Computational modeling of flow-induced anisotropy of polar ice for the EDML deep drilling site, Antarctica: The effect of rotation recrystallization and grain boundary migrationS. BARGMANN, H. SEDDIK and R. GREVE AbstractIn this contribution we model flow-induced anisotropy of polar ice in order to gain a better understanding for the underlying microstructure and its influence on the deformation process. In particular, a continuum-mechanical, anisotropic flow model that is based on an anisotropic flow enhancement factor (CAFFE model) is applied. The polycrystalline ice is regarded as a mixture whose grains are characterized by their orientation. The approach is based on two distinct scales: the underlying microstructure influences the macroscopic material behavior and is taken into account phenomenologically. To achieve this, the orientation mass density is introduced as a mesoscopic field, i.e. it depends on a mesoscopic variable (the orientation) in addition to position and time. The classical flow law of Glen is extended by a scalar, but anisotropic enhancement factor. Four different effects (local rigid body rotation, grain rotation, rotation recrystallization, grain boundary migration) influencing the evolution of the grain orientations are taken into account. All modeling parameters are either measurable in or derivable from field observations or laboratory experiments. A finite volume method is chosen for the discretization procedure. Numerical results simulating the ice flow at the site of the EPICA ice core in Dronning Maud Land (referred to as EDML), Antarctica, are presented. They go beyond earlier results by Seddik et al. (J. Glaciol. 2008; 54(187):631-642) in which only local rigid body rotation and grain rotation were accounted for. By comparing simulated and observed fabrics, we come up with reference values for the parameters in the constitutive equations for rotation recrystallization and grain boundary migration. Down to 2045 m depth, good agreement can be achieved; however, further down the observed fabric cannot be reproduced well due to numerical issues. Additionally, we study the influence of the two superposed deformation regimes of vertical compression and simple shear separately and demonstrate that the numerical problems are due to the predominant shear regime near the bottom, whereas vertical compression only produces stable results everywhere. International Journal for Numerical and Analytical Methods in Geomechanics 36 (7), 892-917 (2012). |
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