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United States Department of Agriculture

Agricultural Research Service

Title: Mean Flow and Turbulence Structure over Fixed Ripples and the Ripple-Dune Transition

Authors
item Bennett, Sean
item Best, James - UNIVERSITY OF LEEDS

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: January 1, 1996
Publication Date: N/A

Interpretive Summary: Detailed measurements of downstream and vertical flow velocities and their turbulent fluctuations in a unidirectional current were obtained using laser Doppler anemometry over two fixed ripple bedforms in a laboratory channel. High-density sampling allowed construction of maps of flow and turbulence parameters which illustrate that: (1) downstream and vertical flow velocities are affected by bed morphology in near-bed regions only; (2) the greatest turbulent velocities occur within the flow separation zone, along the front-side of the ripple bedform; and (3) numerous and very energetic ejection events (turbulent bursts) occur just downstream of flow separation and in mid-depth regions. Whilst ripples and dunes may be considered to be similar, when compared to dunes ripples have: (1) a greater degree of flow uniformity; and (2) a separation zone which is restricted to near-bed regions and has much lower associated turbulence intensities. A schematic model is presented describing the processes responsible for the transition from ripples to dunes and the changing dynamics of the flow separation zone.

Technical Abstract: Detailed measurements of downstream and vertical flow velocities and their turbulent fluctuations in a unidirectional current were obtained using laser Doppler anemometry over two fixed ripples in a laboratory channel. High-density sampling allowed construction of contour maps of flow and turbulence parameters which illustrate that: (1) time-averaged downstream and vertical flow velocities are affected by bed morphology through topographically-induced acceleration and deceleration in near-bed regions only; (2) the maximum root-mean-square values of the downstream and vertical velocity components and Reynolds stress occur within the flow separation zone, along the separation cell free shear layer and at reattachment; (3) positive vertical skewness values and high-magnitude quadrant-2 events (ejections) occur both along the free shear layer and in mid-depth regions; and (4) low-magnitude quadrant-4 events (sweeps) occur near flow reattachment. Whilst ripples and dunes may be considered morphologically and dynamically similar, when compared to dunes ripples have: (1) a greater degree of flow uniformity; (2) a separation zone shear layer which is restricted to near-bed regions and has much lower associated turbulence intensities; and (3) much lower Reynolds stresses at flow reattachment. A schematic model is presented detailing the physical and fluid dynamic mechanisms responsible for the transition from ripples to dunes and the changing dynamics of the flow separation zone.

Last Modified: 10/23/2014
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