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Among the different types of flows encountered in practical applications, the physics of turbulent separated flows and turbulence induced secondary motion are not fully understood despite the large amount of previous experimental and numerical work. The objectives of this work are to study theoretically and computationally the conditions at the separation and reattachment point, the numerical simulation of turbulence induced secondary motion in non-circular ducts, and to provide a comprehensive test of different RANS models of these types of flow. In a theoretical study of flow separation, a Lagrangian approach was first used to derive an Eulerian criterion, which associates separation and reattachment points to a critical point in the eigenvalues of the Cauchy-Green tensor. A turbulent separated boundary layer under the influence of an adverse pressure gradient was simulated using DNS and LES techniques. A bootstrapping method was used to obtain high fidelity results at a relatively high Reynolds number with which the performance of some of the most commonly used eddy-viscosity turbulence models was evaluated. The DNS and LES results were used to assess the consistency of the different terms in the k−e , ζ −f , k −ω and Spalart-Allmaras models. Different wall-modelling techniques were employed for the calculation of separated boundary layers. The exact values of the modelled terms were calculated using the reference DNS and LES dataset. These results were used for both a priori and a posteriori tests. It was determined that the eddy-viscosity assumption works well, and that anisotropic effects are not significant in separated boundary layer. For the secondary flow calculation in non-circular ducts, direct numerical simulations of turbulent flow in square and skewed ducts were carried out to determine the effect of the duct (rhombus) included angle on both the mean and turbulence energy budgets. Two skewed ducts, with included angles of 30 and 60 degrees, were simulated. The capability of different turbulence models to predict the secondary velocity field was investigated. Results obtained from a non-linear stress-strain constitutive relation was found to be fairly accurate for the flows at the range of Reynolds number considered in this study. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2010-11-26 13:52:18.361

Les écoulements décollés sont fortement instationnaires, l'objectif de cette thèse a été de localiser et d'identifier les phénomènes à la source de ces instationnarités et de comprendre les processus physiques permettant le transfert de l'information de ces zones sources au reste de l'écoulement. Pour ce faire une analyse des résultats issus de simulations numériques a été réalisée. En étudiant la corrélation et la cohérence entre les positions de choc et les fluctuations de pression, l'interaction a pu être séparée en plusieurs parties distinctes. A l'aide de la théorie des caractéristiques définissant les directions et les cinématiques de propagation de l'information, les liens spatio-temporels entre ces différentes régions ont pu être déterminés. Les résultats de ces études couplés avec ceux issus des expériences ont montré clairement que les phénomènes se produisant à l'intérieur de la zone de recirculation existant en aval du choc de décollement gouvernent la dynamique de la totalité de l'interaction, aussi bien à basse fréquence qu'à moyenne fréquence. Ainsi les mouvements de choc apparaissent comme le miroir des phénomènes se produisant à l'intérieur de la zone décollée. Une représentation équivalente en fluide non visqueux permettant une description du comportement instationnaire de l'interaction a aussi été proposée. / Separated flow are often strongly unsteady; the aim of this thesis is to localize and identify the sources of the unsteadiness and to understand the physical phenomena governing the information transfer from these source zones to the rest of the flow. To do this, data used for this analysis have been obtained from numerical simulations (LES). Both cross-correlation and coherence between shock motion and pressure fluctuations have shown that the interaction can be split in several distinct zones. The theory of characteristics is used to define the information paths and the propagation velocities, so that the space-time links between these regions have been determined. Both numerical and experimental studies have clearly shown that phenomena present within the recirculation buble govern the whole of the interaction, at low and intermediate frequencies. Indeed the shock motions appears as the mirror of phenomena present in the separated zone. An inviscid equivalent scenario has been proposed to represent the interaction.