A numerical model is presented of vertically homogeneous shallow flows with variable horizontal density. The governing equations represent mass and momentum conservation of a liquid-species mixture, and mass conservation of the species within a control volume. Here, the term species refers to material transported with the liquid flow. For example, when the species is taken to be suspended sediment, the model provides an idealised simulation of hyper-concentrated sediment-laden flows. The volumetric species concentration acts as an active scalar, allowing the species dynamics to influence the flow structure. The model can simulate flows driven by depth and density differences in the horizontal. The governing equations are written in a deviatoric, hyperbolic form to facilitate their solution by means of a Godunov-type finite volume scheme appropriate for flows containing sharp fronts. The deviatoric governing equations ensure that flux gradient and source terms are balanced (and there is no need for further numerical balancing). The numerical model is first verified for constant density cases, for which the governing equations reduce to the conventional coupled shallow water and species transport equations. Close agreement between numerical predictions and benchmark test solutions illustrate the model's ability to capture rapidly-varying flow features over uniform and non-uniform bathymetries. For variable-density cases, analytical steady-state solutions are derived for two simple cases, one with uniform bathymetry and the other with sinusoidal bathymetry. Detailed parameter studies are then undertaken to examine the effects of varying the initial density and depth in different regions. The shock-capturing scheme resolves all sharp features in the flow such as bore, shear waves, shock diamond like features, contact discontinuities and locally intense vortices. These interesting and novel nonlinear features are unique to variable density flows. The validated numerical model is applied to an idealised case of a hyperconcentrated sediment-laden debris-type flow along a tributary entering a river. The predicted evolution of the free surface flow field is qualitatively similar to observations of an actual debris flows into a river connected to the Upper Yangtze.

M.Ing. / The aim of this dissertation is to apply numerical aerodynamic principles to the characterization of an alternative stepped aerofoil concept. The accurate and efficient determination of the aerodynamic forces caused by the relative fluid motion and the consequent lift and drag coefficients are essential for the characterization of new aerofoils. The numerical method used is in the form of a Computational Fluid Dynamics code, which integrates the Navier-Stokes equations through finite-volume dictretization principals. A two-dimensional approximate analysis procedure is used together with a two-equation turbulence approximation in the form of the "standard" k-c turbulence model. Available software is used and adapted where applicable. A suitable method for comparing wing section characteristics as a function of profile geometry and attitude is developed in this thesis. This is achieved by first refining a numerical test case and quantifying the influences of model parameters such as grid design, boundary conditions and solution variables. Alternative geometrical aerofoil concepts can then be characterized by employing the same principles. This thesis contains selected results of hundreds such numerical simulations, all of which were necessary to refine the test case and eventually characterize the aerofoils. The proposed wing section geometry, incorporating a rearward-facing step shows some improvement in aerodynamic performance over a standard reference case. Geometrical variations of the step concept are also investigated and can later be used in an optimization procedure. A transient simulation approach is employed for unsteady cases and flow visualization is done in order to learn more about the unique aerodynamic action of the proposed concept. Experimental results obtained in a wind tunnel for the pressure around the investigated aerofoils are used to verify numerical results. Further development in the numerical approach may include the use of additional, more advanced turbulence models. This may allow the research of more complex phenomena such as stall and also broader ranges of Reynolds numbers in more detail. To complete the characterization process, the moment coefficients should also be included.

The Fraser River plume is the brackish surface layer formed when the Fraser River discharges into the Strait of Georgia. Two approaches to understanding the dynamics of the plume are discussed. Initially, a series of field observations was carried out in the plume. These consisted mainly of CSTD profiles and current profiles in the upper 10-20 meters of the water column. Also, a surface current meter was installed for 34 days at the mouth of the Fraser River. The principal conclusions of the field observations are: the plume is strongly sheared in the vertical and strongly stratified; this vertical structure is most apparent in the vicinity of the river mouth, and around the time of maximum river discharge (near low water in the Strait); and that the water moving outward from the river mouth subsequently acquires velocities and salinities appropriate to the water beneath it with length and time scales for this change of order 50 km and 8 hours. The plume thickness varies between 0 and 10 meters; the salinity varies from 0 to that of the water beneath it (approx. 25 ‰); and the difference between the plume velocity and that of the water beneath it varies from up to 3.5 m/sec to 0 m/sec, and is typically of order 0.5 m/sec over much of the plume area. Inspired by the field data, a model of the thin upper layer was developed. The independent variables are the two components of transport in the upper layer, the thickness of the layer, and the integrated salinity in the upper layer. The bottom of the upper layer has been tentatively defined by an isopycnal surface. The mixing across this interface is modelled by an upward flux of salt water (entrainment), and a downward flux of brackish water (termed depletion in this work). The dynamical effects included in this model are: the local time derivative; the field accelerations; the buoyant spreading pressure gradient (including the effects of salinity on the density field); the entrainment of tidally moving water and the loss by the depletion mechanism of water with the plume momentum; the frictional stress between the plume and the water beneath it; the forcing due to the baroclinic tidal slopes; and the Coriolis force. Subsets of the full model equations are examined, to clarify certain aspects of the plume dynamics. Preliminary results from the numerical solution of the full model eguations are presented, and a comparison is made between the paths of lagrangian trackers produced by the model and drogue tracks observed in the plume. Future improvements to the model are discussed. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

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No description available.

Free jets are naturally unstable. As a result the jet which is initially laminar becomes turbulent. During this transition process, large-scale structures are formed and fluid is "induced" to join the jet from the surroundings. This induction of fluid is defined as entrainment and depends greatly upon the jet geometry. In particular, an elliptical jet has been found to entrain at a higher rate than its circular counterpart. Numerical simulations of elliptical jets have been conducted on NRL’s parallel CM-5 and CM-200 supercomputers using the Flux Corrected Transport algorithm and initialized with the results of a linear stability analysis with the objective of investigating the entrainment process and explaining the dependence of the entrainment on jet geometry. Through the medium of scientific visualization, mechanisms responsible for entrainment have been identified and associated with the results of a linear stability analysis to suggest passive means by which entrainment can be enhanced. In particular, it was found that increasing the aspect ratio, thinning the shear layer, and non-uniformly distributing the shear layer serve to increase entrainment. / Master of Science

The need for a validation technique for computational fluid dynamics (CFD) codes in STOVL applications has led to research efforts to apply infrared thermal imaging techniques to visualize gaseous flow fields. Specifically, a heated, free-jet test facility was constructed. The gaseous flow field of the jet exhaust was characterized using an infrared imaging technique in the 2 to 5.6μm wavelength band as well as conventional pitot tube and thermocouple methods. These infrared images are compared to computer-generated images using the equations of radiative exchange based on the temperature distribution in the jet exhaust measured with the thermocouple traverses. Temperature and velocity measurement techniques, infrared imaging, and the computer model of the infrared imaging technique are presented and discussed. From the study, it is concluded that infrared imaging techniques coupled with the radiative exchange equations applied to CFD models are a valid method to qualitatively verify CFD codes used in STOVL applications. / Master of Science

Suspension flows in varied settings and at different concentrations of particles are studied theoretically using various modeling techniques. Particulate suspension flows are dispersion of particles in a continuous medium and their properties are a consequence of the interplay among hydrodynamic, buoyancy, interparticle and Brownian forces. The applicability of continuum modeling techniques to suspension flows at different particle concentration was assessed by studying systems at different time and length scales. The first two studies involve the use of modeling techniques that are valid in systems where the forces between particles are negligible, which is the case in dilute suspension flows. In the first study, the growth and progradation of deltaic geologic bodies from the sedimentation of particles from dilute turbidity currents is modeled using the shallow water equations or vertically averaged equations of motions coupled with a particle conservation equation. The shallow water model provides a basis for extracting grain size and depositional history information from seismic data. Next, the Navier-Stokes equations of motion and the convection-diffusion equation are used to model suspension flow in a biomedical application involving the flow and reaction of drug laden nanovectors in arteries. Results from this study are then used prescribe the best design parameters for optimal nanovector uptake at the desired sites within an artery. The third study involves the use of macroscopic two phase models to describe concentrated suspension flows where interparticle hydrodynamic forces cannot be neglected. The isotropic form of both the diffusion-flux and the suspension balance models are solved for a buoyant bidisperse pressure-driven flow system. The model predictions are found to compare fairly well with experimental results obtained previously in our laboratory. Finally, the power of discrete type models in connecting macroscopic observations to structural details is demonstrated by studying a system of aggregating colloidal particles via Brownian dynamics. The results from the simulations match experimental shear rheology and also provide a structural explanation for the observed macroscopic behavior of aging. / text

In this work, flow through synthetic arrangements of contacting spheres is studied as a model problem for porous media and packed bed type flows. Direct numerical simulations are performed for moderate pore Reynolds numbers in the range, 10 ≤ Re ≤ 600, where non-linear porescale flow features are known to contribute significantly to macroscale properties of engineering interest. To first choose and validate appropriate computational models for this problem, the relative performance of two numerical approaches involving body conforming and non-conforming grids for simulating porescale flows is examined. In the first approach, an unstructured solver is used with tetrahedral meshes, which conform to the boundaries of the porespace. In the second approach, a fictitious domain formulation (Apte et al., 2009. J Comput. Phys. 228 (8), 2712-2738) is used, which employs non-body conforming Cartesian grids and enforces the no-slip conditions on the pore boundaries implicitly through a rigidity constraint force. Detailed grid convergence studies of both steady and unsteady flow through prototypical arrangements of spheres indicate that for a fixed level of uncertainty, significantly lower grid densities may be used with the fictitious domain approach, which also does not require complex grid generation techniques. Next, flows through both random and structured arrangements of spheres are simulated at pore Reynolds numbers in the steady inertial ( 10 ≲ Re ≲ 200) and unsteady inertial (Re ≈ 600) regimes, and used to analyze the characteristics of porescale vortical structures. Even at similar Reynolds numbers, the vortical structures observed in structured and random packings are remarkably different. The interior of the structured packings are dominated by multi-lobed vortex rings structures that align with the principal axes of the packing, but perpendicular to the mean flow. The random packing is dominated by helical vortices, elongated parallel to the mean flow direction. The unsteady dynamics observed in random and structured arrangements are also distinct, and are linked to the behavior of the porescale vortices. Finally, to investigate the existence and behavior of transport barriers in packed beds, a numerical tool is developed to compute high resolution finite-time Lyapunov exponent (FTLE) fields on-the-fly during DNS of unsteady flows. Ridges in this field are known to correspond to Lagrangian Coherent Structures (LCS), which are invariant barriers to transport and form the skeleton of time dependent Lagrangian fluid motion. The algorithm and its implementation into a parallel DNS solver are described in detail and used to explore several flows, including unsteady inertial flow in a random sphere packing. The resulting FTLE fields unambiguously define the boundaries of dynamically distinct porescale features such as counter rotating helical vortices and jets, and capture time dependent phenomena including vortex shedding at the pore level. / Graduation date: 2013

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1979. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Paul David Bayless. / M.S.