Hot-wire measurements taken in the turbulent wake of a flat plate are presented. Symmetrical and antisymmetrical perturbations at various amplitudes and frequencies were introduced into the wake by small flap oscillations. As predicted by linear stability theory, the sinuous (antisymmetric) mode was observed to be more significant than the varicose (symmetric) mode. When the amplitude of the perturbation was low, the spatial development of the introduced coherent perturbation was predicted well by linear stability theory. At high forcing levels, the wake spreading showed dramatic deviations from the well known square-root behavior of the unforced case. Measured coherent Reynolds stresses changed sign in the neighborhood of the neutral point of the perturbation, as predicted by the linear theory. However, the linear theory failed to predict the disturbance amplitude and transverse shapes close to the neutral point. Some nonlinear aspects of the evolution of instabilities in the wake are discussed. Theoretical predictions of the mean flow distortion and the generation of the first harmonic are compared to experimental measurements. Given the unforced flow and the amplitude of the fundamental wave, the mean flow distortion and the amplitude of the first harmonic are predicted remarkably well.

Numerical model for simulating sediment transport in unsteady flow is incomplete in several aspects: first of all, the numerical schemes have been proved suitable for the simulation of flow over rigid bed needs to be reevaluated for unsteady flow over mobile bed; secondly, existing non-equilibrium sediment transport models are empirically developed and therefore lack of consistency regarding the evaluation of the non-equilibrium parameters; thirdly, the sediment transport in various applications have unique features which needs to be considered in the models. Sediment transport in unsteady flows was studied using analytical and numerical methods. A one dimensional (1D) finite volume method (FVM) model was developed. Five popular numerical schemes were implemented into the model and their performances were evaluated under highly unsteady flow condition. A novel physically-based non-equilibrium sediment transport model was established to describe the non-equilibrium sediment transport process. Infiltration effects on flow and sediment transport was included to make the model applicable to simulate irrigation induced soil erosion in furrows. The Laursen (1958) formula was adopted and modified to calculate the erodibility of fine-grain sized soil, and then verified by laboratory and field datasets. The numerical model was applied to a series of simulations of sediment transport in highly unsteady flow including the dam break erosional flow, flash flood in natural rivers and irrigation flows and proved to be applicable in various applications. The first order schemes were able to produce smooth and reasonably accurate results, and spurious oscillations were observed in the simulated results produced by second order schemes. The proposed non-equilibrium sediment transport model yielded better results than several other models in the literatures. The modified Laursen (1958) formula adopted was applicable in calculating the erodibility of the soil in irrigation. Additionally, it was indicated that the effect of the jet erosion and the structural failure of the discontinuous bed topography cannot be properly accounted for due to the limitation of 1D model. The comparison between the simulated and measured sediment discharge hydrographs indicated a potential process associated to the transport of the fine-grain sized soil in the irrigation furrows.

The largely unstudied extension of ultrasonic circulation measurement techniques (UCMT) to determine instantaneous lift in unsteady and three-dimensional flows has been addressed in this work. A combined analytical-numerical-experimental approach was undertaken with the goal of developing methods to properly convert the measurable time-dependent bound circulation to instantaneous lift force in unsteady flows. The measurement of mean sectional lift distribution along structure spans in three-dimensional flows was also studied. An unsteady correction method for thin airfoils was developed analytically and validated numerically (with finite element solutions) to properly convert bound circulation to instantaneous lift based on unsteady potential flow theory. Results show that the unsteady correction method can provide increased accuracy for unsteady lift prediction over the Kutta-Joukowski method used in previous unsteady flow studies. The unsteady correction model generally should be included for instantaneous lift prediction as long as the bound circulation is time-dependent. Using the same framework, we also studied determination of instantaneous lift forces on stationary bluff bodies (circular cylinders) at low Reynolds number (Re=100). Various force models, including an approximate vortex force model, were studied. A new unsteady model, similar to that developed for the thin airfoils, using instantaneous bound circulation values, was proposed. Another important issue studied in this thesis is the effect of acoustic path sensitivity on bound circulation determination, which we found to be crucial for accurately predicting the instantaneous lift in both unsteady flat plate and cylinder flows. Proper path selection should take into account the location of boundary layers, attached and shed vortices. These findings will be useful in future experimental design of UCMT, PIV and LDV methods. Finally, we used the UCMT method to experimentally study the mean spatial lift distribution along structures. Low Reynolds number low aspect ratio (AR) wings that have application in micro-aerial-vehicles (MAV) were studied. The spanwise circulation (lift) distribution along the MAV wings exhibits a peak (maximum), and deviates from predictions of Prandtl's lifting line theory. Although only 'linear' lift (due to bound circulation) was measured, comparison with force balance results showed that reasonable integrated lift values on low Re, low AR wings can be obtained using UCMT.

Gravel bed braided rivers are distinctive natural environments that provide a wide range of key environmental, economic and recreational services. There is, however, a growing concern that over the twentieth century, an increasing number of braided rivers have metamorphosed into wandering or single thread channels, representing a loss of key habitats, geodiversity and amenity. While in some situations, shifts in channel pattern may be unambiguously linked to abrupt changes in flow or sediment supply, the lack of a theoretical basis underpinning the development and maintenance of braiding makes identification of the cause and effect of channel metamorphosis hazardous. A growing body of research has suggested that the transition between channel patterns may depend on the poorly understood interaction between the flow regime, sediment supply and vegetation colonisation. Such interactions are governed by critical thresholds, due to changes in flow resistance and bank strength associated with the distribution, form and intensity of vegetation colonisation. Subtle changes in flow or sediment supply that promote vegetation growth or indeed remove it through inundation or attrition. This can lead to complex non-linear shifts in the balance of forces that govern sediment transport and bedform morphodynamics, ultimately resulting in one-way changes in channel morphology. There is, therefore, a critical need to develop a quantitative understanding of these feedbacks in order to design sustainable river management programmes that seek to optimize the ecological and socio-economic benefits these rivers offer. During the last three decades, significant advances in the understanding of the morphodynamics of braided rivers have been made through a combination of field and physical experimentation. More recently, the emerging field of numerical modelling has created a new avenue to investigate the processes that govern channel dynamics. While this methodology offers significant promise through the construction of virtual experiments that examine the spectrum of drivers and responses of river systems, such models require careful and critical evaluation before they can be used to guide management practice. The wider goal of this research is to explore the application of a numerical modelling to investigate the feedbacks associated with the development and maintenance of braiding. Specifically, the state-of-the-art numerical model, BASEMENT, was used to examine channel responses to steady, and unsteady flow regimes, with and without the representation of vegetation. The research focuses on four main contributions: 1. The development of a systematic framework to quantify the evolving form and processes of braided rivers that can be used as part of a comprehensive approach to model validation. 2. Simulation of braiding development and maintenance using BASEMENT under steady flow conditions. Model simulations based on the natural prototype of the braided River Feshie were used to examine the sensitivity of emergent channel morphologies to the model parameterisation, focusing in particular on the representation of bank erosion and gravity-driven sediment transport. A novel multi6metric framework for model validation is presented and the results demonstrate the critical importance of lateral bank migration processes in order to maintain braided morphologies under steady flow. 3. A critical evaluation of the simulation of braiding under different form of steady and unsteady flow regimes is presented. These experiments investigate how the morphodynamics of braiding vary under energetically-normalised flow regimes characterized by differences in hydrograph form (peak discharge and duration). This experiment provides a novel insight into the role of flow variation in the maintenance of braiding. 4. Finally, the feedback between flow regimes, sediment transport and vegetation growth are examined using a novel model of vegetation colonisation and die- back. Four scenarios are presented, a no-vegetation model, one based on low growth rate, one based on an intermediate growth rate, and finally a high growth rate parameterisation. These simulations provide a clear insight into the non-linear processes driving channel evolution and demonstrate how subtle changes in the balance between flow frequency and vegetation growth can lead to divergent channel patterns. In summary, this thesis aims to advance our understanding of the morphodynamics of braided rivers and the role numerical models may have in helping to interrogate their behaviour and governing controls. It is hoped that this work may contribute, albeit in a small way, to advancing the science that promotes the sustainability of these fascinating and valuable environments.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERO / Includes bibliographical references. / by Saeed Farokhi. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Michael E. McClurken. / Ph.D.

The present study is an integral part of a broader study focused on the design and implementation of self-cleaning culverts, i.e., configurations that prevent the formation of sediment deposits after culvert construction or cleaning. Sediment deposition at culverts is influenced by many factors, including the size and characteristics of material of which the channel is composed, the hydraulic characteristics generated under different hydrologic events, the culvert geometry design, channel transition design, and the vegetation around the channel. The multitude of combinations produced by this set of variables makes the investigation of practical situations challenging. In addition to the above considerations, the field observations, and the laboratory and numerical experiments have revealed additional complexities of the flow and sediment transport through culverts that further increase the dimensions of the investigation. The flow complexities investigated in this study entail: flow non-uniformity in the areas of transition to and from the culvert, flow unsteadiness due to the flood wave propagation, and the complex correlation between the flow and sediment hydrographs produced during storm events. To date, the literature contains no systematic studies on sediment transport through multi-box culverts. Similarly, there is limited knowledge about the non-uniform, unsteady sediment transport in channels of variable geometry. Furthermore, there are few readily useable numerical models that can reliably simulate flow and sediment transport in such complex situations. Given the current state of knowledge, the main goal of the present study is to investigate the above flow complexities in order to provide the needed insights for optimizing the culvert design. The research was phased so that field observations were conducted first to understand the culvert behavior in Iowa landscape. Modeling through complementary hydraulic model and numerical experiments was subsequently carried out to gain the practical knowledge for the development of the self-cleaning culvert designs.

Unsteady flow separation represents a highly complex and important area of study within fluid mechanics. The extent of separation and specific time scales over which it occurs are not fully understood and has significant consequences in numerous industrial applications such as helicopters, jet engines, hydroelectric turbines and wind turbines. A direct consequence of unsteady separation is the erratic movement of the separation point which causes highly dynamic and unpredictable loads on an airfoil. Current computational models underestimate the aerodynamic loads due to the inaccurate prediction of the emergence and severity of unsteady flow separation especially in response to a sudden change in the effective angle of attack. To capture the complex flow phenomena over wind turbine blades during stall development, a scaled three-dimensional non-rotating blade model is designed to be dynamically similar to a rotating full-scale NREL 5MW wind turbine blade. A time-resolved particle image velocimetry (PIV) investigation of flow behavior during the stall cycle examines the processes of stall development and flow reattachment. The flow fields are examined through the application of Eulerian techniques such as proper orthogonal decomposition and empirical mode decomposition to capture unsteady separation characteristics within the flow field. Then, for a higher order description, coherent structures such as vortices and material lines are resolved to fully characterize the flow during a full pitching cycle in a Lagrangian framework. The Eulerian and Lagrangian methods described in the present analysis is extended to investigate the spanwise characteristics within the root section of a three dimensional airfoil. Furthermore, statistical information of the separation point is pursued along four spanwise positions during two cases of unsteady separation. The results of the study describe a critical role of surface vorticity accumulation in unsteady separation and reattachment. Evaluation of the unsteady characteristics of the shear layer reveal evidence that the build-up and shedding of surface vorticity directly influence the dynamic changes in separation point. The quantitative characterization of surface vorticity and shear layer stability enables improved aerodynamic design, but also has broader implications on the larger discipline of unsteady fluid dynamics.

Wave Number Selection and Defect Dynamics in Patterns with Hexagonal Symmetry Denis B. Semwogerere 108 Pages Directed by Dr. Michael F. Schatz We report quantitative measurements of wave number selection, secondary instability and defect dynamics in hexagonal patterns. A novel optical technique ("thermal laser writing") is used to imprint initial patterns with selected characteristics in a B뮡rd-Marangoni convection experiment. Initial patterns of ideal hexagons are imposed to determine the band of stable-pattern wave numbers. For small values of control parameter epsilon the measured stable band is found to agree quantitatively with theoretical predictions at the low-wave-number side of the band, and qualitatively at the high-wave-number side. Long-wavelength perturbations of ideal hexagonal patterns suggested by theory are imposed for epsilon=0.46 and their growth rates are measured to investigate the mechanisms of secondary instability. Our results suggest a transverse-phase instability limits stable hexagons at low wave number while a longitudinal-phase instability limits high-wave-number hexagons. Initial patterns containing an isolated penta-hepta defect are imprinted to study defect propagation directions and velocities. The experimental results agree well with theoretical predictions. The experimental investigations are discussed in the context of patterns with hexagonal symmetry formed under nonequilibrium external driving conditions.

Thesis (Ph. D.)--Missouri University of Science and Technology, 2008. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed May 12, 2008) Includes bibliographical references.