In this study, a comprehensive approach including mathematical, numerical and experimental study has been taken in order to develop new models for describing free surface flow behavior in porous media. The study suggested that modeling free-surface flow in porous media is possible using a single equation capable of showing proper transition between inertial and classical Darcian flow, based on the similarity distribution functions of depth and velocity. The developed integral model inherits both the flow regimes as depicted in the analysis. For both laminar and turbulent flows through porous media, the integral models give satisfactory results. Also the proposed algorithm for numerical simulation is capable of solving various problems of free-surface flow through porous media. This study adds a new dimension to fluid flow in porous media by replacing Darcy's equation with new models that are capable of representing both Darcy and non-Darcy flow behaviors. These are new nonlinear ordinary differential equations inherited both the flow regimes investigated. Integral formulations for unsteady depth distribution, velocity and front speed under constant water level and constant flux discharge inlet conditions have been developed based on similarity law. The formulations presented provide additional analytical insight about the intrusion dynamics. It is pointed out that, based on the self-similarity analysis, the temporal intrusion processes can be categorized into the inertia-pressure (IP) and the pressure-drag (PD) regimes. The early inertia-pressure regime is followed by the pressure-drag regime. In addition, the integral models proposed can be successfully used for the solution of a host of other nonlinear problems that admit self-similarity. The analytical and numerical solutions for constant inlet water level condition are verified with experimental observations. The unsteady distributions of flow depth, inflow velocity and front speeds are compared for various porous media characterized by its corresponding porosity and permeability. Analyses indicate that the integral models clearly represent the nonlinear flow behavior in porous media both in laminar and turbulent flow conditions. The integral model results are in agreement with those obtained by similarity solution for the temporal change of velocity, depth at inlet and front positions. The thesis also presents a computational fluid dynamics (CFD) model developed for the analysis of unsteady free-surface flows through porous media. Vertical two-dimensional numerical simulations are carried out for the free-surface flow inside the porous media governed by a set of Navier-Stokes equations extended for porous media flow. This model includes the convective and local inertia terms along with viscous diffusion term and resistance term comprising Darcy's linear resistance and Forchheimer's inertial resistance terms. The Finite volume method is applied using constrained interpolated propagation (CIP) method and highly simplified marker and cell (HSMAC) type pressure solver for the numerical solution. The evolution of moving free surface is governed by volume of fluid (VOF) method, adapted for the flow through porous media. To prevent the spurious oscillation and generate diffusion-free sharp interface, a third order monotone upstream-centered schemes for conservation laws (MUSCL) type total variation diminishing (TVD) schemes is used to solve the VOF convection equation. The power law derivation and validation for the general flux inflow condition are made for a channel having a backward facing step. The result of theoretical analysis is compared with that of the numerical simulation and it shows a good agreement. The model can be a tool for the proposition of some empirical flow relationships using multivariate correlation. In the case of rapid vertical infiltration of water through a vertical column filled with porous media, a number of experiments and analytical investigations are carried out to see the effect of acceleration in the intrusion process. It is concluded that the conventional infiltration models like Green-Ampts infiltration model cannot account for the acceleration effect in the case of high velocity flow. It is revealed that it takes certain time for intruding water to be accelerated to its peak velocity before decreasing to almost constant velocity. The investigations are made for two different cases: constant water level and variable water level above the porous media. For porous media having low permeability, the effect of acceleration was not so significant. In the case of dam break flow over horizontal porous strata, the model is applied to a complicated domain regarding both geometry and flow boundary conditions. Single set of governing equation is implemented to simulate the complex phenomenon. The model shows its capability in simulating the flow where interface between pressurized and open channel flow moves forward. The vertical acceleration has a significant effect on the rapid vertical infiltration which the shallow water equations cannot account for. In particular, it is shown that vertical two dimensional numerical solution that couples the fluid and solid systems simultaneously at macroscopic scale are feasible and extremely beneficial, shedding a new light into the phenomena unavailable otherwise. It is also found that the proposed numerical model can be used for the determination of storm water storage in porous sub-base in a typical road section. The capability of the model is assessed by using the unsteady inflow condition so as to simulate the condition during high precipitation. The model could be a promising tool for planners and decision makers for effective drainage calculations to mitigate urban flood. The model successfully simulates the free surface flow in the bulk fluid as well as in the porous region. The velocities and stresses are assumed to be continuous at the interface of free and porous media so that a single set of governing equations could be solved. The robustness of the model is demonstrated by the capability of the numerical approach proposed in this thesis. / Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第14916号 / 工博第3143号 / 新制||工||1471(附属図書館) / 27354 / UT51-2009-M830 / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 細田 尚, 教授 戸田 圭一, 准教授 岸田 潔 / 学位規則第4条第1項該当

Eyewall replacement cycle (ERC) is frequently observed during the evolution of intensifying Tropical Cyclones (TCs). Although intensely studied in recent years, the underlying mechanisms of ERC are still poorly understood, and the forecast of ERC remains a great challenge. To advance our understanding of ERC and provide insights in improvement of numerical forecast of ERC, a series of numerical simulations is performed to investigate ERCs in TC-like vortices on a f-plane. The simulated ERCs possess key features similar to those observed in real TCs including the formation of a secondary tangential wind maximum associated with the outer eyewall. The Sawyer-Eliassen equation and tangential momentum budget analyses are performed to diagnose the mechanisms underlying the secondary eyewall formation (SEF) and ERC. Our diagnoses reveal crucial roles of outer rainband heating in governing the formation and development of the secondary tangential wind maximum and demonstrate that the outer rainband convection must reach a critical strength relative to the eyewall before SEF and the subsequent ERC can occur. A positive feedback among low-level convection, acceleration of tangential winds in the boundary layer, and surface evaporation that leads to the development of ERC and a mechanism for the demise of inner eyewall that involves interaction between the transverse circulations induced by eyewall and outer rainband convection are proposed. The tangential momentum budget indicates that the net tendency of tangential wind is a small residual resultant from a large cancellation between tendencies induced by the resolved and sub-grid scale (SGS) processes. The large SGS contribution to the tangential wind budget explains different characteristics of ERC shown in previous numerical studies and poses a great challenge for a timely correct forecast of ERC. The sensitivity experiments show that ERCs are strongly subjected to model physics, vortex radial structure and background wind. The impact of model physics on ERC can be well understood with the interaction among eyewall/outer rainband heating, radilal inflow in the boundary layer, surface layer turbulent processes, and shallow convection in the moat. However, further investigations are needed to fully understand the exhibited sensitivities of ERC to vortex radial structure and background wind.

No description available.

In 2008 flooding occurred over a majority of Iowa, damaging homes, displacing residents, and taking lives. In the wake of this event, the Iowa Flood Center (IFC) was charged with the investigation of distributed flood mitigation strategies to reduce the frequency and magnitude of peak flows in Iowa. This dissertation is part of the several studies developed by the IFC and focused on the application of a coupled physics based modeling platform, to quantify the coupled benefits of distributed flood mitigation strategies on the reduction of peak flows in an agricultural watershed. Additional investigation into tile drainage and terraces, illustrated the hydrologic impact of each commonly applied agricultural practice. The effect of each practice was represented in numerical simulations through a parameter adjustment. Systems were analyzed at the field scale, to estimate representative parameters, and applied at the watershed scale. The impact of distributed flood mitigation wetlands reduced peak flows by 4 % to 17 % at the outlet of a 45 km2 watershed. Variability in reduction was a product of antecedent soil moisture, 24-hour design storm total depth, and initial structural storage capacity. The highest peak flow reductions occurred in scenarios with dry soil, empty project storage, and low rainfall depths. Peak flow reductions were estimated to dissipate beyond a total drainage area of 200 km2, approximately 2 km downstream of the small watershed outlet. A numerical tracer analysis identified the contribution of tile drainage to stream flow (QT/Q) which varied between 6 % and 71 % through an annual cycle. QT/Q responded directly to meteorological forcing. Precipitation driven events produced a strong positive logarithmic correlation between QT/Q and drainage area. The addition of precipitation into the system saturated near surface soils, increased lateral soil water movement, and reduced the contribution of instream tile flow. A negative logarithmic trend in QT/Q to drainage area persisted in non-event durations. Simulated gradient terraces reduced and delayed peak flows in subcatchments of less than 3 km2 of drainage area. The hydrographs were shifted responding to rainfall later than non-terraced scenarios, while retaining the total volumetric outflow over longer time periods. The effects of dense terrace systems quickly dissipated, and found to be inconsequential at a drainage area of 45 km2. Beyond the analysis of individual agricultural features, this work assembled a framework to analyze the feature at the field scale for implementation at the watershed scale. It showed large scale simulations reproduce field scale results well. The product of this work was, a systematic hydrologic characterization of distributed flood mitigation structures, pattern tile drainage, and terrace systems facilitating the simulation of each practices in a physically-based coupled surface-subsurface model.

No description available.

Controlling ignition timing and flame stabilization is one of the most outstanding challenges limiting the development of modern, efficient and low-emission compression ignition engines (CIEs). In this study, the role of turbulence on two-stage ignition dynamics and subsequent flame stabilization at diesel engine conditions is assessed by performing direct numerical simulations in a simplified inflow-outflow premixed configuration. The thermochemical conditions are chosen to match those of the most reactive mixture in the Engine Combustion Network’s n-dodecane Spray A flame (temperature of 813 K, pressure of 60 atm, equivalence ratio of 1.3, and with 15% vol. O2 in the ambient gas). Inflow velocities 4 to 16 times larger than the laminar flame speed are considered. As a result, in the absence of turbulence, ignition and flame stabilization are controlled by advection and chemistry, diffusion being negligible. Ignition delays match those of the homogeneous reactor and both the cool flame, due to low-temperature chemistry (LTC), and the hot flame, due to high-temperature chemistry (HTC), are spontaneous ignition fronts. Turbulence alters this picture in two ways. First, the second-stage (HTC) ignition delay is increased considerably, in contrast with the first-stage (LTC) ignition delay, which remains virtually unaffected. Second, a sufficiently high turbulence intensity makes the cool spontaneous ignition front transition to a cool deflagration which moves upstream to the inlet, while the hot flame is pushed downstream, still stabilized by spontaneous ignition. The latter phenomenon is caused by the reduced reactivity of LTC products as the cool flame transitions from spontaneous ignition to deflagration. Further increasing the turbulence intensity leads to both cool and hot flames transitioning to deflagrations. For the hot flame, the mechanism governing this transition is the increase in magnitude of progress variable gradient under increased turbulence or reduced inflow velocity, while in cool flames it is mainly due to the reduction in chemical source terms. In addition to turbulence intensity, the role of inflow velocity, integral length scale, and oxygen concentration level on this transition is assessed and modeling challenges are discussed. Finally, a chemical explosive mode analysis is provided to further characterise the ignition and transition phenomena. The present results highlight important fundamental roles of turbulence expected to modulate CIE combustion dynamics.

This work focuses on simulation of core blowing into the core boxes. Mechanical production of cores is now a normal part of any foundry. But can never predict whether core can make right the first time. This situation can be used as a helper QuikCAST simulation program that is able to handle the issue and deal with and can find the optimal basis.

The ability of coated particles of enriched uranium dioxide (UO2) fuel to withstand high temperatures and contain the fission products in the case of a loss of cooling event is a vital passive safety measure over traditional nuclear fuel requiring active safety systems to provide cooling. As a possible solution towards enhancing the safety of light-water reactors (LWRs), it is envisaged that the fuel in the form of loose-coated particles in a helium atmosphere can be introduced inside Silicon-Carbide nuclear reactor fuel cladding tubes of the fuel elements. The coated particles in this investigation were treated as a bed from where heat was transferred to the cladding tube by means of helium gas and the gas movement was by natural convection. Hence, it is proposed that light-water reactors (LWR) could be made safer by redesigning the fuel in the fuel assembly (see Fig. 1.3b). As a first step towards the implementation of this proposal, a proper understanding of the mechanisms of heat transfer, fluid flow and pressure drop through a packed bed of spheres during natural convection was of utmost importance. Such an understanding was achieved through a review of existing literature on porous media. However, most heat transfer correlations and models in heated packed beds are for forced convectional conditions and as such characterise porous media as a function of Reynolds number only rather than expressing media heat transfer performance as a function of thermal properties of the bed in combination with the various components of the overall heat transfer. The media heat transfer performance considered as a function of thermal properties of the bed in the proposed design is found to be a more appropriate approach than the media as a function of Reynolds number. The quest to examine the particle-to-fluid heat transfer characteristics expected in the proposed new fuel design led to implementing this research work in three phases, namely experimental, theoretical and numerical simulation. An experimental investigation of fluid-to-particle natural convection heat transfer characteristics in packed beds heated from below was carried out. Captured data readings from the experiment were analysed and heat transfer characteristics in the medium evaluated by applying the first principle heat transfer concept. A basic unit cell (BUC) model was developed for the theoretical analysis and applied to determine the heat transfer coefficient, h, of the medium. The model adopted a concept in which a single unit of the packed bed was analysed and taken as representative of the entire bed; it related the convective heat transfer effect of the flowing fluid with the conduction and radiative effect at the finite contact spot between adjacent unit cell particles. As a result, the model could account for the thermophysical properties of sphere particles and the heated gas, the interstitial gas effect, gas temperature, contact interface between particles, particle size and particle temperature distribution in the investigated medium. Although the heat transfer phenomenon experienced in the experimental set-up was a reverse case of the proposed fuel design, the study with the achievement in the validation with the Gunn correlation aided in developing the appropriate theoretical relations required for evaluating the heat transfer characteristics in the proposed nuclear fuel design. A slender geometrical model mimicking the proposed nuclear fuel in the cladding was numerically simulated to investigate the heat transfer characteristics and flow distribution under the natural convective conditions anticipated in beds of randomly packed spheres (coated fuel particles) using a commercial code. Random packing of the particles was achieved by discrete element method (DEM) simulation with the aid of Star CCM+ while particle-to-particle and particle-to-wall contacts were achieved through the combined use of the commercial code and a SolidWorks CAD package. Surface-to-surface radiative heat transfer was modelled in the simulation reflecting real-life application. The numerical results obtained allowed for the determination of parameters such as particle-to-fluid heat transfer coefficient, Nusselt number, Grashof number and Rayleigh number. These parameters were of prime importance when analysing the heat transfer performance of a fixed bed reactor. A comparison of three approaches indicated that the application of the CFD combined with the BUC model gave a better expression of the heat transfer phenomenon in the medium mimicking the heat transfer in the new fuel design / Thesis (PhD)--University of Pretoria, 2016. / Mechanical and Aeronautical Engineering / PhD / Unrestricted

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19240号 / 工博第4075号 / 新制||工||1628(附属図書館) / 32239 / 京都大学大学院工学研究科化学工学専攻 / (主査)教授 山本 量一, 教授 宮原 稔, 教授 松坂 修二 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

In order to study inlight ice adhesion at the droplet-scale, a strategy is presented to simulate the impact and solidification of a supercooled water droplet on a cooled substrate. Upon impact, nucleation is assumed to occur instantaneously, and properties of the droplet are chosen to account for the nucleation process. Simulations are performed in ANSYS Fluent using a coupled Volume of Fluid and Level-Set method to capture the air-water interface and an Enthalpy-Porosity method to capture the liquid-solid interface. Calibration of a simulation parameter, Amush, is performed in order to match experimental data for different surface types and surface temperatures. The calibrated simulation strategy is applied to low-speed, inlight icing conditions. The effects of surface variation and droplet diameter variation are investigated, providing insight into the icephobicity of superhydrophobic surfaces. Numerical results suggest that large droplets (approximately 200 micron-diameter) will freeze and adhere to a superhydrophobic surface.