Observations of Coherent Flow Structures Over Subaqueous High‐ and Low‐ Angle Dunes

Abstract

Large‐scale coherent flow structures (CFSs) above dunes are the dominant source of flow resistance and constitute the principal mechanism for sediment transport and mixing in sand bed river and estuarine systems. Based on laboratory observations, CFS formation has been previously linked to flow separation downstream of high‐angle dunes with lee slopes of ~30°. How CFSs form in natural, deep rivers and estuaries where dunes exhibit lower lee slopes and intermittent flow separation is not well understood. Here we present particle image velocimetry measurements from an experiment where dune lee slope was systematically varied (30°, 20°, and 10°), while other geometric and hydraulic parameters were held constant. We show that CFSs form downstream of all three dune geometries from shear layer vortices in the dune lee. The mode of CFS formation undergoes a low‐frequency oscillation with periods of intense vortex shedding interspersed with periods of rare vortex shedding. Streamwise alignment of several vortices during periods of intense shedding results in wedge‐shaped CFSs that are advected above the dune stoss side. Streamwise length scales of wedge‐shaped CFS correspond to large‐scale motions (LSMs). We hypothesize that the advection of LSM over the dune crest triggers the periods of intense shedding in the dune lee. LSMs are weaker and smaller above low‐angle dunes; however, the low‐frequency oscillation in CFS formation periods persists. The formation of smaller and weaker CFS results in a reduction of flow resistance over low‐angle dunes.

The Impact of Nonequilibrium Flow on the Structure of Turbulence Over River Dunes

“…The quantification of reattachment length (Figure ) indicates that a measure of flow magnitude and the shape of velocity profiles provide a more universal description of reattachment length for a given dune geometry. Parameterization of this relationship into models such as those of Paarlberg et al () and Lefebvre et al () will require additional work measuring velocity profiles and median reattachment lengths across a range of dune geometries, including low‐angle dunes, as well as considering the inclusion of brink‐point shape into dune morphology (Kwoll et al, , ).…”

“…Experiments on the ripple-dune transition (Bennett & Best, 1996;Fernandez et al, 2006;Frias & Abad, 2013;Schindler & Robert, 2005) showed that during the transition, the location and intensity of flow separation (Figure 1, point 2), the turbulent wake (Figure 1), and the height of the internal boundary layer (Figure 1, point 4) vary in their spatial location and intensity. Best and Kostaschuk (2002) and Kwoll et al (2016Kwoll et al ( , 2017 have also shown that reductions in the leeside slope angle lessen the intensity and temporal consistency of flow separation (Figure 1, point 2), while Bennett et al (1998) and Naqshband et al (2016) examined the transition of dunes to upper stage plane bed and demonstrated the collapse of the flow separation feedback. All these experiments indicate deviations from the equilibrium dune flow description ( Figure 1) due to alteration of the bed morphology.…”

“…Balachandar et al (2007) showed that reattachment length increased when flow depth (Y) was lowered below the equilibrium flow depth, indicating that under nonequilibrium conditions flow separation and reattachment are dependent on both flow conditions and bedform shape. In addition, experiments such as those of Fernandez et al (2006) and Kwoll et al (2016Kwoll et al ( , 2017 demonstrate that bedform geometry can change reattachment length even under consistent flow boundary conditions. This variety of causal mechanisms indicates a multiplicity of controls upon reattachment length.…”

A PIV-based method to measure spatial gradients in bedload transport over a dune

“…The fluid is seeded with tracer particles that are assumed to follow the flow dynamics. PIV has proven suitable for measurements of flow velocity [17][18][19] and provided detailed information on flow structures, such as large-scale coherent flow structures above dunes [18]. Tools for the application of PIV are readily available.…”

Low-angle dunes in the Changjiang (Yangtze) Estuary: Flow and sediment dynamics under tidal influence

“…Estimation from local velocity profiles over the trough overpredicts total shear stress, while that from the crest fits well with the result of spatial averaging (Figure 4-1). It indicates that although the accuracy of technique of spatial averaging are relevant to bedform characteristics (lee-side angle), flow separation and flow strength Kwoll et al, 2017], for low-angle dunes, shear stress over the crest can roughly represent total stress of spatially averaged.…”