  We present a series of experiments investigating the flow regimes and repose
angles of highly concave particle packings in a rotating drum. By varying grain
geometry from spherical to highly non-convex shapes, adjusting frictional
properties and the particle number of branches, we examine how these parameters
and the drum speed influence the flow behavior. Our study identifies two
distinct flow regimes: the rolling regime, where granular matter exhibits
solid-like behavior near the walls and flows like a liquid near the free
surface, and the slumping regime, characterized by cyclic avalanches and solid
body rotations. Using quantitative criteria such as the repose angle difference
and the area ratio of particle packings, we construct phase diagrams
delineating the cross-over between these regimes. Our findings highlight the
significant effects of particle concavity, friction, and rotation speed on the
flow dynamics of granular materials, providing new insights into the mechanical
behaviors of \emph{meta-granular matter}.
