Post on 06-Aug-2018
Dunes: characteristics & scaling
•Dunes scale with flow depth….. ldune ~ 5-7Y•Dune height ~ 0.33Y•Dunes associated with macroturbulence….
’boils’ on the flow surface• So, since macroturbulence scales as:
•Tb = fU/Y ~ 5-7•the same as burst scaling?
downstream
vertical
lateral
ve
locit
y,
cm
s-1
ADCP field quantification: June 1999
flow
flow
flow
dunes ~ 2m high
BUT• gravel dunes• burst scaling - Strouhal law
St=fY/U ~ 0.2• so bedload flux is key?
suspension effects
using PIV
- straight-crested 2D dune ..….concentrate on
two areas -
….examine the temporal characteristics of the
flow field over fixed dunes….
i) leesideii) upper stoss
I: flow separation, vortex shedding and its effects
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t R, N m-2
0s
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tR,
Dune Lee
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u, m s-1
v, m s-1 t R, N m
-2
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tR,
Dune Stoss
• Shear layer ‘flapping’
• Kelvin-Helmholtz instabilities shed along shear layer
• Ejections of fluid reach surface during flapping
• Ejections generate inrushes at downstream crest that have tR > 6-9x average tR
• link between separation zone dynamics and magnitude and location of downstream
sediment transport
summary & implications
~ 30m
upwellinglarge waves at
downstream edge
II: topology of dune-related macroturbulence –
some field observations
Vortex ring interaction with flow
surface (after Sarpkaya, 1996)
vortex ring
Planform view of surface as vortex ring
approaches
flow
flow
Previous observations of dune-related
macroturbulence interacting with the
water flow surface
(after Babakaiff & Hickin, 1996)
Morphology of coherent vortices behind dunes
(after Nezu & Nakagawa, 1993)
Dune
Flow
Separated vortices
Dune
Cre
st
3-D
Inte
ractio
n
Low-speed fluid
Recirculation
Reattachment
Large-scale vortex
I: vortex approaching surface
DUNE-
GENERATED
VORTEX
Schematic of vortex interaction with a free
surface - based on field
& flume observations
from Patel, Lin and Yue
www.iihr/uiowa/projects/turbulentdune/
LES
simulations
Flow patterns one
grid cell below free
surface
Red: upwelling
Blue: downwelling
flow
shear layer development from lower leeside
shear layers from other
changes in bed slopeintermittent separation
(‘transitory stall’)
‘wake’ stacking
sediment ejected into suspension
at ‘separation’ and ‘reattachment’ regions
flow deceleration
in leeside
sediment suspension along shear layers
from upstream dunes
large events may erupt at flow surface
• many natural alluvial dunes are ‘low-angle’ (flow resistance implications)
• dynamics dominated by intermittent separation and shedding
• control of sediment transport by separation zone/ejection-inrush dynamics
summary & implications
Ideas:
• they aren’t - they are part of a continuum?• wave instabilities of different size?• form controlled by different scales of CFS?• dunes evolve from ‘rogue’ ripples -therefore influence flow field?
Why are ripples and dunes separate forms?
References
Dunes:Babakaiff, C.S. and Hickin, E.J. 1996: Coherent flow structures in the Squamish River estuary, BC, Canada, In: Coherent Flow Structures, 321-342.Bennett,S.J. & Best, J.L. 1995 Mean flow and turbulence structure over fixed two-dimensional dunes….., Sedimentology, 42, 491-513Best, J.L. 2005 The fluid dynamics of river dunes: a review and some future research directions. J. Geophysical Research, Earth Surface, 110, F04S02, doi:10.1029/2004JF000218. Best, J.L. and Kostaschuk, R.A. 2002 An experimental study of turbulent flow over a low-angle dune, J. Geophysical Research, 107, C9, 3135-3153.*Jackson, R.G. 1976 Sedimentological and fluid dynamic implications of turbulent bursting… J.Fluid Mechanics, 77, 531-560.Muller, A. and Gyr, A. 1982 Visualization of the mixing layer behind dunes, In: Mechanics of Sediment Transport, 41-48Nelson, J.M. et al. 1993 Mean flow and turbulence fields over two-dimensional bedforms, Water Resources Research, 29, 3935-3953