  We performed large-eddy simulations of the flow over a typical
two-dimensional dune geometry at laboratory scale (the Reynolds number based on
the average channel height and mean velocity is 18,900) using the Lagrangian
dynamic eddy-viscosity subgrid-scale model. The flow separates at the dune
crest and reattaches downstream on the bed (at x=5.7h). A favorable pressure
gradient accelerates the flow over the stoss-side (the upward-sloping region
for x > 8h) and an unfavorable gradient for x < 8h decelerates the flow over
the lee-side of the dune. Due to the separation of the flow, a shear layer is
generated after the crest that expands in the wake region towards the next
dune. The outer-layer turbulence structures are visualized through isosurfaces
of pressure fluctuations colored by distance to the surface. Spanwise vortices
are generated in the shear layer separating from the crest due to the
Kelvin-Helmholtz instability. They are convected downstream and either interact
with the wall or rise to the surface, taking the form of large horseshoe-like
structures. These structures may undergo an intense distortion, become
one-legged, or be completely destroyed. As they grow to dimensions comparable
to (or larger than) the flow depth, a strong ejection occurs between their
legs. The interaction between two large coherent structures may result in
merging, or in dissipation of the weaker one. Surviving eddies grow and rise
along the shear layer emanating from the dune crest; they tilt downward and,
eventually, their tips touch the surface. When the legs of the horseshoe are
close to the surface, they create an upwelling, which expands and weakens. The
legs of the vortex loop remain coherent for a longer time.
