Turbulent Energy Transport in Nonradiative Accretion Flows

Just as correlations between fluctuating radial and azimuthal velocitiesproduce a coherent stress contributing to the angular momentum transport inturbulent accretion disks, correlations in the velocity and temperaturefluctuations produce a coherent energy flux. This nonadvective energy flux isalways of secondary importance in thin radiative disks, but cannot be neglectedin nonradiative flows, in which it completes the mean field description ofturbulence. It is, nevertheless, generally ignored in accretion flow theory,with the exception of models explicitly driven by thermal convection, where itis modeled phenomenologically. This flux embodies both turbulent thermalconvection as well as wave transport, and its presence is essential for aproper formulation of energy conservation, whether convection is present ornot. The sign of the thermal flux is likely to be outward in real systems, butthe restrictive assumptions used in numerical simulations may lead to inwardthermal transport, in which case qualitatively new effects may be exhibited. Wefind, for example, that a static solution would require inward, not outward,thermal transport. Even if it were present, thermal convection would beunlikely to stifle accretion, but would simply add to the outward rotationalenergy flux that must already be present.Comment: Final Version, 15 pages, no figs., AAS Latex macros v5.0, to appear ApJ 2004, n2, 60