Hydrostatic equilibrium

About: Hydrostatic equilibrium is a research topic. Over the lifetime, 2451 publications have been published within this topic receiving 62172 citations.

Ductile creep, compaction, and rate and state dependent friction within major fault zones

Abstract: The shear traction on major strike-slip faults during earthquakes is much lower than that expected on a frictionally sliding surface in equilibrium with hydrostatic pressure. The low shear traction is explained if the fluid pressure at the time of the earthquake is much greater than hydrostatic pressure. Ductile creep within mostly sealed fault zones compacts the matrix and thus increases fluid pressure between earthquakes. Frictional dilatancy during earthquakes decreases fluid pressure below hydrostatic, and over the earthquake cycle, the fault zone is in long-term equilibrium with the country rock. This ductile mechanism is formally unified with rate and state theory for time-dependent friction when the difference between a critical porosity where the rock loses all strength and the actual porosity of cracks is used as a state variable. This choice is justified by percolation theory of mostly broken lattices. Time-dependent behavior associated with changes in normal traction in the laboratory is explained by the formalism. Instability (earthquakes) sometimes occurs in the numerical experiments. However, fairly small amounts of frictional dilatancy during initial frictional creep decrease fluid pressure and preclude unstable sliding. Two coupled mechanisms for producing dilatancy on faults once an instability is well underway are evident. (1) Expansion of pore fluids associated with frictional heating increases fluid pressure offsetting the effects of increased pore volume during earthquakes. There is some tendency for pore volume increase to balance fluid expansion so that fluid pressure stays relatively constant. (2) Production of isolated voids that do not immediately decrease fluid pressure throughout the fault zone during earthquakes can occur to the extent that the fault zone is not significantly strengthened. Although the extent of both processes is constrained by energy considerations, the variation of fluid pressure during earthquakes is not yet well enough understood to predict stress drop from observable material properties.

LBM-DEM modeling of fluid-solid interaction in porous media

Abstract: SUMMARY Three porous media flow problems, in which the fluid mechanical interactions are critical, are studied in a mesoscopic–microscopic coupling system In this system, fluid flow in the pore space is explicitly modeled at mesoscopic level by the lattice Boltzmann method, the geometrical representation and the mechanical behavior of the solid skeleton are modeled at microscopic level by the particulate distinct element method (DEM), and the interfacial interaction between the fluid and the solids is resolved by an immersed boundary scheme In the first benchmark problem, the well-known and frequently utilized Ergun equation is validated in periodic particle and periodic pore models In the second problem, the upward seepage problem is simulated over three stages: The settlement of the column of sphere under gravity loading is measured to illustrate the accuracy of the DEM scheme; the system is solved to hydrostatic state with pore space filled with fluid, showing that the buoyancy effect is captured correctly in the mesoscopic–microscopic coupling system; then, the flow with constant rate is supplied at the bottom of the column; the swelling of the ground surface and pore pressure development from the numerical simulation are compared with the predictions of the macroscopic consolidation theory In the third problem, the fluid-flow-induced collapse of a sand arch inside a perforation cavity is tested to illustrate a more practical application of the developed system Through comparing simulation results with analytical solutions, empirical law and physical laboratory observations, it is demonstrated that the developed lattice Boltzmann–distinct element coupling system is a powerful fundamental research tool for investigating hydromechanical physics in porous media flow Copyright © 2012 John Wiley & Sons, Ltd

Weighing Galaxy Clusters with Gas. II. On the Origin of Hydrostatic Mass Bias in ΛCDM Galaxy Clusters

Abstract: The use of galaxy clusters as cosmological probes hinges on our ability to measure their masses accurately and with high precision. Hydrostatic mass is one of the most common methods for estimating the masses of individual galaxy clusters, which suffer from biases due to departures from hydrostatic equilibrium. Using a large, mass-limited sample of massive galaxy clusters from a high-resolution hydrodynamical cosmological simulation, in this work we show that in addition to turbulent and bulk gas velocities, acceleration of gas introduces biases in the hydrostatic mass estimate of galaxy clusters. In unrelaxed clusters, the acceleration bias is comparable to the bias due to non-thermal pressure associated with merger-induced turbulent and bulk gas motions. In relaxed clusters, the mean mass bias due to acceleration is small ( 3%), but the scatter in the mass bias can be reduced by accounting for gas acceleration. Additionally, this acceleration bias is greater in the outskirts of higher redshift clusters where mergers are more frequent and clusters are accreting more rapidly. Since gas acceleration cannot be observed directly, it introduces an irreducible bias for hydrostatic mass estimates. This acceleration bias places limits on how well we can recover cluster masses from future X-ray and microwave observations. We discuss implications for cluster mass estimates based on X-ray, Sunyaev-Zel'dovich effect, and gravitational lensing observations and their impact on cluster cosmology.