[1] In highly curved river bends, flow may separate at the inner bank
to create a recirculation eddy with weak upstream flow. Very little is
known about how recirculation eddies connect with the downstream flow
or how the latter is affected by their presence. We investigate these
questions using three-dimensional time-averaged computational fluid
dynamics models of two natural bends with inner-bank separation. Test
measurements of velocity in one bend agree well with the simulation.
Common points in the two simulations are that ( 1) an outer-bank helix
is only present in the upstream part of the bend, ( 2) maximum
near-bank velocities are highest here rather than beyond the apex as in
most bends, ( 3) reverse flow extends farther across the channel at the
surface than the bed, and ( 4) flow within the recirculation eddy has a
three-dimensional structure, linked with that in the outer-bank free
stream. Substantial differences in detail between the two bends appear
to be due to differences in upstream planform, manifested through the
lateral distribution of inflow velocity.