  We investigate the performance of wall-modeled LES for external aerodynamics
in the NASA Juncture Flow. We characterize the errors in the prediction of mean
velocity profiles and pressure coefficient for three different locations over
the aircraft: the upstream region of the fuselage, the wing-body juncture, and
the wing-body juncture close to the trailing-edge. The last two locations are
characterized by strong mean-flow three-dimensionality and separation. The
message conveyed by our error analysis is that WMLES performs as expected in
regions where the flow resembles a zero-pressure-gradient flat plate boundary
layer. However, there is a clear decline of the current models in the presence
of wing-body junctions and, more acutely, in separated zones. The slow
convergence to the solution in these regions renders the brute-force
grid-refinement approach to improve the accuracy of the solution unfeasible.
The results reported above pertain to the mean velocity profile predicted using
the typical grid resolution for external aerodynamics applications, i.e., 5--20
points per boundary-layer thickness. The prediction of the pressure coefficient
is below 5\% error for all grid sizes considered, even when boundary layers
were marginally resolved. The latter accuracy can be attributed to the
outer-layer nature of the mean pressure, which becomes less sensitive to flow
details within the turbulent boundary layer. Finally, we show that
boundary-layer-conforming grids (i.e., grids maintaining a constant number of
points per boundary-layer thickness) allow for a more efficient distribution of
grid points and smaller errors. Our results suggest that novel modeling venues
encompassing physical insights, together with numerical and gridding
advancements, must be exploited to attain predictions within the tolerance
required for Certification by Analysis.
