Ripples and dunes
Flow over ripples: KEY features ripple size independent of flow depth l ~ 1000d deceleration in leeside topographic acceleration over stoss flow separation in leeside shear layer development Q2 events along shear layer.. Kelvin-Helmholtz instabilities effects limited to < ~0.4Y.so, what does this look like & imply??
Flow over a ripple
Methodology: fixed ripple laser Doppler anemometry many points quadrant analysis
Ripples cease to exist in coarse sands - why? Year 3 EARS 3072/GEOG 3430 Alluvial Flow.
Grass, 1970
Roughness effects on flow separation Leeder, 1980
Year 3 EARS 3072/GEOG 3430 Alluvial Flow.
Dunes: characteristics & scaling Dunes scale with flow depth.. l dune ~ 5-7Y Dune height ~ 0.33Y Dunes associated with macroturbulence. boils on the flow surface So, since macroturbulence scales as: T b = fu/y ~ 5-7 the same as burst scaling?
T b =fu/y~5 Jackson, 1975, 1976
flow Dune-related macroturbulence interacting with the flow surface - Jamuna River, Bangladesh boil
Flow over dunes
using LDA
flow over dunes..
Field Study: Fraser River, British Columbia
Echo sounder trace Downstream river flow 2 dune with ~10 leeside compound dune metres 0 50
ADCP field quantification: June 1999 velocity, cm s -1 downstream flow dunes ~ 2m high lateral flow vertical flow
so, ideas on how this causes dune scaling? suspension effects
suspension effects BUT gravel dunes burst scaling - Strouhal law St=fY/U ~ 0.2 so bedload flux is key?
Dinehart 1992
I: flow separation, vortex shedding and its effects ii) upper stoss using PIV i) leeside.examine the temporal characteristics of the flow field over fixed dunes. - straight-crested 2D dune...concentrate on two areas -
upstream flow mean ms -1 leeside: downstream downstream velocity downstream
mean leeside: vertical velocity vertical velocity
upper stoss: downstream velocity flow
upper stoss: vertical velocity flow
m s -1 blue ~ 40 cm s -1 white ~ 25 cm s -1 downstream velocity m s -1 blue - 5 cm s -1 (towards bed) red - 3 cm s -1 (away from bed) vertical velocity upper stoss
downstream Reynolds velocity stress one frame at 15 Hz vertical velocity
summary & implications 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 R > 6-9x average R link between separation zone dynamics and magnitude and location of downstream sediment transport
II: topology of dune-related macroturbulence some field observations ~ 30m upwelling large waves at downstream edge
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Previous observations of dune-related macroturbulence interacting with the water flow surface (after Babakaiff & Hickin, 1996) Vortex ring interaction with flow surface (after Sarpkaya, 1996) flow vortex ring flow Planform view of surface as vortex ring approaches
Morphology of dune-related macroturbulent ejections (after Müller & Gyr, 1983, 1996) dune crest
Morphology of coherent vortices behind dunes (after Nezu & Nakagawa, 1993) Flow Large-scale vortex Low-speed fluid Dune Crest 3-D Interaction Separated vortices Dune Recirculation Reattachment
Schematic of vortex interaction with a free surface - based on field & flume observations DUNE- GENERATED VORTEX I: vortex approaching surface
Shear with mean flow II: vortex tip interacts with free surface
Vortex tubes developing III: vortex leg interaction with free surface
IV: vortex tube development
Flow patterns one grid cell below free surface Red: upwelling Blue: downwelling from Patel, Lin and Yue www.iihr/uiowa/projects/turbulentdune/ LES simulations
Flow and CFS over dunes - modelling Omidyeganeh and Piomelli, 2010: LES simulations
Omidyeganeh and Piomelli, 2010 horseshoe vortices
Omidyeganeh and Piomelli, 2010 boil nearing the surface vertical vortices
III: the influence of dune leeside angle slipface angle (angle of repose ~ 30 ) Jamuna River
water surface flow dune leeside
a LDA study
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flow flow in the leeside
Height above bed, y/ytot % time flow reversal intermittent separation zone
flow large events may erupt at flow surface sediment suspension along shear layers from upstream dunes flow deceleration in leeside wake stacking intermittent separation ( transitory stall ) shear layers from other changes in bed slope shear layer development from lower leeside sediment ejected into suspension at separation and reattachment regions
summary & implications 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
Why are ripples and dunes separate forms? 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?
Shear layer velocity gradients & therefore turbulence?
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, 491-513 Best, 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, 41-48 Nelson, J.M. et al. 1993 Mean flow and turbulence fields over twodimensional bedforms, Water Resources Research, 29, 3935-3953
References General: Grass, A.J. 1971 Structural features of turbulent flow over smooth & rough boundaries, J. Fluid Mechanics, 223-256. Smith, C.R. 1996 Coherent flow structures in smooth wall boundary layers, In: Coherent Flow Structures in Open Channels Ripples: Bennett,S.J. & Best, J.L. 1996 Mean flow and turbulence structure over fixed ripples, In: Coherent Flow Structures in Open Channels Leeder, M.R. 1980 J. Geol. Soc. London, 137, 423-429