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Abstract

Some basic aspects of turbulence, transition to turbulence, and turbulence modelling, are summarized in Chapter 1 (Introduction). Emphasis is put on the increasing understanding of turbulent phenomena made possible by recent advances in the theory of dynamical systems; on the concept of "coherent structure"; and on the parallel evolution of computing power and computational fluid dynamics.

Chapter 2 is a survey of the turbulence modelling technique known as Large Eddy Simulation (LES). The equations governing fluid flow and scalar transport are introduced; the direct simulation of turbulent flows and its limitations are briefly outlined; and the concept of LES, with the related topics of decomposition, filtering and subgrid modelling, is discussed. The state of the art in LES is reviewed in the last three sections, under the separate headings of proposed subgrid models; wall boundary conditions; and applications presented in the literature. An attempt is made to give the most complete and updated possible account of the subject; work carried out from the early 'Seventies up to now is considered. Emphasis is put more on physical models and corresponding performances than on numerical methods and computational details.

In Chapter 3, the finite-volume numerical techniques used in the present work are presented and discussed. Emphasis is put on those aspects and options which bear more relevance for the accuracy and quality of the results, such as the pressure-velocity coupling algorithm, the discretization of advective terms and the treatment of centred (co-located), body-fitted grids. The architecture and the basic features of the computer code Harwell-FL0W3D, Release 2, are outlined, while the modifications introduced in order to implement the Smagorinsky subgrid model and the appropriate boundary conditions for LES are described in detail.

In Chapters 4-6 results are presented and discussed for the basic geometries studied.

Chapter 4 deals with the flow between indefinite parallel plates (plane channel), one of which heated with a uniform heat flux. For this basic geometry, a detailed study is presented on the influence of numerical options (grid size, time step and time-stepping method, pressure-velocity coupling, discretization of the advective terms); model parameters (Smagorinsky constant, subgrid Prandtl number, near-wall damping); Reynolds number; and alternative wall boundary conditions. The issues of initial conditions, numerical transients and statistical processing of the results are also discussed with some depth.

In Chapter 5, computations are presented for a plane channel having one of the walls roughened by transverse square ribs. An extensive literature review of experimental and numerical studies on this geometry is included. The parametrical study is limited here to the influence of grid size and Reynolds number; LES results are presented in detail for a reference case, and are compared with experimental flow and heat transfer data.

Chapter 6 is dedicated to the geometry of cross-corrugated ducts, representative of storage-type air preheaters for fossil-fuelled power stations. Flow and heat transfer predictions from direct simulation and LES are presented; they are compared with experimental results and with numerical predictions obtained by a standard and a low-Reynolds number version of the k-s turbulence model.

Finally, Chapter 7 summarizes the main conclusions which can be drawn from the above studies. Emphasis is put on the basic issue of LES applicability to engineering problems of practical interest, and of its feasibility using a commercial, general-purpose (though highly sophisticated) computer code. A critical comparison with more conventional turbulence modelling approaches is outlined, and 'weak spots', or issues requiring further clarifications, are pointed out for future studies.

The work includes an extensive bibliography with almost 400 references, and an appendix on the tensorial formulation of the governing equations of fluid dynamics in general domains.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses School of Science & Technology > Engineering

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