Flow separation in meander bends
Most investigations of river meanders have
concentrated on classical bends that have a helical flow structure
through the pool, which is carried through into the entrance of the
next bend. However, it
is known that flow separation can occur at the outer bank before the
bend apex or at the inner bank after the apex and that this results
in the breakdown of the classical flow model. Although some of the controls and the implications of flow
separation are now known, the frequency of occurrence,
three-dimensional (3-D) flow structure, sediment dynamics, controls
on separation presence, and full geomorphological implications are
poorly understood.
This thesis uses a combination of fieldwork and
3-D numerical flow modeling (computational fluid dynamics, CFD) to
investigate meander bends where flow separation and recirculation
zones are present. A
reconnaissance survey of over 600 bends reveals that flow separation
at the inner or outer bank is common along the two lowland rivers
investigated and provides tentative indications of the controls. Investigations in a sub-set of 22 bends indicate that certain
characteristics of both the channel geometry and the bend inflow are
important in generating flow separation. Detailed fieldwork combined with validated and verified
time-averaged CFD modeling in three bends with separation zones is
used to identify and examine the 3-D flow fields. The simulations reveal that the 3-D flow in bends with
separation differs considerably from the classical flow model. They also reveal considerable differences between the flow
structures in bends with inner-bank and outer-bank separation.
The turbulent flow structures produced by the
shear layer between the downstream and recirculating flow are
investigated in the field (figure 4) and attempts at modeling
transient flow structures in a simple open channel flow expansion
are detailed.
The geomorphological, sedimentological, and
ecological implications of both the time-averaged and the transient
flow structures within the bends are examined and discussed.
The figures on the right show the field sites
and the results from the field validated numerical flow modeling.
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