Flow over ripples: KEY features ripple size independent of flow depth l ~ 1000d deceleration in leeside topographic acceleration over stoss flow

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1 Ripples and dunes

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4 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??

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6 Flow over a ripple

7 Methodology: fixed ripple laser Doppler anemometry many points quadrant analysis

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12 Ripples cease to exist in coarse sands - why? Year 3 EARS 3072/GEOG 3430 Alluvial Flow.

13 Grass, 1970

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15 Roughness effects on flow separation Leeder, 1980

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17 Year 3 EARS 3072/GEOG 3430 Alluvial Flow.

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19 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?

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21 T b =fu/y~5 Jackson, 1975, 1976

22 flow Dune-related macroturbulence interacting with the flow surface - Jamuna River, Bangladesh boil

23 Flow over dunes

24 using LDA

25 flow over dunes..

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31 Field Study: Fraser River, British Columbia

32 Echo sounder trace Downstream river flow 2 dune with ~10 leeside compound dune metres 0 50

33 ADCP field quantification: June 1999 velocity, cm s -1 downstream flow dunes ~ 2m high lateral flow vertical flow

34 so, ideas on how this causes dune scaling? suspension effects

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36 suspension effects BUT gravel dunes burst scaling - Strouhal law St=fY/U ~ 0.2 so bedload flux is key?

37 Dinehart 1992

38 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 -

39 upstream flow mean ms -1 leeside: downstream downstream velocity downstream

40 mean leeside: vertical velocity vertical velocity

41 upper stoss: downstream velocity flow

42 upper stoss: vertical velocity flow

43 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

44 downstream Reynolds velocity stress one frame at 15 Hz vertical velocity

45 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

46 II: topology of dune-related macroturbulence some field observations ~ 30m upwelling large waves at downstream edge

47 flow

48 flow

49 flow

50 flow flow flow

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52 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

53 Morphology of dune-related macroturbulent ejections (after Müller & Gyr, 1983, 1996) dune crest

54 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

55 Schematic of vortex interaction with a free surface - based on field & flume observations DUNE- GENERATED VORTEX I: vortex approaching surface

56 Shear with mean flow II: vortex tip interacts with free surface

57 Vortex tubes developing III: vortex leg interaction with free surface

58 IV: vortex tube development

59 Flow patterns one grid cell below free surface Red: upwelling Blue: downwelling from Patel, Lin and Yue LES simulations

60 Flow and CFS over dunes - modelling Omidyeganeh and Piomelli, 2010: LES simulations

61 Omidyeganeh and Piomelli, 2010 horseshoe vortices

62 Omidyeganeh and Piomelli, 2010 boil nearing the surface vertical vortices

63 III: the influence of dune leeside angle slipface angle (angle of repose ~ 30 ) Jamuna River

64 water surface flow dune leeside

65 a LDA study

66 flow

67 flow

68 flow

69 flow

70 flow flow in the leeside

71 Height above bed, y/ytot % time flow reversal intermittent separation zone

72 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

73 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

74 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?

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78 Shear layer velocity gradients & therefore turbulence?

79 References Dunes: Babakaiff, C.S. and Hickin, E.J Coherent flow structures in the Squamish River estuary, BC, Canada, In: Coherent Flow Structures, Bennett,S.J. & Best, J.L Mean flow and turbulence structure over fixed two-dimensional dunes.., Sedimentology, 42, Best, J.L The fluid dynamics of river dunes: a review and some future research directions. J. Geophysical Research, Earth Surface, 110, F04S02, doi: /2004jf Best, J.L. and Kostaschuk, R.A An experimental study of turbulent flow over a low-angle dune, J. Geophysical Research, 107, C9, Jackson, R.G Sedimentological and fluid dynamic implications of turbulent bursting J.Fluid Mechanics, 77, Muller, A. and Gyr, A Visualization of the mixing layer behind dunes, In: Mechanics of Sediment Transport, Nelson, J.M. et al Mean flow and turbulence fields over twodimensional bedforms, Water Resources Research, 29,

80 References General: Grass, A.J Structural features of turbulent flow over smooth & rough boundaries, J. Fluid Mechanics, Smith, C.R Coherent flow structures in smooth wall boundary layers, In: Coherent Flow Structures in Open Channels Ripples: Bennett,S.J. & Best, J.L Mean flow and turbulence structure over fixed ripples, In: Coherent Flow Structures in Open Channels Leeder, M.R J. Geol. Soc. London, 137,