An experimental study of turbulent flow over a low‐angle dune

Abstract

[1] Many large, sand bed alluvial channels are dominated by dunes that possess lowangle lee sides, often <10°, which play a critical role in the transportation of sediment and generation of significant bed form roughness. Despite the fact that these low-angle dunes are very common in such channels many current models of dune flow dynamics are based on bed forms with an angle of repose slip face that generates a zone of permanent separated flow in the dune lee. Study of flow associated with low-angle dunes in the field is inherently difficult since it is usually both hard to measure very near the bed and hard to quantify the nature of turbulence over these bed forms. Results from a detailed scale model experimental study of flow over a low-angle dune, which is based on a prototype dune from the Fraser River, Canada, present a necessary link between flume and field studies and document the origins of macroturbulence associated with these bed forms. Two-dimensional laser Doppler anemometer measurements over a low-angle dune (maximum lower lee side slope = 14°) show that dune morphology exerts a dominant control on the turbulent flow, causing flow deceleration in the lower lee and development of an intermittent layer of shear at the interface with the higher velocity flow above. The scale model confirms that permanent flow separation does not occur over low-angle dunes but, instead, is replaced by a small region (here $7% of the dune wavelength in length) of intermittent flow reversal, which may be present for up to 4% of the time. Shear layers generated along this small zone of decelerated and/or separated flow in the lower lee have a much smaller velocity differential than is characteristic of shear layers generated by flow separation in the lee of angle of repose dunes. Turbulence production associated with lowangle dunes is dominated by eddies generated along this shear layer, which produce highly variable horizontal and vertical velocities and large Reynolds stresses in this region. These results show that macroturbulence associated with low-angle dunes is generated by intermittent separation or shear layer generation due to velocity gradients established in the zone of lee side flow expansion. Velocity profiles and maps of turbulence structure from the scale model are in reasonable agreement with field measurements from low-angle dunes in natural sand bed rivers. These results highlight the need to consider the temporal evolution and intermittency of shear layer behavior, often very near the bed, when interpreting the generation of macroturbulence and dispersal of sediment associated with low-angle dunes.INDEX TERMS: 1824 Hydrology: Geomorphology (1625); 4235 Oceanography: General: Estuarine processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; KEYWORDS: dune, low-angle leeside, intermittent flow separation Citation: Best, J., and R. Kostaschuk, An experimental study of turbulent flow over a low-angle dune,

Morphodynamics of river dunes : suspended sediment transport along mobile dunes and dune development towards upper stage plane bed

“…Smith and McLean [1977] found that, in the Columbia River, asymmetric dunes with flow separation occurred when bed load was the dominant transport mechanism while symmetric dunes without flow separation zone (low-angle dunes) developed when most sand is transported in suspension. They suggested that the steep lee sides of asymmetric dunes are maintained by avalanching of bed load down the lee slope, whereas the much lower angle lee sides of symmetric dunes result from deposition of sand from suspension in the lee side and trough between dunes [Best and Kostaschuk, 2002]. This was also concluded from the field measurements performed by Kostaschuk and Villard [1996], Kostaschuk [2000] and Amsler et al, [2003].…”

“…Fluid and sediment dynamics over dunes are controlled by dune geometry and in particular the dune lee side angle [Kostaschuk and Villard, 1996;Best and Kostaschuk, 2002;Best, 2005a;Venditti, 2013]. In flumes, interaction between flow and sediment transport gives rise to high-angle dunes with long, gentle upstream stoss side slopes and short lee sides with steep angles often reaching the angle of repose (~30°).…”

“…Although due the turbulent nature of the flow associated with dunes, it is likely that turbulent sediment fluxes may have a significant contribution to the total sediment fluxes, their exact distribution along the dune bed and their contribution to dune migration are poorly understood mainly due to the difficulties of measuring them quantitively. This gap of knowledge is partly attributed to the fact that a majority of the experimental studies associated with dunes have focused on flow and sediment dynamics above fixed beds [e.g., Van Mierlo and de Ruiter, 1988;Lyn, 1993;Nelson et al, 2003;McLean et al, 1999;Cellino and Graf, 2000;Best and Kostaschuk, 2002;Maddux et al, 2003;Kleinhans, 2004;Best, 2005a,b;Venditti, 2007].…”

“…As highlighted by Parsons and Best [2013], sediment transport and the exact nature of lee side deposition processes are key to predicting dune migration. Coleman and Nikora [2011] attributed this gap of knowledge partly to the fact that most of the experimental studies associated with dunes have focused on flow and sediment dynamics above fixed beds [e.g., Cellino and Graf, 2000;Best and Kostaschuk, 2002;Kleinhans, 2004;Best, 2005a,b;Venditti, 2007]. The advantage of utilizing fixed bedforms is that they allow detailed flow measurements without the complications of both a migrating and changing bedform and the difficulties of flow measurement in the presence of sediment transport over a fully mobile bed [Best and Kostaschuk, 2002].…”

Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 2. Flow structures

“…The flow separates at the crest and reattaches at the beginning of the stoss side (Kostaschuk 2000). Note that in large sand-bed alluvial channels dunes may possess low-angle lee side and intermittent separation of the flow at the crest (Roden 1998;Best & Kostaschuk 2002); here we study typical dune models with an angle of repose slip face where the flow permanently separates at the crest (Roden 1998;Kostaschuk 2000). In the first part of this paper (Omidyeganeh & Piomelli 2013), the past research on two-and three-dimensional dunes was reviewed, and thus here we limit ourselves to a brief summary.…”