  We investigate the ability of discontinuous Galerkin (DG) methods to simulate
under-resolved turbulent flows in large-eddy simulation. The role of the
Riemann solver and the subgrid-scale model in the prediction of a variety of
flow regimes, including transition to turbulence, wall-free turbulence and
wall-bounded turbulence, are examined. Numerical and theoretical results show
the Riemann solver in the DG scheme plays the role of an implicit subgrid-scale
model and introduces numerical dissipation in under-resolved turbulent regions
of the flow. This implicit model behaves like a dynamic model and vanishes for
flows that do not contain subgrid scales, such as laminar flows, which is a
critical feature to accurately predict transition to turbulence. In addition,
for the moderate-Reynolds-number turbulence problems considered, the implicit
model provides a more accurate representation of the actual subgrid scales in
the flow than state-of-the-art explicit eddy viscosity models, including
dynamic Smagorinsky, WALE and Vreman. The results in this paper indicate new
best practices for subgrid-scale modeling are needed with high-order DG
methods.
