Hydrostatic equilibrium

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

NCAR global General Circulation Model of the atmosphere

Abstract: This paper describes a model of the general circulation of the earth’s atmosphere which has been developed and experimented with, since 1964, at the National Center for Atmospheric Research (NCAR), Boulder, Colo. A distinguishing feature of the NCAR model is that the vertical coordinate is height rather than pressure, though hydrostatic equilibrium is maintained in the system. In fact, the dynamical framework of the model is very similar to the one proposed by L. F. Richardson in 1922. Various physical processes in the atmosphere, such as energy transfer due to solar and terrestrial radiation, small-scale turbulence and convection, etc., are incorporated in the model. An explicit prediction of the moisture field is avoided. Instead, it is assumed that the atmosphere is completely saturated by water vapor. Thus, the release of latent heat of condensation can be computed. In addition to a description of the model, the equations for the zonal mean and eddy energy are presented. Finally, a baroclinic stability analysis of the model is made in order to gain an insight into the finite-difference formulation of the present model. Long term (over 100 days) numerical integrations are being performed successfully with a two-layer version of the present model. Details of finite-difference schemes and the results of numerical calculations will be described in a separate article. “Perhaps some day in the dim future it will be possible to advance the computations faster than the weather advances and at a cost less than the saving to mankind due to the information gained. But that is a dream.” [From Weather Prediction by Numerical Process by L. F. Richardson, 1922.1

Galaxy clusters at the edge: temperature, entropy, and gas dynamics near the virial radius

Abstract: Recently, Suzaku has produced temperature and entropy profiles, along with profiles of gas density, gas fraction, and mass, for multiple galaxy clusters out to approximately the virial radius. In this paper, we compare these novel X-ray observations with results from N-body + hydrodynamic adaptive mesh refinement cosmological simulations using the Enzo code. There is excellent agreement in the temperature, density, and entropy profiles between a sample of 24 mostly substructure-free massive clusters in the simulated volume and the observed clusters. This supports our previous contention that clusters have "universal" outer temperature profiles. Furthermore, it appears that the simplest adiabatic gas physics used in these Enzo simulations is adequate to model the outer regions of these clusters without other mechanisms (e.g., non-gravitational heating, cooling, magnetic fields, or cosmic rays). However, the outskirts of these clusters are not in hydrostatic equilibrium. There is significant bulk flow and turbulence in the outer intracluster medium created by accretion from filaments. Thus, the gas is not fully supported by thermal pressure. The implications for mass estimation from X-ray data are discussed.

Depth-dependent resistance of granular media to vertical penetration.

Abstract: We measure the quasistatic friction force acting on intruders moving downwards into a granular medium. By utilizing different intruder geometries, we demonstrate that the force acts locally normal to the intruder surface. By altering the hydrostatic loading of grain contacts by a sub-fluidizing airflow through the bed, we demonstrate that the relevant frictional contacts are loaded by gravity rather than by the motion of the intruder itself. Lastly, by measuring the final penetration depth versus airspeed and using an earlier result for inertial drag, we demonstrate that the same quasistatic friction force acts during impact. Altogether this force is set by a friction coefficient, hydrostatic pressure, projectile size and shape, and a dimensionless proportionality constant. The latter is the same in nearly all experiments, and is surprisingly greater than one.

Evolution of the merger-induced hydrostatic mass bias in galaxy clusters

Abstract: In this work, we examine the effects of mergers on the hydrostatic mass estimate of galaxy clusters using high-resolution Eulerian cosmological simulations. We utilize merger trees to isolate the last merger for each cluster in our sample and follow the time evolution of the hydrostatic mass bias as the systems relax. We find that during a merger, a shock propagates outward from the parent cluster, resulting in an overestimate in the hydrostatic mass bias. After the merger, as a cluster relaxes, the bias in hydrostatic mass estimate decreases but remains at a level of ?5%-10% with 15%-20% scatter within r 500. We also investigate the post-merger evolution of the pressure support from bulk motions, a dominant cause of this residual mass bias. At r 500, the contribution from random motions peaks at 30% of the total pressure during the merger and quickly decays to ~10%-15% as a cluster relaxes. Additionally, we use a measure of the random motion pressure to correct the hydrostatic mass estimate. We discover that 4?Gyr after mergers, the direct effects of the merger event on the hydrostatic mass bias have become negligible. Thereafter, the mass bias is primarily due to residual bulk motions in the gas which are not accounted for in the hydrostatic equilibrium equation. We present a hydrostatic mass bias correction method that can recover the unbiased cluster mass for relaxed clusters with 9% scatter at r 500 and 11% scatter in the outskirts, within r 200.

Testing chameleon gravity with the Coma cluster

Abstract: We propose a novel method to test the gravitational interactions in the outskirts of galaxy clusters. When gravity is modified, this is typically accompanied by the introduction of an additional scalar degree of freedom, which mediates an attractive fifth force. The presence of an extra gravitational coupling, however, is tightly constrained by local measurements. In chameleon modifications of gravity, local tests can be evaded by employing a screening mechanism that suppresses the fifth force in dense environments. While the chameleon field may be screened in the interior of the cluster, its outer region can still be affected by the extra force, introducing a deviation between the hydrostatic and lensing mass of the cluster. Thus, the chameleon modification can be tested by combining the gas and lensing measurements of the cluster. We demonstrate the operability of our method with the Coma cluster, for which both a lensing measurement and gas observations from the X-ray surface brightness, the X-ray temperature, and the Sunyaev-Zel'dovich effect are available. Using the joint observational data set, we perform a Markov chain Monte Carlo analysis of the parameter space describing the different profiles in both the Newtonian and chameleon scenarios. We report competitive constraints on the chameleon field amplitude and its coupling strength to matter. In the case of f(R) gravity, corresponding to a specific choice of the coupling, we find an upper bound on the background field amplitude of |fR0| < 6 × 10−5, which is currently the tightest constraint on cosmological scales.