Debris flows, avalanches and other geophysical mass flows pose a significant hazard to settlements in or near mountainous regions. Understanding the physical processes that govern these flows is an essential part of hazard assessment and mitigation strategies. This thesis addresses two aspects of geophysical mass flows: flow self-channelisation due to the formation of lateral levees, and granular shocks, which occur when a rapidly-moving debris flow or avalanche collides with an obstacle. We present the results of large-scale debris flow experiments in which the flow is channelised by coarse-particle levees that form at the flow margins. The flow surface velocities are measured with high speed overhead photography, and the deposits both sampled to obtain the grain size distribution and excavated to recover the deposited locations of tracer pebbles that were introduced in to the flow. We propose a model, supported by evidence from the large-scale experiments, that describes in detail the size segregation and kinematic transport processes responsible for the deposition of lateral levees. The second problem addressed in the thesis concerns granular shocks, or jumps, which are rapid changes in the depth and velocity of granular avalanches. We investigate these through experiments in which a falling jet of granular material impacts on an inclined plane, generating a steady granular jump, which is either teardrop-shaped or 'blunted'. Numerical solutions of a depth-averaged flow model agree quantitatively with many of the observed flow features. We use this model show that the transition between the teardrop-shaped and blunted jump regimes corresponds to a transition between two shock reflection structures, known as a regular and a Mach shock reflection. On planes inclined at a shallow angle, we demonstrate a wide variety of unsteady and channelised flows, which occur due to the complex interaction between flowing and stationary regions of granular material.

This study focuses on the labor market outcomes of immigration flows. To obtain a reliable view, I try to find evidence from the agricultural sector, whereby around half of the workers are undocumented. In recessionary periods, the labor demand might shift to the left in an unobservable manner. The reasons mainly lie that the steady demand for major fresh vegetables.1 Besides, most of the foreign-born farmworkers are seasonal. Hence, the job opportunities might maintain a similar level as at ordinary times. In other words, the agricultural sector might hold additional job vacancies while other sectors are facing a rising unemployment rate at recessionary times. During recessions, the undocumented immigrants could be crowded out by documented workers who were laid off from other sectors, or they could be unaffected because less than two percent of the US's native-born labor force would like working on farms. This study addresses: (1) what are the compositional changes of foreign-born farmworkers? (2) What are the changes in hours worked of foreign-born farmworkers? And (3) what are the changes in stays of those farmworkers if they enter the US at recessionary times? This study reveals that during recessions, the share of documented foreign-born farmworkers, the share of newcomers, and the share of undocumented newcomers decreases.3 The number of hours worked rises for both foreign-born documented and undocumented agricultural workers. Shorter duration spells are observed if foreign-born farmworkers enter the US during recessions, especially for foreign-born documented workers. These findings suggest a possible downsized labor supply in recessions and employed agricultural workers could choose to work more hours if they want to.4 Also, during recessions, foreign-born documented agricultural workers tend to shorten their stays. Overall, these findings together demonstrate that recessions deter immigration flows.

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This dissertation considers the genesis and dynamics of interfacial instability in vertical laminar gas-liquid flows, using as a model the two-dimensional channel flow of a thin falling film sheared by counter-current gas. The methodology is linear stability theory by means of Orr-Sommerfeld analysis together with direct numerical simulation of the two-phase flow in the case of nonlinear disturbances. The influence of two main flow parameters on the interfacial dynamics, namely the film thickness and pressure drop applied to drive the gas stream, is investigated. To make contact with existing studies in the literature, the effect of various density and viscosity contrasts as well as surface tension is also examined. Energy budget analyses based on the Orr-Sommerfeld theory reveal various coexisting unstable modes (interfacial, shear, internal) in the case of high density contrasts, which results in mode coalescence and mode competition, but only one dynamically relevant unstable interfacial mode for low and intermediate density contrast. Furthermore, high viscosity contrast and increases in surface tension lead to some amount of mode competition for thin film. A study of absolute and convective instability for low density contrast shows that the system is absolutely unstable for all but two narrow regions of the investigated parameter space. These regions are extended at intermediate density contrast and exhibit only small changes with increased viscosity contrast or surface tension. Direct numerical simulations of the system with low density contrast show that linear theory holds up remarkably well upon the onset of large-amplitude waves as well as the existence of weakly nonlinear waves. For high density contrasts corresponding more closely to an air-water-type system, linear stability theory is also successful at determining the most-dominant features in the interfacial wave dynamics at early-to-intermediate times. Nevertheless, the short waves selected by the linear theory undergo secondary instability and the wave train is no longer regular but rather exhibits chaotic motion. Furthermore, linear stability theory also predicts when the direction of travel of the waves changes - from downwards to upwards. The practical implications of this change in terms of loading and flooding is discussed. The change in direction of the wave propagation is represented graphically for each investigated system in terms of a flow map based on the liquid and gas flow rates and the prediction carries over to the nonlinear regime with only a small deviation. Besides the semi-analytical and numerical analyses, experiments with an practically relevant setup and flow system have been carried out to benchmark and validate the models developed in this work.

The prediction of two-phase fluid-solid (gas-solid and liquid-solid) flow remains a major challenge in many engineering and industrial applications. Numerical modeling of these flows is complicated and various studies have been conducted to improve the model performance. In the present work, the two-fluid model of Bolio et al. (1995), developed for dilute turbulent gas-solid flows, is employed to investigate turbulent two-phase liquid-solid flows in both a vertical pipe and a horizontal channel. <p>Fully developed turbulent gas-solid and liquid-solid flows in a vertical pipe and liquid-solid (slurry) flow in a horizontal channel are numerically simulated. The momentum equations for the fluid and solid phases were solved using the finite volume technique developed by Patankar (1980). Mean and fluctuating velocities for both phases, solids concentration, and pressure drop were predicted and compared with the available experimental data. In general, the mean velocity predictions for both phases were in good agreement with the experimental data for vertical flow cases, considered in this work. <p>For dilute gas-solid vertical flows, the predictions were compared with the experimental data of Tsuji et al. (1984). The gas-phase fluctuating velocity in the axial direction was significantly under-predicted while the results for the solids fluctuating velocity were mixed. There was no data to compare the solids concentration but the profiles looked realistic. The pressure drop was observed to increase with increasing Reynolds number and mass loading when compared with the data of Henthorn et al. (2005). The pressure drop first decreased as particle size increased and then started increasing. This behaviour was shown by both experimental data and model predictions. <p>For the liquid-solid flow simulations the mean velocity profiles for both phases, and the liquid-phase turbulence kinetic energy predictions (for dilute flow case), were in excellent agreement with the experimental data of Alejbegovic et al. (1995) and Sumner et al. (1990). The solids concentration profiles were poorly predicted, especially for the lighter particles. The granular temperature profiles, accounting for the solids velocity fluctuations, for the dilute flow case failed to agree with the data, although they captured the overall trend. The liquid-solid pressure drop predictions, using the present model, were only successful for some particles. <p>The solids concentration predictions for the horizontal flow case were similar to the experimental measurements of Salomon (1965), except for a sharp peak at the bottom wall and the opposite curvature. The mixture velocity profiles were asymmetric, due to the addition of particles, and were similar to the experimental data, though only a partial agreement was observed between the predictions and the data.<p>A conclusion from this work is that the present model, which was developed for dilute gas-solid flows, is inadequate when liquid-solid flows are considered. Further improvements, such as including the interstitial fluid effects while computing the liquid-phase stress, are needed to improve the predictive capability of this two-fluid model.

The prediction of two-phase fluid-solid (gas-solid and liquid-solid) flow remains a major challenge in many engineering and industrial applications. Numerical modeling of these flows is complicated and various studies have been conducted to improve the model performance. In the present work, the two-fluid model of Bolio et al. (1995), developed for dilute turbulent gas-solid flows, is employed to investigate turbulent two-phase liquid-solid flows in both a vertical pipe and a horizontal channel. <p>Fully developed turbulent gas-solid and liquid-solid flows in a vertical pipe and liquid-solid (slurry) flow in a horizontal channel are numerically simulated. The momentum equations for the fluid and solid phases were solved using the finite volume technique developed by Patankar (1980). Mean and fluctuating velocities for both phases, solids concentration, and pressure drop were predicted and compared with the available experimental data. In general, the mean velocity predictions for both phases were in good agreement with the experimental data for vertical flow cases, considered in this work. <p>For dilute gas-solid vertical flows, the predictions were compared with the experimental data of Tsuji et al. (1984). The gas-phase fluctuating velocity in the axial direction was significantly under-predicted while the results for the solids fluctuating velocity were mixed. There was no data to compare the solids concentration but the profiles looked realistic. The pressure drop was observed to increase with increasing Reynolds number and mass loading when compared with the data of Henthorn et al. (2005). The pressure drop first decreased as particle size increased and then started increasing. This behaviour was shown by both experimental data and model predictions. <p>For the liquid-solid flow simulations the mean velocity profiles for both phases, and the liquid-phase turbulence kinetic energy predictions (for dilute flow case), were in excellent agreement with the experimental data of Alejbegovic et al. (1995) and Sumner et al. (1990). The solids concentration profiles were poorly predicted, especially for the lighter particles. The granular temperature profiles, accounting for the solids velocity fluctuations, for the dilute flow case failed to agree with the data, although they captured the overall trend. The liquid-solid pressure drop predictions, using the present model, were only successful for some particles. <p>The solids concentration predictions for the horizontal flow case were similar to the experimental measurements of Salomon (1965), except for a sharp peak at the bottom wall and the opposite curvature. The mixture velocity profiles were asymmetric, due to the addition of particles, and were similar to the experimental data, though only a partial agreement was observed between the predictions and the data.<p>A conclusion from this work is that the present model, which was developed for dilute gas-solid flows, is inadequate when liquid-solid flows are considered. Further improvements, such as including the interstitial fluid effects while computing the liquid-phase stress, are needed to improve the predictive capability of this two-fluid model.

This thesis reports an experimental investigation of low Reynolds number particle-laden turbulent flows in a horizontal plane channel. Experiments were conducted over a smooth wall and over two rough surfaces made from sand grain and gravel of relative roughness k/h ≈ 0.08 and 0.25, respectively, where k is the roughness height and h is the channel half-height. The flow was loaded with small solid particles with diameters less than 1/10 of the length scale of the energy-containing eddies, and whose concentrations decreased with time due to sedimentation. A novel particle image velocimetry (PIV) method that employed colour filtering for phase discrimination was used to measure the velocities of the fluid and solid particles. Over the smooth wall, the particles mean velocity, turbulence intensities and Reynolds shear stress matched those of the unladen flow very well. There were substantial differences between particle and fluid profiles over the rough wall, which include more rapid reduction in the particle mean velocity and significantly larger turbulence intensities and Reynolds shear stress compared to the unladen flow values. Stratification of the particle concentration led to attenuation of the fluid wall-normal turbulence intensity. This effect was nullified by the roughness perturbation leading to collapse of the wall-normal turbulence intensities over the rough wall. The streamwise turbulence intensity also collapsed over the rough wall but it was found that particles augmented the fluid Reynolds shear stress due to enhanced correlation between the rough wall streamwise and wall-normal velocity fluctuations. A quadrant decomposition of the fluid Reynolds shear stress also revealed corresponding enhancements in ejections and sweeps, the dominant contributors to the Reynolds shear stress, over the rough wall. Based on two-point correlations between the velocity fluctuations and between the velocity fluctuations and swirling strength, it was concluded that both wall roughness and particles modified the turbulence structure by increasing the size of the larger-scale structures. The idea of eddies growing from the wall, thereby enhancing communication between the inner layer and outer parts of the flow, has implications for wall-layer models that assume that the outer layer is detached from the turbulence in the inner region.

Thesis (Ph. D.)--University of Hawaii at Manoa, 1994. / Includes bibliographical references (leaves 222-247). / Microfiche. / xvii, 246 p. ill. (some col.), maps 29 cm