scholarly journals Premelting increases the rate of regelation by an order of magnitude

2019 ◽  
Vol 65 (251) ◽  
pp. 518-521 ◽  
Author(s):  
ALAN W. REMPEL ◽  
COLIN R. MEYER
Keyword(s):  
Melting Temperature ◽  
Effective Stress ◽  
Temperature Gradients ◽  
Liquid Pressure ◽  
Equilibrium Melting Temperature ◽  
Temperature Changes ◽  
Transition Wavelength ◽  
Order Of Magnitude ◽  
Pressure Melting ◽  
Ice Pressure

ABSTRACTGlacier sliding over small obstacles relies on melting on their upstream sides and refreezing downstream. Previous treatments have appealed to ‘pressure melting’ as the cause of the spatial variations in melting temperature that drive thisregelationprocess. However, we show that typical liquid pressure variations across small obstacles are negligible and therefore variations in ice pressure closely approximate variations in effective stress. For a given change in effective stress, the equilibrium melting temperature changes by an order of magnitude more than when the pressure of ice and liquid both change by an equal amount. In consequence, the temperature gradients that drive heat flow across small obstacles are larger than previously recognized and the rate of regelation is faster. Under typical conditions, the transition wavelength at which ice deformation and regelation contribute equally is of m-scale, several times longer than previous predictions, which have been reported to underestimate field inferences.

scholarly journals Experimental constraints on subglacial rock friction

2019 ◽  
Vol 60 (80) ◽  
pp. 37-48 ◽  
Author(s):  
Dougal D. Hansen ◽  
Lucas K. Zoet
Keyword(s):  
Shear Stress ◽  
Effective Stress ◽  
Frictional Resistance ◽  
Viscous Drag ◽  
Rock Friction ◽  
Rock Fragments ◽  
Order Of Magnitude ◽  
Pressure Melting ◽  
Sliding Dynamics ◽  
Basal Shear

AbstractSubglacial rock friction is an important control on the sliding dynamics and erosive potential of hard-bedded glaciers, yet it remains largely unconstrained. To explore the relative influence of basal melt rate, effective stress and ice temperature on frictional resistance, we conducted abrasion experiments in which limestone beds were slid beneath a fixed slab of ice laden with granitic rock fragments. Shear stress scales linearly with melt rate and cryostatic stress, confirming that both viscous drag and effective stress are first-order controls on the contact force in drained conditions. Furthermore, temperature gradients in the ice increase the contribution of viscous drag on basal shear stress. In all experiments, the relationship between melt rate and shear stress is best explained by a model that accounts for the effects of regelation and viscous creep on the bed-normal drag force. We interpret this to mean fluid flow around entrained clasts contributed to basal drag even at subfreezing temperatures. Incorporating premelting dynamics into the Watts/Hallet model for subglacial rock friction, we find that the predicted debris-bed drag decreases by approximately an order of magnitude, with a corresponding ~3.5 × increase in the transition radius. This is lower than we observe for ice slightly below the pressure melting point.

Fluid pressure response to undrained compression in saturated sedimentary rock

Geophysics ◽  
1986 ◽  
Vol 51 (4) ◽  
pp. 948-956 ◽  
Author(s):  
Douglas H. Green ◽  
Herbert F. Wang
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Wave Attenuation ◽  
Fluid Pressure ◽  
Confining Pressure ◽  
Pressure Response ◽  
Additional Parameter ◽  
Seismic Wave Velocity ◽  
Specific Storage ◽  
Order Of Magnitude

The pore pressure response of saturated porous rock subjected to undrained compression at low effective stresses are investigated theoretically and experimentally. This behavior is quantified by the undrained pore pressure buildup coefficient, [Formula: see text] where [Formula: see text] is fluid pressure, [Formula: see text] is confining pressure, and [Formula: see text] is the mass of fluid per unit bulk volume. The measured values for B for three sandstones and a dolomite arc near 1.0 at zero effective stress and decrease with increasing effective stress. In one sandstone, B is 0.62 at 13 MPa effective stress. These results agree with the theories of Gassmann (1951) and Bishop (1966), which assume a locally homogeneous solid framework. The decrease of B with increasing effective stress is probably related to crack closure and to high‐compressibility materials within the rock framework. The more general theories of Biot (1955) and Brown and Korringa (1975) introduce an additional parameter, the unjacketed pore compressibility, which can be determined from induced pore pressure results. Values of B close to 1 imply that under appropriate conditions within the crust, zones of low effective pressure characterized by low seismic wave velocity and high wave attenuation could exist. Also, in confined aquifer‐reservoir systems at very low effective stress states, the calculated specific storage coefficient is an order of magnitude larger than if less overpressured conditions prevailed.

Thermodynamics of Crystallization in High Polymers. Natural Rubber

1955 ◽  
Vol 28 (3) ◽  
pp. 718-727 ◽  
Author(s):  
Donald E. Roberts ◽  
Leo Mandelkern
Keyword(s):  
Natural Rubber ◽  
Melting Temperature ◽  
Low Temperatures ◽  
Rapid Heating ◽  
Heat Of Fusion ◽  
Low Molecular Weight ◽  
Heating Rates ◽  
Equilibrium Melting Temperature ◽  
Clapeyron Equation ◽  
Melting Temperatures

Abstract The existence of an equilibrium melting temperature, T0m, at 28 ± 1°, for unstretched natural rubber has been established, using dilatometric methods. The lower melting temperatures previously observed are a consequence of the low temperatures of crystallization and the rapid heating rates employed. From melting point studies of mixtures of the polymer with low molecular-weight diluents, the heat of fusion per repeating unit, ΔHu has been evaluated as 15.3 ± 0.5 cal./g. The values of ΔHu and T0m have then been combined with data of other workers to obtain the following information concerning natural rubber: (1) The variation of melting temperature with applied hydrostatic pressure has been calculated from the Clapeyron equation to be 0.0465° C/atm. (2) The degree of erystallinity resulting from maintaining a sample at 0° until the rate of crystallization is negligible has been calculated, by three independent methods, to be in the range 26 to 31 per cent. (3) Analysis of the stress-strain-temperature relationship has indicated that crystallization is the cause of the large internal energy changes that are observed at relatively high elongations.

scholarly journals Laboratory Simulations Of Glacial Abrasion: Comparison With Theory

1990 ◽  
Vol 36 (124) ◽  
pp. 304-314 ◽  
Author(s):  
Neal R. Iverson
Keyword(s):  
Water Film ◽  
Theoretical Models ◽  
Stress Concentrations ◽  
Creep Theory ◽  
Ice Flow ◽  
Rock Fragments ◽  
Roughness Elements ◽  
Ice Velocity ◽  
Pressure Melting ◽  
Ice Pressure

AbstractGlacial abrasion was simulated in experiments in which a small artificial glacier bed was pushed beneath a fixed ice block under pressure. The experiments provide a means of testing theoretical models of abrasion, particularly those factors that govern the magnitude of stress concentrations beneath abrading rock fragments. In preliminary experiments, vertical ice flow around a sphere mounted on the bed was studied. In subsequent experiments, marble tablets were pushed beneath granitic rock fragments frozen into the base of the ice block. Unlike previous abrasion experiments, the sliding velocity was realistic (25 mm d−1), and ice near the bed was at the pressure-melting temperature. Resultant striations closely resemble those observed on glaciated bedrock.As predicted by Hallet (1979), the component of the ice velocity towards the bed strongly influenced stresses beneath fragments, and classical regelation and creep theory provided an approximate estimate of the downward drag force on fragments. Half of the rock fragments rotated significantly, accounting for 10–50% of their motion relative to the bed and influencing abrasion rates and the shear stress supported along the ice-bed interface. Striation patterns indirectly suggest that fragment rotations were inhibited by increases in ice pressure, which presumably increased the drag on roughness elements on fragment surfaces. This may have resulted from a reduction in the thickness of the water film around fragments, facilitated by leakage of water from the bed.

scholarly journals Temperature measurements and heat transfer in near-surface snow at the South Pole

1997 ◽  
Vol 43 (144) ◽  
pp. 339-351 ◽  
Author(s):  
Richard E. Brandt ◽  
Stephen G. Warren
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Lower Boundary ◽  
Water Vapor Transport ◽  
South Pole ◽  
Near Surface ◽  
Temperature Changes ◽  
Surface Snow ◽  
Order Of Magnitude ◽  
The Difference

AbstractTo study near-surface heat flow on the Antarctíc ice sheet, snow temperatures were measured at South Pole Station to a depth of 3 m at 15 min intervals during most of 1992. Solar heating and water-vapor transport were negligible during the 6 month Winter, as was inter-grain net thermal radiation, leaving conduction as the dominant heat-transport mechanism. The rate of temperature change at depth over 15 min intervals was smaller than that at the surface, by one order of magnitude at 20 cm depth and two orders of magnitude at 1 m depth. A finite-difference model, with conduction as the only heat-transfer mechanism and measured temperatures as the upper and lower boundary conditions, was applied to foursets of three thermistors each. The thermal conductivity was estimated as that which minimized the difference between modeled and measured 15 min changes in temperatures at the center thermistor. The thermal conductivity obtained at shallow depths (above 40 cm) was lower than that given by existing parameterizations based on density, probably because the snow grains were freshly deposited, cold and poorly bonded. A model using only vertical conduction explains on average 87% ofthe observed 15 min temperature changes at less than 60 cm depth and 92% below 60 cm. The difference between modeled andmeasured temperature changes decreased with depth. The discrepancies between model and observation correlated more strongly with the air-snow temperature difference than with the product of that difference with the square of the wind speed,suggesting that the residual errors are due more to non-vertical conduction and to sub-grid-scale variabilis of the conductivity than to windpumping. The residual heating rate not explained by the model of vertical conduction exceeds 0.2 W m−3only in the top 60 cm of the near-surface snow.

Finite Element Analysis of Engine Bore Distortions During Boring Operation

1993 ◽  
Vol 115 (4) ◽  
pp. 379-384 ◽  
Author(s):  
N. N. Kakade ◽  
J. G. Chow
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Piston Ring ◽  
Experimental Studies ◽  
Temperature Gradients ◽  
Element Analysis ◽  
Temperature Changes ◽  
Simulation Procedure ◽  
Finite Element Package ◽  
Element Technique

Bore geometry is the major factor affecting oil comsumption, piston ring wear, and frictional losses in an engine. As such, auto industries have been constantly striving to develop better machining technologies to produce engine bores with greater precision. Experimental studies have shown that the bore distortion as a result of machining is mainly caused by temperatures and stresses created during cutting. Consequently, optimization of machining conditions so as to minimize both bore temperature gradients as well as mechanical stresses while machining should lead to the production of better bore geometry. This research develops a model aimed at simulating bore distortions caused by temperature changes and stresses generated during machining using finite element technique. The commercial finite element package ANSYS has been used along with the CAD package I-DEAS to simulate the boring process on DEC-VAX computers. The simulation procedure developed can be used to obtain a better understanding of the boring process, in particular, to determine distortion trends for different cutting conditions.

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