The interaction of oblique shocks and oblique compression waves with a laminar boundary layer on an adiabatic flat plate is analyzed by solving the Navier-Stokes equations in conservation-law form numerically. The numerical scheme is based on the Beam and Warming’s implicit method with approximate factorization. We examine the flow of Bethe-Zel’dovich-Thompson (BZT) fluids at pressures and temperatures on the order of those of the thermodynamic critical point. A BZT fluid is a single-phase gas having specific heat so large that the fundamental derivative of gas dynamics, Γ, is negative over a finite range of pressures and temperatures. The equation of state is the well-known Martin-Hou equation. The main result is the demonstration that the natural dynamics of BZT fluids can suppress boundary layer separation. Physically, this suppression can be attributed to the decrease in adverse pressure gradients associated with the disintegration of compression discontinuities in BZT fluids. / Ph. D.

The two-dimensional, confined, laminar flow over a downstream facing step was studied using a finite element, Navier-Stokes equation solver. The weak form of the stationary, incompressible Navier-Stokes equations in primitive variable form was obtained using the conventional Galerkin technique for mixed problems. Biquadratic Lagrange interpolating polynomials were used to construct the basis functions that generated the finite-dimensional subspace containing the approximate solutions to the velocity field, while the pressure field was represented by a discontinuous, piecewise-linear approximation. This particular combination of solution subspaces was previously shown in a mathematically rigorous fashion to yield stable, consistent solutions to the Navier-Stokes equations. The results of the computations were benchmarked against the experimental data of Denham and Patrick, and also compared to earlier calculations by Ecer and Thomas, both of whom utilized alternative, unconventional formulations. These comparisons indicate that with the proper choice of basis functions, a conventional Galerkin scheme can yield results that are in as good and in many cases better agreement with the available experimental data than those of unconventional schemes that rely upon an infusion of artificial dissipation to enhance their numerical stability. The computational algorithm was also used to ascertain the cause of the noticeable lack of development and skewness that characterized the experimental data of Denham and Patrick both at and upstream of the step. The results of this study indicated that as suspected by Denham and Patrick, the skewness as well as the lack of development of the velocity profiles near the step were caused by the geometry of the test apparatus upstream of the step rather than by the presence of the step itself. The numerical experiments conducted here have been carefully documented so as to facilitate future comparisons intended to assess the relative efficiency of the present method of computation. / Doctor of Philosophy

Supersonic slot injection problems were studied by a finite volume method. The numerical technique used is the upwind method of Roe’s flux difference splitting (FDS) with vertical line Gauss-Seidel relaxation applied to the thin layer Navier-Stokes equations. To test the accuracy of the numerical methods without the complications and uncertainties of turbulence modeling, two sample cases were chosen with laminar flows. The sample problems were the compressible laminar boundary layer flow over a flat plate and the laminar boundary layer - shock interaction problem. For these problems, both the results from Roe’s FDS and van Leer’s flux vector splitting (FVS) are compared with exact solutions and experimental data. For the sample problems, comparisons showed that Roe’s FDS method is more accurate than van Leer’s FVS method. Because of the very complicated wave patterns and strong viscous-inviscid interaction produced by supersonic slot injection, an adaptive grid based on the equidistribution law was combined with the solution algorithm. The results from Roe’s FDS method with the adaptive grid showed good results for the supersonic slot injection over a flat plate. For the slot injection over a 10-degree wedge surface case, there is a significant difference between the numerical and experimental wall pressure distribution. Some potential reasons for the discrepancy including 3D effects and/or transition in the reattachment region in the experiments and possibly a need for a much finer grid in the calculations are discussed. / Ph. D.

A Prandtl-type eddy viscosity model including the first-order effect of turbulence structure has been developed to deal with curved free-shear flows. The model is generalized to account for the effect of arbitrary cross-section of the jets injected from a various nozzle configurations into a uniform crossflow. The model is implemented as a module of a general purpose finite-element computer code. The finite-element procedures used here follow from a Galerkin type variational formulation with the penalty approximation for pressure in a consistent manner, with which a significant savings in computational time and storage are achieved. In order to simulate complicated 3-D turbulent flow with a restricted computational space and modest mesh, a slip condition is employed to model the wall flow and stress-free conditions are used for the farfield and outflow boundaries. Numerical predictions are performed for three problems: a single circular jet in a crossflow, a single streamwise aligned rectangular (aspect ratio 4) jet in a crossflow, and dual side-by-side rectangular jets in a crossflow, all at a jet-to-crossflow velocity ratio 4, which is important for V/STOL and other applications. The prediction of the mean velocity components of the circular jet case is in excellent agreement with the measured data except for the near wall region. The surface pressure comparison is very good except for the viscous wake region right behind the nozzle due to flow separation. The current pressure prediction is as good as any inviscid solution given by singularity or panel method with empirically tuned jet properties. No mean flowfield comparison is made for the single rectangular jet case due to the lack of available measured data. Surface pressure comparison is consistently very good, especially for the region near the front corners of the nozzle where the large negative peaks appear. The agreement for this case seems to be even better than the circular jet case, and the reason is, as observed in the surface velocity vector plot, the different vortex structure and mixing in the vicinity of the nozzle. For the dual jets case, the surface pressure prediction is still in a very good agreement, and the mean velocity comparison shows better agreement as the mesh is refined. The flowfield is found to be more complicated than the circular jet case due to the jet interaction, and further mesh refinement is needed for the complete resolution of the jet/wake flowfield. However, if the surface pressure prediction is the major concern, as in the V/STOL applications, the current size of computational space along with numerical strategies adopted here can serve that purpose effectively. Finally, the mean velocity and the pressure prediction obtained here for rectangular jet(s) are the first known to this author, and will provide useful information for the 3-D, complex, turbulent, free shear flow computations. / Ph. D.

A half-equation model of turbulence has been developed to described the eddy viscosity distribution of two and three-dimensional turbulent free shear flows. The model is derived by integrating the parabolized transport equation for the turbulence kinetic energy over the cross section of the flow. The Prandtl-Kolmogrov hypothesis is used to obtain an ordinary differential equation for the eddy viscosity. The model is used in a general purpose finite element procedure using primitive variables. The penalty function method is used, in a generalized Galerkin weak formulation of the Navier-Stokes equations, to enforce the conservation of mass. In this procedure the pressure does not explicitly appear, this significantly reducing the computation time when compared to the velocity-pressure approach. Numerical solution are obtained for four problems: a round jet issuing from a wall into still surroundings, a three-dimensional square jet issuing from a wall into still surroundings, a uniform flow past a free running propeller, and a shear flow past a free running propeller. An actuator disk with variable radial distribution of thrust and torque is used to model the propeller. The numerical solution in the far field of the round jet agrees very well with the analytical similar solution. Very good agreement between prediction and experiments is observed for the square jet problem. A simplified analysis of the flow past a propeller is used to provide the initial value of the eddy viscosity. Numerical experiments on the uniform flow past a thrusting disk confirmed the validity of the analysis and illustrated the effect of the initial value of the initial value of the eddy viscosity. For both propeller flows, agreement between predictions and experiments is excellent for both the axial and swirl velocity components at two stations located at x/D = 0.025 and 0.23. The quality of the swirl prediction is a major improvement over previous analyses. Pressure predictions are obtained for the first time, and are in reasonable agreement with the experiments. The radial velocity prediction is in fair agreement with the experiments at the station x/D = 0.025 .The discrepancy between the finite element solutions and the experiments at the station x/D = 0.23, for the pressure an the radial velocity are attributed to the presence of the body housing the propeller drive train. The body is not included in the present study. The complex three-dimensional nature of the shear flow past the propeller is very well captured in the simulation. / Doctor of Philosophy

Dans cette thèse, nous étudions des modèles mathématiques concernant certains problèmes d'écoulement de fluide à densité variable. Le premier chapitre résume l'ensemble de la thèse et se concentre sur les résultats obtenus, la nouveauté et la comparaison avec la littérature existante. Dans le deuxième chapitre, nous étudions la stabilisation locale des équations non homogènes de Navier-Stokes dans un canal 2d autour du flot de Poiseuille. Nous concevons un contrôle feedback de la vitesse qui agit sur l'entrée du domaine de sorte que la vitesse et la densité du fluide soient stabilisées autour du flot de Poiseuille, à condition que la densité initiale soit donnée par une constante additionnée d'une perturbation dont le support se situe loin du bord latéral du canal. Dans le troisième chapitre, nous étudions un système couplant les équations de Navier-Stokes compressibles à une structure élastique située à la frontière du domaine fluide. Nous prouvons l'existence locale de solutions solides pour ce système couplé. Dans le quatrième chapitre, notre objectif est d'étudier la nulle- contrôlabilité d'un problemè d'interaction fluide-structure linéarisé dans un canal bi dimensional. L'écoulement du fluide est ici modélisé par les équations de Navier-Stokes compressibles. En ce qui concerne la structure, nous considérons une poutre de type Euler-Bernoulli amortie située sur une partie du bord. Dans ce chapitre, nous établissons une inégalité d'observabilité pour le problème considéré d'interaction fluid-structure linéarisé qui constitue le premier pas vers la preuve de la nulle contrôlabilité du système. / In this thesis we study mathematical models concerning some fluid flow problems with variable density. The first chapter is a summary of the entire thesis and focuses on the results obtained, novelty and comparison with the existing literature. In the second chapter we study the local stabilization of the non-homogeneous Navier-Stokes equations in a 2d channel around Poiseuille flow. We design a feedback control of the velocity which acts on the inflow boundary of the domain such that both the fluid velocity and density are stabilized around Poiseuille flow provided the initial density is given by a constant added with a perturbation, such that the perturbation is supported away from the lateral boundary of the channel. In the third chapter we prove the local in time existence of strong solutions for a system coupling the compressible Navier-Stokes equations with an elastic structure located at the boundary of the fluid domain. In the fourth chapter our objective is to study the null controllability of a linearized compressible fluid structure interaction problem in a 2d channel where the structure is elastic and located at the fluid boundary. In this chapter we establish an observability inequality for the linearized fluid structure interaction problem under consideration which is the first step towards the direction of proving the null controllability of the system.

Numerical solution of differential equations having multitude of scales in the solution field is one of the most challenging research areas, but highly demanded in scientific and industrial applications. One of the natural approaches for handling such problems is to separate the scales and approximate the solution of the segregated scales with appropriate numerical method. Variational multiscale method (VMS) is a predominant method in the paradigm of scale separation schemes. In our work we have used the VMS technique to develop a numerical scheme for computations of turbulent flows in time-dependent domains. VMS allows separation of the entire range of scales in the flow field into two or three groups, thereby enabling a different numerical treatment for the different groups. In the context of computational fluid dynamics(CFD), VMS is a significant new improvement over the classical large eddy simulation (LES). VMS does away with the commutation errors arising due to filtering in LES. Further, in a three-scale VMS approach the model for the subgrid scale can be contained to only a part of the resolved scales instead of effecting the entire range of resolved scales. The projection based VMS scheme that we have developed gives a robust and efficient method for solving problems of turbulent fluid flows in deforming domains, governed by incompressible Navier {Stokes equations. In addition to the existing challenges due to turbulence, the computational complexity of the problem increases further when the considered domain is time-dependent. In this work, we have used an arbitrary Lagrangian-Eulerian (ALE) based VMS scheme to account for the domain deformation. In the proposed scheme, the large scales are represented by an additional tensor valued space. The resolved large and small scales are computed in a single unified equation, and the effect of unresolved scales is confined only to the resolved small scales, by using a projection operator. The popular Smagorinsky eddy viscosity model is used to approximate the effects of unresolved scales. The used ALE approach consists of an elastic mesh update technique. Moreover, a computationally efficient scheme is obtained by the choice of orthogonal finite element basis function for the resolved large scales, which allows to reformulate the ALE-VMS system matrix into the standard form of the NSE system matrix. Thus, any existing Navier{Stokes solver can be utilized for this scheme, with modifications. Further, the stability and error estimates of the scheme using a linear model of the NSE are also derived. Finally, the proposed scheme has been validated by a number of numerical examples over a wide range of problems.

This thesis is concerned with the further development and analysis of a class of Computational Fluid Dynamics (CFD) methods for the numerical simulation of compressible flows on unstructured grids, known as Residual Distribution (RD).<p>The RD method constitutes a class of discretization schemes for hyperbolic systems <p>of conservation laws, which forms an attractive alternative to the more classical Finite Volume methods, particularly since it allows better representation of the flow physics by genuinely multi-dimensional upwinding and offers second-order accuracy on a compact stencil.<p><p>Despite clear advantages of RD schemes, they also have some unexpected anomalies in common with Finite Volume methods and an attempt to resolve them is presented. The most notable anomaly is the violation of the entropy condition, which as a consequence allows unphysical expansion shocks to exist in the numerical solution. In the thesis the genuinely multi-dimensional character of this anomaly is analyzed and a multi-dimensional entropy fix is presented and shown to avoid expansion shocks. Another infamous anomaly is the carbuncle phenomenon, an instability observed in many numerical solutions with strong shocks, such as the bow shock on a blunt body in hypersonic flow. The occurence of the carbuncle phenomenon with RD methods is analyzed and a novel formulation for a shock fix, based on an anisotropic diffusion term added in the shock layer, is presented and shown to cure the anomaly in 2D and 3D hypersonic flow problems.<p><p>In the present work an effort has been made also to an objective and quantitative assessment of the merits of the RD method for typical aerodynamical engineering applications, such as the transonic flow over airfoils and wings.<p>Validation examples including inviscid, laminar as well as high Reynolds number turbulent flows <p>and comparisons against results from state-of-the-art Finite Volume methods are presented.<p>It is shown that the second-order multi-dimensional upwind RD schemes have an accuracy which is at least as good as second-order FV methods using dimension-by-dimension upwinding and that their main advantage lies in providing excellent monotone shock capturing. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Numerical solutions of the Navier-Stokes Equations became more popular in recent decades with increasingly accessible and powerful computational resources. Simulations in reconstructed or artificial pore geometries are often performed to gain insight into microscopic fluid flow structures or are used for upscaling quantities of interest, like hydraulic conductivity. A physically adequate representation of pore-scale flow fields requires analysis of large domains. We solve the incompressible NSE in artificial ordered and random pore-space structures. A simple cubic and face-centred packings of spheres placed in a square duct are analysed. For the fluid flow simulations of random media, packings of spheres, icosahedra, and cubes forming unconsolidated sediments are generated using a rigid body simulation software. The Direct Numerical Simulation method is used for the solution of the NSE implemented in the open-source computational fluid dynamics software OpenFOAM. The influence of the number of spheres in ordered packings, the mesh type, and the mesh resolution is investigated for fluid flow up to Reynolds numbers of 100 based on the spheres' diameter. The random media mesh generation method relies on approximate surface reconstruction. The resulting tetrahedral meshes are then used for steady-state simulations and refined based on an a-posteriori error estimator. The fluid flow simulation results can further be used twofold: 1) They provide homogenized hydro-mechanical properties of the analysed medium for the larger meso and macro groundwater flow simulations. A concept of one-way binding for large-scale simulations is presented. 2) Visualisation: A post-processing image rendering technique was employed in interactive and still image visualisation environments allowing better overview over local fluid flow structures. The ogs FEM code for the solution of large-scale groundwater processes was inspected for computational efficiency. The conclusions drawn from this analysis formed the~basis for the implementation of the~new version of the code---ogs6. The improvements include comparison of linear algebra software realisations and an implementation of optimized memory access patterns in FEM-local assembler part.

Two numerical models have been developed to describe the volumetric changes during solidification in pure metals and alloys and to predict shrinkage defects in the castings of general three-dimensional configuration. The first model is based on the full system of the Continuity, Navier-Stokes and Enthalpy Equations. Volumetric changes are described by introducing a source term in the Continuity Equation which is a function of the rate of local phase transformation. The model is capable of simulating both volumetric shrinkage and expansion. The second simplified shrinkage model involves the solution of only the Enthalpy Equation. Simplifying assumptions that the feeding flow is governed only by gravity and solidification rate and that phase transformation proceeds only from liquid to solid allowed the fluid flow equations to be excluded from consideration. The numerical implementation of both models is based on an existing proprietary general purpose CFD code, FLOW-3D, which already contains a numerical algorithm for incompressible fluid flow with heat transfer and phase transformation. An important part of the code is. the Volume Of Fluid (VOF) algorithm for tracking multiple free surfaces. The VOF function is employed in both shrinkage models to describe shrinkage cavity formation. Several modifications to FLOW-3D have been made to improve the accuracy and efficiency of the metal/mould heat transfer and solidification algorithms. As part of the development of the upwind differencing advection algorithm used in the simulations, the Leith's method is incorporated into the public domain twodimensional SOLA code. It is shown that the resulting scheme is unconditionally stable despite being explicit.