  For more than a decade, the so-called shearing box model has been used to
study the fundamental local dynamics of accretion discs. This approach has
proved to be very useful because it allows high resolution and long term
studies to be carried out, studies that would not be possible for a global
disc.
  Localised disc studies have largely focused on examining the rate of enhanced
transport of angular momentum, essentially a sum of the Reynolds and Maxwell
stresses. The dominant radial-azimuthal component of this stress tensor is, in
the classic Shakura-Sunayaev model, expressed as a constant alpha times the
pressure. Previous studies have estimated alpha based on a modest number of
orbital times. Here we use much longer baselines, and perform a cumulative
average for alpha. Great care must be exercised when trying to extract
numerical alpha values from simulations: dissipation scales, computational box
aspect ratio, and even numerical algorithms all affect the result. This study
suggests that estimating alpha becomes more, not less, difficult as
computational power increases.
