Reflection tomography of time-lapse GPR data for studying dynamic unsaturated flow phenomena

Ground-penetrating radar (GPR) reflection tomography algorithms allow non-invasive monitoring of water content changes resulting from flow in the vadose zone. The approach requires multi-offset GPR data that are traditionally slow to collect. We automate GPR data collection to reduce the survey time significantly, thereby making this approach to hydrologic monitoring feasible. The method was evaluated using numerical simulations and laboratory experiments that suggest reflection tomography can provide water content estimates to within 5 % vol vol−1–10 % vol vol−1 for the synthetic studies, whereas the empirical estimates were typically within 5 %–15 % of measurements from in situ probes. Both studies show larger observed errors in water content near the periphery of the wetting front, beyond which additional reflectors were not present to provide data coverage. Overall, coupling automated GPR data collection with reflection tomography provides a new method for informing models of subsurface hydrologic processes and a new method for determining transient 2-D soil moisture distributions

Fire performance of phase change material enhanced plasterboard

Sustainable construction materials are increasingly being used to reduce the carbon footprint of modern buildings. These materials have the potential to change the fire dynamics of compartments by altering the compartment energy balance however there is little quantitative understanding of how these materials behave in the event of a real fire. The changes in fire dynamics may be due to increased fuel load in a compartment, reduced time to failure or promotion of flame spread. The objective of this research is to quantify how Phase Change Materials (PCMs) perform in realistic fire scenarios. It was found that a plasterboard product containing microencapsulated PCMs will behave similarly to a charring solid and have the potential to contribute significant fuel to a compartment fire but that they maintain integrity for the duration of flaming period. The critical heat flux for this product was determined in the cone calorimeter to be 17.5 ± 2.5 kW m−2, the peak heat release rate and mass loss rate ranged from 60.2 kW m−2 to 107 kW m−2 and 1.88 g s−1 m−2 to 8.47 g s−1 m−2 respectively for exposures between 20 kW m−2 and 70 kW m−2. Sample orientation was found to increase the peak heat release rate by up to 25%, whilst having little to no effect on the mass loss rate. These parameters, in addition to the in-depth temperature evolution and ignition properties, can be used as design criteria for balancing energy savings with quantified fire performance

Numerical time-dependent partial differential equations for scientists and engineers

It is the first text that in addition to standard convergence theory treats other necessary ingredients for successful numerical simulations of physical systems encountered by every practitioner. The book is aimed at users with interests ranging from application modeling to numerical analysis and scientific software development. It is strongly influenced by the authors research in in space physics, electrical and optical engineering, applied mathematics, numerical analysis and professional software development. The material is based on a year-long graduate course taught at the University of Arizona since 1989. The book covers the first two-semesters of a three semester series. The second semester is based on a semester-long project, while the third semester requirement consists of a particular methods course in specific disciplines like computational fluid dynamics, finite element method in mechanical engineering, computational physics, biology, chemistry, photonics, etc. The first three chapters focus on basic properties of partial differential equations, including analysis of the dispersion relation, symmetries, particular solutions and instabilities of the PDEs; methods of discretization and convergence theory for initial value problems. The goal is to progress from observations of simple numerical artifacts like diffusion, damping, dispersion, and anisotropies to their analysis and management technique, as it is not always possible to completely eliminate them. In the second part of the book we cover topics for which there are only sporadic theoretical results, while they are an integral part and often the most important part for successful numerical simulation. We adopt a more heuristic and practical approach using numerical methods of investigation and validation. The aim is teach students subtle key issues in order to separate physics from numerics. The following topics are addressed: Implementation of transparent and absorbing boundary conditions; Practical stability analysis in the presence of the boundaries and interfaces; Treatment of problems with different temporal/spatial scales either explicit or implicit; preservation of symmetries and additional constraints; physical regularization of singularities; resolution enhancement using adaptive mesh refinement and moving meshes. Self contained presentation of key issues in successful numerical simulation Accessible to scientists and engineers with diverse background Provides analysis of the dispersion relation, symmetries, particular solutions and instabilities of the partial differential equations

Resolving precipitation induced water content profiles by inversion of dispersive GPR data: A numerical study

Surface-based ground-penetrating radar (GPR) measurements have significant potential for monitoring dynamic hydrologic processes at multiple scales in time and space. At early times during infiltration into a soil, the zone above the wetting front may act as a low-velocity waveguide that traps GPR waves, thereby causing dispersion and making interpretation of the data using standard methods difficult. In this work, we show that the dispersion is dependent upon the distribution of water within the waveguide, which is controlled by soil hydrologic properties. Simulations of infiltration were performed by varying the n-parameter of the Mualem–van Genuchten equation using HYDRUS-1D; the associated GPR data were simulated to evaluate the influence of dispersion. We observed a notable decrease in wave dispersion as the sharpness of the wetting front profile decreased. Given the sensitivity of the dispersion effect to the wetting front profile, we also evaluated whether the water content distribution can be determined through inversion of the dispersive GPR data. We found that a global grid search combined with the simplex algorithm was able to estimate the average water content when the wetted zone is divided into 2 layers. This approach was incapable, however, of representing the gradational nature of the water content distribution behind the wetting front. In contrast, the shuffled complex evolution algorithm was able to constrain a piece-wise linear function to closely match the shallow gradational water content profile. In both the layered and piece-wise linear case, the sensitivity of the dispersive data dropped sharply below the wetting front, which in this case was around 20 cm, i.e., twice the average wavelength, for a 900 MHz GPR survey. This study demonstrates that dispersive GPR data has significant potential for capturing the early-time dynamics of infiltration that cannot be obtained with standard GPR analysis approaches