Finite-element Modelling of Haupapa/Tasman Glacier's Basal Sliding Events

<p>The rate of ice loss from glaciers and ice caps is a major source of uncertainty in predicting sea level rise out to 2100. Improving the predictive capability of ice flow models will, in part, require a more robust coupling of climate to long-term and short-term variability in glacial discharge. An ongoing concern is the role that surface melting and rainfall plays in accelerating glacier flow. Rapid drainage of surface water to the base of a glacier or ice sheet is thought to elevate basal water pressure, reduce basal friction, and thereby increases sliding speed. Here, we present several rain-induced speed-ups of Haupapa/Tasman Glacier, South Island, New Zealand, recorded by GNSS (Global Navigation Satellite System) instruments. Observed speed-up events involve large vertical offsets (up to ~53 cm) and large horizontal accelerations of up to twenty-four times background velocity. Due to it's pronounced sliding events, Haupapa/Tasman Glacier offers a useful case study for investigating the processes that govern the sliding behaviour of large glaciers prone to increasing meltwater variability as a cause of enhanced mass loss in a warming climate. The observed correspondence of vertical displacement and horizontal acceleration in this study suggests that the rapid growth of water-filled cavities at the bed controls basal motion during speed-ups. However, sliding laws that relate changes in basal velocity to changes in water pressure do not account for cavity growth. To investigate the processes governing a typical speed-up event, we use a finite-element modelling approach combined with a commonly-used sliding law to recreate internal deformation and basal sliding of Haupapa/Tasman Glacier during rain-induced acceleration. In general, we find peak velocities can only be achieved when basal water pressure exceeds ice overburden and velocity at the glacier sides is allowed to exceed that observed by a GNSS unit situated near the margins. The sliding law requires a more complete treatment of cavity growth under rapid water pressure changes to better capture basal acceleration observed at Haupapa/Tasman Glacier.</p>

Finite-element Modelling of Haupapa/Tasman Glacier's Basal Sliding Events

<p>The rate of ice loss from glaciers and ice caps is a major source of uncertainty in predicting sea level rise out to 2100. Improving the predictive capability of ice flow models will, in part, require a more robust coupling of climate to long-term and short-term variability in glacial discharge. An ongoing concern is the role that surface melting and rainfall plays in accelerating glacier flow. Rapid drainage of surface water to the base of a glacier or ice sheet is thought to elevate basal water pressure, reduce basal friction, and thereby increases sliding speed. Here, we present several rain-induced speed-ups of Haupapa/Tasman Glacier, South Island, New Zealand, recorded by GNSS (Global Navigation Satellite System) instruments. Observed speed-up events involve large vertical offsets (up to ~53 cm) and large horizontal accelerations of up to twenty-four times background velocity. Due to it's pronounced sliding events, Haupapa/Tasman Glacier offers a useful case study for investigating the processes that govern the sliding behaviour of large glaciers prone to increasing meltwater variability as a cause of enhanced mass loss in a warming climate. The observed correspondence of vertical displacement and horizontal acceleration in this study suggests that the rapid growth of water-filled cavities at the bed controls basal motion during speed-ups. However, sliding laws that relate changes in basal velocity to changes in water pressure do not account for cavity growth. To investigate the processes governing a typical speed-up event, we use a finite-element modelling approach combined with a commonly-used sliding law to recreate internal deformation and basal sliding of Haupapa/Tasman Glacier during rain-induced acceleration. In general, we find peak velocities can only be achieved when basal water pressure exceeds ice overburden and velocity at the glacier sides is allowed to exceed that observed by a GNSS unit situated near the margins. The sliding law requires a more complete treatment of cavity growth under rapid water pressure changes to better capture basal acceleration observed at Haupapa/Tasman Glacier.</p>

Basic Creep-Fatigue Models Considering Cavitation

Cavitation plays a central role during creep-fatigue. During recent years, fundamental models for initiation and growth of creep cavities that do not involve any adjustable parameters have been developed. These models have successfully been used to predict creep rupture data for austenitic stainless steels again without using adjustable parameters. However, it appears that basic models have not yet been applied to creep-fatigue assessments. A summary of the fundamental cavitation models is given. A model for monotonous deformation is transferred to cyclic loading. The parameter values are kept except that the dynamic recovery constant is raised due to increased interactions between dislocations during cycling. This model is successfully compared with observed LCF and TMF hysteresis loops. A new model for cavity growth due to plastic deformation is presented. The model is formulated in such a way that the condition for constrained growth is automatically satisfied. In this way, it is avoided to overestimate the growth rate.

Impact of surface viscosity on the stability of a droplet translating through a stagnant fluid

This study examines the impact of interfacial viscosity on the stability of an initially deformed droplet translating through an unbounded quiescent fluid. The boundary-integral formulation is employed to investigate the time evolution of a droplet in the Stokes flow limit. The droplet interface is modelled using the Boussinesq–Scriven constitutive relationship having surface shear viscosity $\eta _\mu$ and surface dilatational viscosity $\eta _\kappa$ . We observe that, below a critical value of the capillary number, $Ca_C$ , the initially perturbed droplet reverts to its spherical shape. Above $Ca_C$ , the translating droplet deforms continuously, growing a tail at the rear end for initial prolate perturbations and a cavity for initial oblate perturbations. We find that surface shear viscosity inhibits the tail/cavity growth at the droplet's rear end and increases the $Ca_C$ compared with a clean droplet. In contrast, surface dilatational viscosity increases tail/cavity growth and lowers $Ca_C$ compared with a clean droplet. Surprisingly, both shear and dilatational surface viscosity appear to delay the time at which pinch off occurs, and hence satellite droplets form. Lastly, we explore the combined influence of surface viscosity and surfactant transport on droplet stability by assuming a linear dependence of surface tension on surfactant concentration and exponential dependence of interfacial viscosities on the surface pressure. We find that pressure-thinning/thickening effects significantly affect the droplet dynamics for surface shear viscosity but play a small role for surface dilatational viscosity. We lastly provide phase diagrams for the critical capillary number for different values of the droplet's viscosity ratio and initial Taylor deformation parameter.

Microstructure Evolution in He-Implanted Si at 600 °C Followed by 1000 °C Annealing

Si single crystal was implanted with 230 keV He+ ions to a fluence of 5 × 1016/cm2 at 600 °C. The structural defects in Si implanted with He at 600 °C and then annealed at 1000 °C were investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The microstructure of an as-implanted sample is provided for comparison. After annealing, rod-like defects were diminished, while tangled dislocations and large dislocation loops appeared. Dislocation lines trapped by cavities were directly observed. The cavities remained stable except for a transition of shape, from octahedron to tetrakaidecahedron. Stacking-fault tetrahedrons were found simultaneously. Cavity growth was independent of dislocations. The evolution of observed lattice defects is discussed.

Regression Models Utilization to the Underground Temperature Determination at Coal Energy Conversion

The underground coal gasification represents a technology capable of obtaining synthetic coal gas from hard-reached coal deposits and coal beds with tectonic faults. This technology is also less expensive than conventional coal mining. The cavity is formed in the coal seam by converting coal to synthetic gas during the underground coal gasification process. The cavity growth rate and the gasification queue’s moving velocity are affected by controllable variables, i.e., the operation pressure, the gasification agent, and the laboratory coal seam geometry. These variables can be continuously measured by standard measuring devices and techniques as opposed to the underground temperature. This paper researches the possibility of the regression models utilization for temperature data prediction for this reason. Several regression models were proposed that were differed in their structures, i.e., the number and type of selected controllable variables as independent variables. The goal was to find such a regression model structure, where the underground temperature is predicted with the greatest possible accuracy. The regression model structures’ proposal was realized on data obtained from two laboratory measurements realized in the ex situ reactor. The obtained temperature data can be used for visualization of the cavity growth in the gasified coal seam.

Numerical investigation of unsteady cavitating turbulent flows around a three-dimensional hydrofoil using stress-blended eddy simulation

The mechanism of flow instability, which involves complex gas–liquid interactions and multiscale vortical structures, is one of the hot research areas in cavitating flow. The role of turbulence modeling is crucial in the numerical investigation of unsteady flow characteristics. Although large-eddy simulation (LES) has been used as a reliable numerical method, it is computationally costly. In this work, we used a hybrid Reynolds-averaged Navier–Stokes (RANS) and LES model, that is, stress-blended eddy simulation (SBES), to improve the prediction capability for the cloud cavitating flow. Our hybrid approach introduces a shielding function to integrate the RANS model with the LES applied only regionally, such as to large-scale separated flow regions. The results showed that the periodic shedding of cavity growth, break off, and collapse around a three-dimensional Clark-Y hydrofoil was reproduced in accordance with experimental observations. The lift/drag coefficients, streamwise velocity profiles, and cavity patterns obtained by the SBES model were in better agreement with the experimental data than those obtained by the modified RANS model. The re-entrant jet dynamics responsible for the break off of the attached cavity were discussed. Further analysis of vorticity transportation indicated that the stretching and dilatation terms dominated the development of vorticity around the hydrofoil. In conclusion, the SBES model can be used to predict cavitating turbulent flows in practical engineering applications.

Lagrangian Analysis of Unsteady Partial Cavitating Flow Around a Three-Dimensional Hydrofoil

This study employs an incompressible homogeneous flow framework with a transport-equation-based cavitation model and shear stress transport turbulence model to successfully reproduce the unsteady cavitating flow around a three-dimensional hydrofoil. Cavity growth, development, and break-off during the periodic shedding process are adequately reproduced and match experimental observations. The predicted shedding frequency is very close to the experimental value of 23 ms. By monitoring the motions of the seeding trackers, growth-up of attached cavity and dynamic evolution of U-type cavity are clearly displayed, which indicating the trackers could serve as an effective tool to visualize the cavitating field. Repelling Lagrangian Coherent Structure (RLCS) is so complex that abundant flow patterns are highlighted, reflecting the intricacy of cavity development. The formation of cloud cavities is clearly characterized by the Attracting Lagrangian Coherent Structure (ALCS), where bumbling wave wrapping the whole shedding cavities indicates the rotating transform of cavities and stretching of the wave eyes shows the distortion of vortices. Generation of the re-entrant jet is considered to be not only associated with the adverse pressure gradient due to the positive attack angle, but also the contribution of cloud cavitating flow, based on the observation of a buffer zone between the attached and cloud cavities.

MODELING OF MATERIAL BALANCE FROM THE EXPERIMENTAL UCG

The underground coal gasification is a continually evolving technology, which converts coal to calorific gas. There are many important parameters in this technology, which are difficult to measure. These parameters include the underground cavity growth, amount gasified coal, and the leakage of input and output gaseous components into the surrounding layers during the coal gasification process. Mathematical modeling of this process is one of the possible alternatives for determining these unknown parameters. In this paper, the structure of the mathematical model of laboratory underground coal gasification process from the material balance aspect is presented. The material balance consists of mass components entering and leaving from the UCG process. The paper shows a material balance in the form of a general mass balance and atomic species balance. The material balance was testing by six UCG laboratory experiments, which were realized in two ex-situ reactors.