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Author: Patrick Joseph Staron Publisher: ISBN: Category : Avalanches Languages : en Pages : 372
Book Description
In the presence of a sufficient temperature gradient, snow evolves from an isotropic network of ice crystals to a transversely isotropic system of depth hoar chains. This morphology is often the weak layer responsible for full depth avalanches. Previous research primarily focused on quantifying the conditions necessary to produce depth hoar. Limited work has been performed to determine the underlying reason for the microstructural changes. Using entropy production rates derived from nonequilibrium thermodynamics, this research shows that depth hoar forms as a result of the snow progressing naturally toward thermal equilibrium. Laboratory experiments were undertaken to examine the evolution of snow microstructure at the macro scale under nonequilibrium thermal conditions. Snow samples with similar initial microstructure were subjected to either a fixed temperature gradient or fixed heat input. The metamorphism for both sets of boundary conditions produced similar depth hoar chains with comparable increases in effective thermal conductivity. Examination of the Gibbs free energy and entropy production rates showed that all metamorphic changes were driven by the system evolving to facilitate equilibrium in the snow or the surroundings. This behavior was dictated by the second law of thermodynamics. An existing numerical model was modified to examine depth hoar formation at the grain scale. Entropy production rate relations were developed for an open system of ice and water vapor. This analysis showed that heat conduction in the bonds had the highest specific entropy production rate, indicating they were the most inefficient part of the snow system. As the metamorphism advanced, the increase in bond size enhanced the conduction pathways through the snow, making the system more efficient at transferring heat. This spontaneous microstructural evolution moved the system and the surroundings toward equilibrium by reducing the local temperature gradients over the bonds and increasing the entropy production rate density. The employment of nonequilibrium thermodynamics determined that the need to reach equilibrium was the underlying force that drives the evolution of snow microstructure. This research also expanded the relevance of nonequilibrium thermodynamics by applying it to a complicated, but well bounded, natural problem.
Author: Patrick Joseph Staron Publisher: ISBN: Category : Avalanches Languages : en Pages : 372
Book Description
In the presence of a sufficient temperature gradient, snow evolves from an isotropic network of ice crystals to a transversely isotropic system of depth hoar chains. This morphology is often the weak layer responsible for full depth avalanches. Previous research primarily focused on quantifying the conditions necessary to produce depth hoar. Limited work has been performed to determine the underlying reason for the microstructural changes. Using entropy production rates derived from nonequilibrium thermodynamics, this research shows that depth hoar forms as a result of the snow progressing naturally toward thermal equilibrium. Laboratory experiments were undertaken to examine the evolution of snow microstructure at the macro scale under nonequilibrium thermal conditions. Snow samples with similar initial microstructure were subjected to either a fixed temperature gradient or fixed heat input. The metamorphism for both sets of boundary conditions produced similar depth hoar chains with comparable increases in effective thermal conductivity. Examination of the Gibbs free energy and entropy production rates showed that all metamorphic changes were driven by the system evolving to facilitate equilibrium in the snow or the surroundings. This behavior was dictated by the second law of thermodynamics. An existing numerical model was modified to examine depth hoar formation at the grain scale. Entropy production rate relations were developed for an open system of ice and water vapor. This analysis showed that heat conduction in the bonds had the highest specific entropy production rate, indicating they were the most inefficient part of the snow system. As the metamorphism advanced, the increase in bond size enhanced the conduction pathways through the snow, making the system more efficient at transferring heat. This spontaneous microstructural evolution moved the system and the surroundings toward equilibrium by reducing the local temperature gradients over the bonds and increasing the entropy production rate density. The employment of nonequilibrium thermodynamics determined that the need to reach equilibrium was the underlying force that drives the evolution of snow microstructure. This research also expanded the relevance of nonequilibrium thermodynamics by applying it to a complicated, but well bounded, natural problem.
Author: Publisher: ISBN: Category : Languages : en Pages : 261
Book Description
Snow microstructure significantly influences the mechanical, thermal, and electromagnetic properties of snow. The microstructure is constantly evolving from the time it is deposited on the surface until it sublimates or melts. The resulting time variant material properties make the study of snow metamorphism of fundamental importance to a wide variety of snow science disciplines. Dry snow metamorphism has traditionally been classified by the thermal gradient encountered in the snowpack. Snow experiencing a predominantly equi-temperature environment develops different micro structure than snow that is subjected to a temperature gradient. As such, previous research has evaluated snow metamorphism based upon select thermal gradient dependent processes, when in reality, there is a continuum of physical processes simultaneously contributing to metamorphism. In previous research, a discrete temperature gradient transition between the two thermal environments has been used to activate separate morphological analyses. The current research focuses on a unifying approach to dry snow metamorphism that is applicable to generalized thermal environments. The movement of heat and mass is not prescribed, but is allowed to develop naturally through modeling of physical processes. Heat conduction, mass conservation, and phase change equations are derived in a simplified two-dimensional approach. Each differential equation is non-linearly coupled to the others through phase change. The microstructural network is then discretized into elements and nodes. Finite difference equations are developed for the network, and numerically solved using iterative techniques. The finite difference model provides a unique platform to study the influence of numerous geometric and thermodynamic parameters relating to dry snow metamorphism. Numerical metamorphism studies in an equi-temperature environment agree well with established trends and published experimental results.
Author: Samuel C. Colbeck Publisher: ISBN: Category : Avalanches Languages : en Pages : 24
Book Description
Grain growth, bond growth and densification of wet snow are described in terms of the distribution of equilibrium temperature in the snow matrix. At high water saturations the equilibrium temperature increases with grain size; hence, small particles melt away as large particles grow. Melting also occurs at the integrain bonds, causing a low strength and rapid densification. At low saturations the equilibrium temperature is determined by the capillary pressure and the particle sizes have only a second order effect. Therefore, grain growth proceeds slowly and, even at large over-burden pressures, no intergrain melting occurs. At low saturations the water 'tension' acts through a finite area, thus large attractive forces exist between the grains, and the strength of the snow matrix is large. (Author).
Author: Daniel August Miller Publisher: ISBN: 9781423510499 Category : Depth hoar Languages : en Pages : 261
Book Description
Snow microstructure significantly influences the mechanical, thermal, and electromagnetic properties of snow. The microstructure is constantly evolving from the time it is deposited on the surface until it sublimates or melts. The resulting time variant material properties make the study of snow metamorphism of fundamental importance to a wide variety of snow science disciplines. Dry snow metamorphism has traditionally been classified by the thermal gradient encountered in the snowpack. Snow experiencing a predominantly equi-temperature environment develops different micro structure than snow that is subjected to a temperature gradient. As such, previous research has evaluated snow metamorphism based upon select thermal gradient dependent processes, when in reality, there is a continuum of physical processes simultaneously contributing to metamorphism. In previous research, a discrete temperature gradient transition between the two thermal environments has been used to activate separate morphological analyses. The current research focuses on a unifying approach to dry snow metamorphism that is applicable to generalized thermal environments. The movement of heat and mass is not prescribed, but is allowed to develop naturally through modeling of physical processes. Heat conduction, mass conservation, and phase change equations are derived in a simplified two-dimensional approach. Each differential equation is non-linearly coupled to the others through phase change. The microstructural network is then discretized into elements and nodes. Finite difference equations are developed for the network, and numerically solved using iterative techniques. The finite difference model provides a unique platform to study the influence of numerous geometric and thermodynamic parameters relating to dry snow metamorphism. Numerical metamorphism studies in an equi-temperature environment agree well with established trends and published experimental results.
Author: Edward R. LaChapelle Publisher: ISBN: Category : Languages : en Pages : 42
Book Description
Spatial and temporal variations of temperature in alpine snow covers have been systematically observed over a period of two winters. Concurrently, snow crystal metamorphism has been monitored in the same snow covers, along with such basic snow properties as density and ram resistance. Near-surface snow temperatures fluctuate widely in response to diurnal weather variations. Below about 25 cm beneath the surface the temperatures change more slowly in response to longer-term weather trends. Mean snow temperatures are colder on north slopes than south ones but mean snow cover temperature gradients are similar on both exposures owing to shallower snow on south slopes. A forest canopy tends to suppress snow surface radiation cooling and hence reduce magnitude of temperature gradients at depth. Metamorphism in snow follows a recrystallization mode with declining mechanical strength when the saturation water vapor pressure gardient exceeds 0.05 mb/cm. Owing to a nonlinear vapor pressure-temperature relationship over ice, this corresponds to the conventional critical temperature gradient of 0.1 C/cm for this metamorphism mode only at snow temperatures close to the melting point. Mean monthly snow temperature gradients can reasonably be estimated from air temperature and snow depth means, but this method can be extended to vapor pressure gradients only if appropriate corrections for non-linearity are introduced. (Author).
Author: Xuan Wang Publisher: ISBN: Category : Languages : en Pages : 242
Book Description
Investigation of snow properties can be applied to understand a range of issues including climate change and snow avalanche prediction. Most of the snow properties, i.e. thermal and mechanical, are directly linked to the microstructure of snowpack and those properties evolve simultaneously with deformation of the snow and its metamorphism. Thus, this dissertation primarily explores the characterization of snow microstructural evolution under the effects of temperature gradient and overburden. Snow structural evolution was monitored by the techniques of scanning electron microscopy (SEM), X-ray computed microtomography (micro-CT) and numerical simulation. The temperature gradient setup constructed for this work, commercial compression stage and commercial cooling stage were used to simulate the natural boundary conditions within snow layers. Of the different types of metamorphism that may occur in snow layers, temperature gradient metamorphism (TGM) is perhaps the most significant one. The snow layer undergoing TGM will lose its strength and transform into a weak layer, which is the microstructural cause of avalanches. 1-D arrays of ice spheres were used as a reproducable approach to observe snow microstructural evolution and investigate the mass transfer process. By controlling temperatures on both sides of ice spheres, different vapor transfer directions were studied. Our experiments demonstrated the mass transfer processes and microstructural evolutions under alternating and unidirectional temperature gradient. We also investigated the effects of temperature gradient on natural snow. The specific surface area (SSA) was used to characterize the TGM. The temperature gradient magnitude and the initial snow type both influence the evolution of the SSA. The trend in the SSA is controlled by two mechanisms, grain growth and the formation of complex surfaces. For the relationship between the structure of snow and its mechanical properties, we focused on investigating how the structure of snow evolves under an applied load. Micro-CT images, complemented with SEM images, demonstrate that the mechanical properties of snow depend on the density, the SSA, and bond formation. During our interrupted compression tests, the SSA decreased more rapidly than that determined for snow metamorphism without an overburden. It is clearly evident that pressure sintering of snow contributes to accelerated sintering and coarsening processes.
Author: Patrick Joseph Staron
Author: Patrick Joseph Staron
Author: 
Author: Samuel C. Colbeck
Author: Daniel August Miller
Author: R. A. Sommerfeld
Author: Bernd R. Pinzer
Author: R. A. Sommerfeld
Author: Edward R. LaChapelle
Author: Xuan Wang
Author: Marcel de Quervain