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The flow structure of interacting barchan dunes

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Title: The flow structure of interacting barchan dunes
Author(s): Palmer, Jessica A.
Advisor(s): Best, James L.
Department / Program: Geology
Discipline: Geology
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: M.S.
Genre: Thesis
Subject(s): Turbulent flow Barchan dunes bedform separation zone reattachment length dune interactions Particle-imaging velocimetry (PIV)
Abstract: High-resolution particle-image velocimetry (PIV) experiments have been conducted over a fixed-bed to examine the effects of interacting barchan dunes on the flow structure. The barchan dune models were based upon an idealized contour map, the shape and dimensions of which were based upon previous empirical studies of dune morphology. The experimental setup comprised two, co-axially aligned barchan dune models that were spaced at different distances apart. Three volumetric ratios (Vr, upstream barchan dune: downstream barchan dune) of 0.025, 0.056, 0.175, replicated the different behaviors of interacting barchan dunes as observed in the field and laboratory by Endo et al. (2004), and were accompanied by an investigation of identically-sized dunes. Models were placed in an Eiffel-type, open-circuit wind tunnel with a working test-section 6090 mm long by 914 mm wide by 457 mm high and a free-stream turbulence intensity of 0.16%. Flow quantification was achieved using particle imaging velocimetry (PIV) at 0.5Hz. PIV measurements of the mean and turbulent flow field were made in the streamwise–wall-normal plane, along the centerline of the barchans(s), at an average Reynolds number of 59,000. The presence of an upstream barchan dune of equal volume to the downstream barchan dune (Vr=1) induces a ‘sheltering effect’ on the flow, manifested by a significantly shorter separation bubble and both reduced streamwise velocity and turbulence intensity in the downstream barchan dune leeside, as compared to an isolated barchan. The volumetric ratio associated with the ‘splitting’ behavior (Vr=0.175) of Endo et al. (2004) shows enhanced turbulence production over the downstream barchan dune leeside, that is proposed to be caused by interacting shear layers from the up- and down- stream dunes. The upstream dune also creates a shear layer in its leeside that extends onto the stoss-side of the downstream barchan, and is proposed to be responsible for dune ‘splitting’. For volumetric ratios associated with ‘ejection’ and ‘absorption’ behaviors (Vr=0.056 and 0.025, respectively), the upstream barchan dune does not significantly impact the flow in the leeside of the downstream barchan dune. In these cases, the small coherent turbulent structures generated from the upstream dune lose their coherence before reaching the downstream barchan crest, or may reach the downstream dune shear layer but are too insignificant in intensity and size to interact with the larger flow structures generated in the leeside of downstream barchan dune. The upstream dune leeside for the ‘ejection’ and ‘absorption’ behaviors is characterized by smaller separation zone lengths, and smaller turbulence intensities, than the ‘splitting’ behavior. In this case, it is speculated the potential for downstream erosion is reduced and the downstream barchan maintains its morphology.
Issue Date: 2010-05-18
URI: http://hdl.handle.net/2142/15972
Rights Information: Copyright 2010 Jessica A. Palmer
Date Available in IDEALS: 2010-05-18
2012-05-19
Date Deposited: May 2010
 

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