Over the last three decades Computational Fluid Dynamics (CFD) has gradually joined the
wind tunnel and flight test as a primary flow analysis tool for aerodynamic designers. CFD
has had its most favorable impact on the aerodynamic design of the high-speed cruise
configuration of a transport. This success has raised expectations among aerodynamicists
that the applicability of CFD can be extended to the full flight envelope. However, the
complex nature of the flows and geometries involved places substantially increased demands
on the solution methodology and resources required. Currently most simulations involve
Reynolds-Averaged Navier-Stokes (RANS) codes although Large Eddy Simulation (LES) and
Detached Eddy Suimulation (DES) codes are occasionally used for component analysis or
theoretical studies. Despite simplified underlying assumptions, current RANS turbulence
models have been spectacularly successful for analyzing attached, transonic flows. Whether
or not these same models are applicable to complex flows with smooth surface separation is
an open question. A prerequisite for answering this question is absolute confidence that
the CFD codes employed reliably solve the continuous equations involved. Too often,
failure to agree with experiment is mistakenly ascribed to the turbulence model rather
than inadequate numerics. Grid convergence in three dimensions is rarely achieved. Even
residual convergence on a given grid is often inadequate. This paper discusses issues
involved in residual and especially grid convergence.