  We report on a computational parameter space study of mixing protocols for a
half-full bi-axial spherical granular tumbler. The quality of mixing is
quantified via the intensity of segregation (concentration variance) and
computed as a function of three system parameters: angles of rotation about
each tumbler axis and the flowing layer depth. Only the symmetric case is
considered in which the flowing layer depth is the same for each rotation. We
also consider the dependence on $\bar{R}$, which parametrizes the concentric
spheroids ("shells") that comprise the volume of the tumbler. The intensity of
segregation is computed over 100 periods of the mixing protocol for each choice
of parameters. Each curve is classified via a time constant, $\tau$, and an
asymptotic mixing value, $bias$. We find that most choices of angles and most
shells throughout the tumbler volume mix well, with mixing near the center of
the tumbler being consistently faster (small $\tau$) and more complete (small
$bias$). We conclude with examples and discussion of the pathological mixing
behaviors of the outliers in the so-called $\tau$-$bias$ scatterplots.
