http://hdl.handle.net/1721.1/7799 2017-06-08T04:22:58Z http://hdl.handle.net/1721.1/109644 Multiphase flow in porous media: the impact of capillarity and wettability from field-scale to pore-scale Zhao, Benzhong Multiphase flow in the context of this Thesis refers to the simultaneous flow of immiscible fluids. It differs significantly from single-phase flow due to the existence of fluid-fluid interfaces, which are subject to capillary forces. Multiphase flow in porous media is important in many natural and industrial processes, including geologic carbon dioxide (CO₂) sequestration, enhanced oil recovery, and water infiltration into soil. Despite its importance, much of our current description of multiphase flow in porous media is based on semi-empirical extensions of single-phase flow theories, which miss key physical mechanisms that are unique to multiphase systems. One challenging aspect of solving this problem is visualization-flow typically occurs inside opaque media and hence eludes direct observation. Another challenging aspect of multiphase flow in porous media is that it encompasses a wide spectrum of length scales-while capillary force is active at the pore-scale (on the order of microns), it can have a significant impact at the field-scale (on the order of kilometers). In this Thesis, we employ novel laboratory experiments and mathematical modeling to study multiphase flow in porous media across scales. The field-scale portion of this Thesis focuses on gravity-driven flows in the subsurface, with an emphasis on application to geological CO₂ storage. We find that capillary forces can slow and stop the migration of a CO₂ plume. The meso-scale portion of this Thesis demonstrates the powerful control of wettability on multiphase flow in porous media, which is manifested in the markedly different invasion protocols that emerge when one fluid displaces another in a patterned microfluidic cell. The pore-scale portion of this Thesis focuses on the impact of wettability on fluid-fluid displacement inside a capillary tube. We show that the contact line movement is strongly affected by wettability, even in regimes where viscous forces dominate capillary forces. Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2017.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 95-104). 2017-01-01T00:00:00Z http://hdl.handle.net/1721.1/109643 Water quality and hydrological assessment and modeling of bioretention basins in tropical cities Wang, Jia, Ph. D. Massachusetts Institute of Technology The bioretention basin is a form of stormwater best management practice that originated in temperate regions and is becoming increasingly popular in tropical regions. Nevertheless, it is a poorly evaluated system in the tropics due to the limited availability of long-term, high-resolution hydrological and multi-component water quality data on completed basins. The resulting gaps in data and knowledge limit the ability of environmental managers to control nonpoint source pollution by stormwater. In this thesis, we provide a survey of basin performance in treating 15 water quality parameters in the Balam Estate Rain Garden in Singapore. We then use a combination of field observations and model simulations to recommend practical engineering strategies to design and manage bioretention basins in the tropics. First, field observations on water quality performance in six sampled events show that removal targets for total nitrogen, total phosphorus, and total suspended solids are generally met only for low-depth rainfall events and not high-depth events. We attribute this to the low influent event mean concentration and weak first flush resulting from the frequent and intense rainfall of the tropical climate. Second, field observations on hydrological performance in 80 events show that a lack of storage capacity and resulting high culvert overflow is the main driver in reducing removal efficiency for large, but still common, storm events. We recommend that design guidelines in the tropics be specified in terms of the more definitive quantity, water quality depth (WQD), instead of average return interval (ARI). Third, we investigate design configurations for improved hydrological performance efficiency by first demonstrating the applicability of the RECHARGE numerical hydrological model for continuous simulation over a half-year period at the field scale. Sensitivity analysis using the model suggests design configurations that improve basin hydrological performance. Lastly, we develop a process-based numerical water quality model using the mathematical formulations from CW2D to describe the bacterial dynamics in contributing to nitrogen removal in the subsurface soil media. Sensitivity analysis using the model produces bioretention basin design curves that are suitable for the tropics. Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2017.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 232-242). 2017-01-01T00:00:00Z http://hdl.handle.net/1721.1/109642 Simulating fluid-solid interaction using smoothed particle hydrodynamics method Pan, Kai, Ph. D. Massachusetts Institute of Technology The fluid-solid interaction (FSI) is a challenging process for numerical models since it requires accounting for the interactions of deformable materials that are governed by different equations of state. It calls for the modeling of large deformation, geometrical discontinuity, material failure, including crack propagation, and the computation of flow induced loads on evolving fluid-solid interfaces. Using particle methods with no prescribed geometric linkages allows high deformations to be dealt with easily in cases where grid-based methods would introduce difficulties. Smoothed Particle Hydrodynamics (SPH) method is one of the oldest mesh-free methods, and it has gained popularity over the last decades to simulate initially fluids and more recently solids. This dissertation is focused on developing a general numerical modeling framework based on SPH to model the coupled problem, with application to wave impact on floating offshore structures, and the hydraulic fracturing of rocks induced by fluid pressure. An accurate estimate of forces exerted by waves on offshore structures is vital to assess potential risks to structural integrity. The dissertation first explores a weakly compressible SPH method to simulate the wave impact on rigid-body floating structures. Model predictions are validated against two sets of experimental data, namely the dam-break fluid impact on a fixed structure, and the wave induced motion of a floating cube. Following validation, this framework is applied to simulation of the mipact of large waves on an offshore structure. A new numerical technique is proposed for generating multi-modal and multi-directional sea waves with SPH. The waves are generated by moving the side boundaries of the fluid domain according to the sum of Fourier modes, each with its own direction, amplitude and wave frequency. By carefully selecting the amplitudes and the frequencies, the ensemble of wave modes can be chosen to satisfy a real sea wave spectrum. The method is used to simulate an extreme wave event, with generally good agreement between the simulated waves and the recorded real-life data. The second application is the modeling of hydro-fracture initiation and propagation in rocks. A new general SPH numerical coupling method is developed to model the interaction between fluids and solids, which includes non-linear deformation and dynamic fracture initiation and propagation. A Grady-Kipp damage model is employed to model the tensile failure of the solid and a Drucker-Prager plasticity model is used to predict material shear failures. These models are coupled together so that both shear and tensile failures can be simulated within the same scheme. Fluid and solid are treated as a single system for the entire domain, and are computed using the same stress representation within a uniform SPH framework. Two new stress coupling approaches are proposed to maintain the stress continuity at the fluid-solid interface, namely, a continuum approach and stress-boundary-condition approach. A corrected form of the density continuity equation is implemented to handle the density discontinuity of the two phases at the interface. The method is validated against analytic solutions for a hydrostatic problem and for a pressurized borehole in the presence of in-situ stresses. The simulation of hydro-fracture initiation and propagation in the presence of in-situ stresses is also presented. Good results demonstrate that SPH has the potential to accurately simulate the hydraulic-fracturing phenomenon in rocks. Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2017.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 97-102). 2017-01-01T00:00:00Z http://hdl.handle.net/1721.1/109641 Spectral modeling of an idealized atmospheric surface layer McColl, Kaighin Alexander Almost all of humanity resides in the atmospheric surface layer (ASL), so its state (e.g., temperature, humidity, wind velocity) is relevant to a range of applications in human health, agriculture, and ecosystem health. However, the ASL is turbulent, and therefore characterized by complex dynamics across a wide range of spatial and temporal scales. Explicitly modelling turbulent motions in the ASL at all scales is computationally expensive and beyond current capabilities. In this thesis, a framework is proposed for parsimoniously modelling a broad range of turbulent motions in wall-bounded turbulent flows such as the ASL, using spectra of turbulent fluctuations as inputs. Turbulent spectra contain information on turbulent motions across scales, and are constrained by theory and observations. By propagating spectra through a cospectral budget, a model of the mean velocity profile (MVP) is obtained. Comparison with a direct numerical simulation (DNS) of a neutral channel flow reveals a good correspondence between the MVPs of the cospectral budget model and DNS, provided the pressure-decorrelation model in the cospectral budget includes established effects of wall-blocking. This work demonstrates that the distribution of turbulent vertical velocity fluctuations (the 'microstate' of the flow) contains sufficient information to generate the MVP (the 'macrostate' of the flow). It also establishes a link between two previously unrelated areas of the turbulence literature: 1) Kolmogorov's scaling of the turbulent energy spectrum, derived for homogeneous, isotropic turbulence and 2) the 'law of the wall' in wall-bounded turbulence. The cospectral budget model is then extended to the case where the wall-bounded flow is heated from below, as in an unstable ASL. The MVP and mean buoyancy profile (MBP) of the cospectral budget model and the DNS agree qualitatively, with remaining differences attributable to neglected terms in the cospectral budget, and the low Reynolds number of the DNS. The normalized turbulent statistics of the heated duct flow DNS agree surprisingly well with ASL measurements, despite the low Reynolds number of the DNS and other differences. Treating the DNS as an idealized ASL, a spectral model is derived to describe the partitioning of turbulent kinetic and potential energy between turbulent transport of heat and momentum in the ASL. The model reproduces observed dissimilarity between turbulent heat and momentum transport in unstable conditions. It attributes the dissimilarity to contributions from large eddies in turbulent heat transport, which are largely ignored in existing ASL parameterizations in weather and climate models. Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2017.; Cataloged from PDF version of thesis.; Includes bibliographical references (pages 115-126). 2017-01-01T00:00:00Z