CEE 595W
“The fluid dynamics
of bedform superimposition”
Jim Best
Earth and
Biosphere Institute
Abstract
Bedform superimposition and
amalgamation are ubiquitous in all river channels, and involve various scales
of superimposition of ripples on dunes, dunes on dunes/bars and unit bars upon
compound bars. Bedform superimposition is critical in
determining the local turbulence field and flow resistance, and can be expected
to radically affect the nature of sediment sorting on the bedform
surface and the delivery of sediment to the downstream bedform
leeside, thus dictating patterns of leeside sorting and stratification. Despite
the importance of bedform superimposition, there has
been little study of the fluid dynamic interactions between bedforms
and how bedform amalgamation may proceed. This talk
will present results from laboratory studies of the superimposition of simple
triangular bedforms and examine changes to the mean
and turbulent flow field at different stages of bedform
superimposition.
Two series of experiments
will be detailed that sought to quantify the flow field over interacting and
combining bedforms. A simple methodology was adopted
in these experiments: at first, isolated, two-dimensional, asymmetrical
triangular roughness elements were inserted in a uniform unidirectional flow,
and the flow was quantified using 2D LDA and PIV. This
revealed features of flow identical to that quantified over single bedforms in many past experimental studies: the flow field
is dominated by fluid acceleration over the crest, flow separation in the
leeside and boundary layer recovery downstream.
Once the flow field over
these solitary forms had been detailed, the modification to the flow caused by
smaller triangular bedforms was studied as the
smaller bedforms were positioned at various
locations, and in various numbers (up to three smaller bedforms),
along the stoss side of the larger bedform. LDA and PIV reveal both the significant
modification to the flow field as superimposition proceeds and the interaction
between the regions of leeside flow separation as the smaller bedform nears the crest of the larger form. At this stage
before amalgamation, the Reynolds stresses within the flow are at their highest
and the superimposed form may be expected to dictate flow (and sediment
transport under mobile bed conditions) at the crest of the larger form. This
talk will examine these changes to the flow fields at various stages of
superimposition, and its effects on the scale and periodicity of turbulence in
the leeside of the larger bedform. Additionally,
these experiments were designed so that the initial large bedform
and the final amalgamation of the two triangular elements produced bedforms whose height: flow depth ratio spanned the ripple:dune transition. These results will also be
discussed in light of previous ideas concerning turbulence across this bedform transition.
Wednesday, April 20, 2005
4:00 p.m.
Room 1518 Hydrosystems Lab
Everyone
Welcome