Experimental Observation of Flow Structure and Resistance over High- and Low-angle Dunes

Abstract:

A prominent control on the flow over dunes in sedimentary environments is the slope of the downstream lee-side. While previous work has focused on steep (~30°), asymmetric dunes with permanent flow separation (‘high-angle dunes’), little is known about dunes with lower lee-slope angles for which flow separation is absent or intermittent (‘low-angle dunes’). Here, we use laboratory experiments to systematically vary and isolate the effect of the dune lee-slope on the turbulent flow field over dunes.

Three sets of fixed dunes with lee-slope angles of 10°, 20° and 30° were separately installed in a 15 m long and 1 m wide flume and subjected to flow 0.20 m deep. At present, no clear hydraulic scaling has been demonstrated for low- and high-angle dunes as both dune configurations occur at the same Froude and Reynolds numbers. However, observations indicate that low-angle dunes are more frequent in environments dominated by suspension of bed material. Therefore, we focus on matching the transport stage between field conditions and our experiments using field observations of bedform morphology and flow stage. Measurements consisted of high-frequency, vertical profiles collected with a Laser Doppler Velocimeter (LDV) along one dune-length and Particle Image Velocimetry (PIV) of the flow field.

We show that the temporal and spatial occurrence of flow separation decreases with dune lee-slope and may be fully absent for lee-slopes <<10°, only. Velocity gradients in the dune leeside depict a free shear layer downstream of the 30° dunes and a weaker shear layer closer to the bed for the 20° and 10° dunes. The decrease in velocity gradients leads to lower turbulence production for gentle lee-slopes. Consequently, flow resistance of dunes decreases with lee-slope; the transition being non-linear. Over the 10°, 20° and 30° dunes, shear stress is 8%, 33% and 90 % greater than a flat bed, respectively. Our results demonstrate that dune shape plays an important, but often ignored role in flow resistance.