Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The stress-pressure relationship in simulations of MRI-induced
  turbulence   We determine how MRI-turbulent stresses depend on gas pressure via a suite of
unstratified shearing box simulations. Earlier numerical work reported only a
very weak dependence at best, results that call into question the canonical
alpha-disk model and the thermal stability results that follow from it. Our
simulations, in contrast, exhibit a stronger relationship, and show that
previous work was box-size limited: turbulent `eddies' were artificially
restricted by the numerical domain rather than by the scale height.
Zero-net-flux runs without physical diffusion coefficients yield a stress
proportional to $P^{0.5}$, where P is pressure. The stresses are also
proportional to the grid length and hence remain numerically unconverged. The
same runs with physical diffusivities, however, give a result closer to an
alpha-disk: the stress is proportional to $P^{0.9}$. Net-flux simulations
without explicit diffusion exhibit stresses proportional to $P^{0.5}$, but
stronger imposed fields weaken this correlation. In summary, compressibility is
important for the saturation of the MRI, but the exact stress-pressure
relationship is difficult to ascertain in local simulations because of
numerical convergence issues and the influence of any imposed flux. As a
consequence, the interpretation of thermal stability behaviour in local
simulations is a problematic enterprise.

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The categories of this paper are: eess
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The categories of this paper are: q-fin
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The categories of this paper are: alg-geom
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The categories of this paper are: atom-ph
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The categories of this paper are: adap-org
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The categories of this paper are: econ
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The categories of this paper are: bayes-an
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The categories of this paper are: chem-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: stat
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The categories of this paper are: ao-sci
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The categories of this paper are: math
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The categories of this paper are: cond-mat
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The categories of this paper are: acc-phys
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The categories of this paper are: patt-sol
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The categories of this paper are: funct-an