamlines and pressure and velocity contours for capturing the unsteadiness due to the vortex formation over the front side glass surface. The iso-surface of Q captures the unsteadiness at the A-Pillar wake and side mirror wake over front side glass surface. It also reveals that the range of length scales captured were limited even at the finest grid resolution. Quantitative analysis compares the mean pressure (Cp) data with JLR results. Probes were located at 51 locations over the front side glass window that showed a good comparison; specifically for the fine grid; with maximum variation incurred at probes located in separation areas. For predicting the wall pressure fluctuations, a total of ten probes were located over the front side glass window surface. The surface pressure (static) data was recorded for 1 sec of flow-time and later imported in MATLAB for post-processing. The results obtained in 1/3rd octave band showed that the large scales were too energetic and small scales are not captured. However, comparing sound pressure levels with the Aero-acoustic Wind Tunnel (AWT); provided by JLR; it is concluded that either the grid is too coarse to resolve higher frequencies or the numerical modelling used is too dissipative to benefits the use of LES.