Adaptive mesh refinement with spectral accuracy for magnetohydrodynamics in two space dimensions

We examine the effect of accuracy of high-order spectral element methods, with or without adaptive mesh refinement (AMR), in the context of a classical configuration of magnetic reconnection in two space dimensions, the so-called Orszag-Tang vortex made up of a magnetic X-point centered on a stagnation point of the velocity. A recently developed spectral-element adaptive refinement incompressible magnetohydrodynamic (MHD) code is applied to simulate this problem. The MHD solver is explicit, and uses the Elsasser formulation on high-order elements. It automatically takes advantage of the adaptive grid mechanics that have been described elsewhere in the fluid context [Rosenberg, Fournier, Fischer, Pouquet, J. Comp. Phys. 215, 59-80 (2006)]; the code allows both statically refined and dynamically refined grids. Tests of the algorithm using analytic solutions are described, and comparisons of the Orszag-Tang solutions with pseudo-spectral computations are performed. We demonstrate for moderate Reynolds numbers that the algorithms using both static and refined grids reproduce the pseudo--spectral solutions quite well. We show that low-order truncation--even with a comparable number of global degrees of freedom--fails to correctly model some strong (sup--norm) quantities in this problem, even though it satisfies adequately the weak (integrated) balance diagnostics.Comment: 19 pages, 10 figures, 1 table. Submitted to New Journal of Physic

Improving the robustness of the control volume finite element method with application to multiphase porous media flow

Control volume finite element methods (CVFEMs) have been proposed to simulate flow in heterogeneous porous media because they are better able to capture complex geometries using unstructured meshes. However, producing good quality meshes in such models is nontrivial and may sometimes be impossible, especially when all or parts of the domains have very large aspect ratio. A novel CVFEM is proposed here that uses a control volume representation for pressure and yields significant improvements in the quality of the pressure matrix. The method is initially evaluated and then applied to a series of test cases using unstructured (triangular/tetrahedral) meshes, and numerical results are in good agreement with semianalytically obtained solutions. The convergence of the pressure matrix is then studied using complex, heterogeneous example problems. The results demonstrate that the new formulation yields a pressure matrix than can be solved efficiently even on highly distorted, tetrahedral meshes in models of heterogeneous porous media with large permeability contrasts. The new approach allows effective application of CVFEM in such models

The Application of 199 Hg NMR and 199m Hg Perturbed Angular Correlation (PAC) Spectroscopy to Define the Biological Chemistry of Hg II : A Case Study with Designed Two- and Three-Stranded Coiled Coils

The use of de novo designed peptides is a powerful strategy to elucidate Hg II –protein interactions and to gain insight into the chemistry of Hg II in biological systems. Cysteine derivatives of the designed Α-helical peptides of the TRI family [Ac-G-(L a K b A c L d E e E f K g ) 4 -G-NH 2 ] bind Hg II at high pH values and at peptide/Hg II ratios of 3:1 with an unusual trigonal thiolate coordination mode. The resulting Hg II complexes are good water-soluble models for Hg II binding to the protein MerR. We have carried out a parallel study using 199 Hg NMR and 199m Hg perturbed angular correlation (PAC) spectroscopy to characterize the distinct species that are generated under different pH conditions and peptide TRI 14L9C/Hg II ratios. These studies prove for the first time the formation of [Hg{(TRI 14L9C) 2 -(TRI 14L9C[bond]H)}], a dithiolate–Hg II complex in the hydrophobic interior of the three-stranded coiled coil (TRI 14L9C) 3 . 199 Hg NMR and 199m Hg PAC data demonstrate that this dithiolate–Hg II complex is different from the dithiolate [Hg(TRI 14L9C) 2 ], and that the presence of third Α-helix, containing a protonated cysteine, breaks the symmetry of the coordination environment present in the complex [Hg(TRI 14L9C) 2 ]. As the pH is raised, the deprotonation of this third cysteine generates the trigonal thiolate–Hg II complex Hg(TRI 14L9C) 3 − on a timescale that is slower than the NMR timescale (0.01–10 14ms). The formation of the species [Hg{(TRI 14L9C) 2 (TRI 14L9C[bond]H)}] is the result of a compromise between the high affinity of Hg II to form dithiolate complexes and the preference of the peptide to form a three-stranded coiled coil.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/57382/1/9178_ftp.pd

Somali Current rings in the eastern Gulf of Aden

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 111 (2006): C09039, doi:10.1029/2005JC003338.New satellite-based observations reveal that westward translating anticyclonic rings are generated as a portion of the Somali Current accelerates northward through the Socotra Passage near the mouth of the Gulf of Aden. Rings thus formed exhibit azimuthal geostrophic velocities exceeding 50 cm/s, are comparable in overall diameter to the width of the Gulf of Aden (250 km), and translate westward into the gulf at 5–8 cm/s. Ring generation is most notable in satellite ocean color imagery in November immediately following the transition between southwest (boreal summer) and northeast (winter) monsoon regimes. The observed rings contain anomalous fluid within their core which reflects their origin in the equator-crossing Somali Current system. Estimates of Socotra Passage flow variability derived from satellite altimetry provide evidence for a similar ring generation process in May following the winter-to-summer monsoon transition. Cyclonic recirculation eddies are observed to spin up on the eastern flank of newly formed rings with the resulting vortex pair translating westward together. Recent shipboard and Lagrangian observations indicate that vortices of both sign have substantial vertical extent and may dominate the lateral circulation at all depths in the eastern Gulf of Aden.This investigation is a component of the Red Sea Outflow Experiment (REDSOX) sponsored by the U.S. National Science Foundation through grants OCE 98-18464 and OCE 04-24647 to the Woods Hole Oceanographic Institution and OCE 98-19506 and OCE 03-51116 to the University of Miami

Constraints from GPS measurements on the dynamics of deformation in Anatolia and the Aegean

We estimate the strength of the lithosphere in Anatolia and the Aegean, and the boundary forces acting upon it, using a dynamical model that treats the lithosphere as a thin fluid sheet deforming in response to variations in gravitational potential energy. This model has one free material parameter, the power law exponent, n, of the vertically averaged rheology of the lithosphere, and two parameters that specify the forces per unit length applied to its edges. Solutions to this model that best fit the velocities of 346 reliable GPS sites require an effective viscosity of the lithosphere of 1022 to 1021 Pa s at strain rates of 10 to 100 nanostrain per year. The best-fitting force at the Arabia-Anatolia boundary is consistent with the lithostatic pressure due to the high topography there, and the force at the Nubia-Aegean boundary is consistent with the contrast in lithostatic pressure across that boundary. No additional force, from “slab rollback” or basal tractions due to convection in the mantle, is required to explain the observations. These results are supported by scaling relations derived from approximate analytical solutions. The inverse relationship between the viscosity of the lithosphere and deviatoric stress produces strong slowly deforming regions in the Southern Aegean and Central Anatolia whose motions resemble those of microplates. The distribution of geodetic strain rates within the region, and the partitioning between normal and strike-slip faulting, are explained by the interplay between boundary conditions, internal variations in gravitational potential energy, and the power law rheology of the lithosphere

Abusive Supervision, Upward Maintenance Communication, and Subordinates\u27 Psychological Distress

This study reanalyzes data from Tepper\u27s (2000) two-wave study regarding the effects of subordinates\u27 perceptions of supervisory abuse to assess previously unexamined relationships. As predicted, we found that subordinates who more rather than less strongly perceived that they had been abused by supervisors tended to use regulative maintenance tactics with higher frequency. Further, the positive relationship between abusive supervision and subordinates\u27 psychological distress was exacerbated by subordinates\u27 use of regulative maintenance communications, and that relationship was reduced by subordinates\u27 use of direct maintenance communication. Theoretical and practical implications are discussed