Global General Relativistic Magnetohydrodynamic Simulations of Accretion Tori

This paper presents an initial survey of the properties of accretion flows in the Kerr metric from three-dimensional, general relativistic magnetohydrodynamic simulations of accretion tori. We consider three fiducial models of tori around rotating, both prograde and retrograde, and nonrotating black holes; these three fiducial models are also contrasted with axisymmetric simulations and a pseudo-Newtonian simulation with equivalent initial conditions to delineate the limitations of these approximations.Comment: Submitted to ApJ. 30 pages, 21 figures. Animations and high-resolution version of figures available at http://www.astro.virginia.edu/~jd5

Global MHD Simulations of Cylindrical Keplerian Disks

This paper presents a series of global three dimensional accretion disk simulations carried out in the cylindrical limit in which the vertical component of the gravitational field is neglected. The simulations use a cylindrical pseudo-Newtonian potential to model the main dynamical properties of the Schwarzschild metric. The disks are initially constant density with a Keplerian angular momentum distribution and contain a weak toroidal or vertical field. These simulations reaffirm many of the conclusions of previous local simulations. The magnetorotational instability grows rapidly and produces MHD turbulence with a significant Maxwell stress which drives accretion. Tightly-wrapped low-$m$ spiral waves are prominent. In some simulations radial variations in Maxwell stress concentrate gas into rings, creating substantial spatial inhomogeneities. There is a nonzero stress at the marginally stable orbit which produces a small decline in specific angular momentum inside the last stable orbit. Detailed comparisons between simulations are used to examine the effects of computational domain and equation of state. Simulations that begin with vertical fields have greater field amplification and higher ratios of stress to magnetic pressure compared with those beginning with toroidal fields. In contrast to MHD, hydrodynamics alone neither creates nor sustains turbulence.Comment: Submitted to the Astrophysical Journal Web version of paper and MPEG animations can be found at http://www.astro.virginia.edu/~jh8h/cylinder

Studies of Connecticut Hardwoods: The Treatment of Advance Growth Arising as a Result of Thinnings and Shelterwood Cuttings

The typical hardwood stand with which the silviculturist in Connecticut has to deal is even-aged in form. The causes which operated in the past to create stands of this character are thoroughly understood and do not require consideration in this study. That even-aged stands, particularly when densely stocked and composed of comparatively intolerant species, require thinning to develop the most vigorous individuals and to obtain maximum production, is an accepted principle of silviculture. The study separates logically into five parts: 1. A system of classification for the woody vegetation. 2. Amount, character, and distribution of the advance growth and of reproduction arising subsequent to the last cutting. 3. Growth in height of the various elements composing the young stand. 4. Conclusions as to the probable composition of the future stand and the relative value of the individuals which it will contain. 5. Treatment advised to improve the future stand

The Keene Forest: A Preliminary Report

In 1913 the Yale School of Forestry came into possession of certain parcels of land located near Keene, New Hampshire. This land amounting to 629.4 acres was presented to the School as a nucleus for a school forest to be used for purposes of instruction and research. Subsequently in March, 1915, additional lots comprising 270.9 acres were purchased with funds contributed by the original donor. The present area totals 900.3 acres and is know as the Keene Forest

Accretion of low angular momentum material onto black holes: 2D magnetohydrodynamical case

We report on the second phase of our study of slightly rotating accretion flows onto black holes. We consider magnetohydrodynamical (MHD) accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum and a weak radial magnetic field. We study accretion flows by means of numerical 2D, axisymmetric, MHD simulations with and without resistive heating. Our main result is that the properties of the accretion flow depend mostly on an equatorial accretion torus which is made of the material that has too much angular momentum to be accreted directly. The torus accretes, however, because of the transport of angular momentum due to the magnetorotational instability (MRI). Initially, accretion is dominated by the polar funnel, as in the hydrodynamic inviscid case, where material has zero or very low angular momentum. At the later phase of the evolution, the torus thickens towards the poles and develops a corona or an outflow or both. Consequently, the mass accretion through the funnel is stopped. The accretion of rotating gas through the torus is significantly reduced compared to the accretion of non-rotating gas (i.e., the Bondi rate). It is also much smaller than the accretion rate in the inviscid, weakly rotating case.Our results do not change if we switch on or off resistive heating. Overall our simulations are very similar to those presented by Stone, Pringle, Hawley and Balbus despite different initial and outer boundary conditions. Thus, we confirm that MRI is very robust and controls the nature of radiatively inefficient accretion flows.Comment: submitted in Ap

The Effect of Resistivity on the Nonlinear Stage of the Magnetorotational Instability in Accretion Disks

We present three-dimensional magnetohydrodynamic simulations of the nonlinear evolution of the magnetorotational instability (MRI) with a non-zero Ohmic resistivity. The properties of the saturated state depend on the initial magnetic field configuration. In simulations with an initial uniform vertical field, the MRI is able to support angular momentum transport even for large resistivities through the quasi-periodic generation of axisymmetric radial channel solutions rather than through the maintenance of anisotropic turbulence. Simulations with zero net flux show that the angular momentum transport and the amplitude of magnetic energy after saturation are significantly reduced by finite resistivity, even at levels where the linear modes are only slightly affected. This occurs at magnetic Reynolds numbers expected in low, cool states of dwarf novae, these results suggest that finite resistivity may account for the low and high angular momentum transport rates inferred for these systems.Comment: 8 figures, accepted for publication in Ap

What's the point of knowing how?

Why is it useful to talk and think about knowledge-how? Using Edward Craig’s discussion of the function of the concepts of knowledge and knowledge-how as a jumping off point, this paper argues that considering this question can offer us new angles on the debate about knowledge-how. We consider two candidate functions for the concept of knowledge-how: pooling capacities, and mutual reliance. Craig makes the case for pooling capacities, which connects knowledge-how to our need to pool practical capacities. I argue that the evidence is much more equivocal. My suggested diagnosis is that the concept of knowledge-how plays both functions, meaning that the concept of knowledge-how is inconsistent, and that the debate about knowledge-how is at least partly a metalinguistic negotiation. In closing, I suggest a way to revise the philosophical concept of knowledge how

Vortices in Thin, Compressible, Unmagnetized Disks

We consider the formation and evolution of vortices in a hydrodynamic shearing-sheet model. The evolution is done numerically using a version of the ZEUS code. Consistent with earlier results, an injected vorticity field evolves into a set of long-lived vortices, each of which has a radial extent comparable to the local scale height. But we also find that the resulting velocity field has a positive shear stress, . This effect appears only at high resolution. The transport, which decays with time as t^-1/2, arises primarily because the vortices drive compressive motions. This result suggests a possible mechanism for angular momentum transport in low-ionization disks, with two important caveats: a mechanism must be found to inject vorticity into the disk, and the vortices must not decay rapidly due to three-dimensional instabilities.Comment: 8 pages, 10 figures (high resolution figures available in ApJ electronic edition