Threshold criteria for incipient sediment motion on an inclined bedform in the presence of oscillating-grid turbulence

Abstract

A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. Here, we report laboratory experiments to investigate the threshold criteria for incipient sediment motion in the presence of oscillating-grid turbulence, with the bed slope inclined at angles between the horizontal and the repose limit for the sediment. A set of nine mono-disperse sediment types was used with size ranges normally associated with either the hydraulically-smooth or transitional regimes. Measurements of the (turbulent) fluid velocity field, in the region between the grid and bedform's surface, were obtained using two-dimensional particle imaging velocimetry. Statistical analysis of the velocity data showed that the turbulence had a anisotropic structure, due to the net transfer of energy from the normal to the tangential velocity components in the near-bed region, and that the fluctuations were dominant compared to the secondary mean flow. The sediment threshold criteria for horizontal bedforms were compared with, and found to be in good qualitative agreement with the standard Shields curve. For non-horizontal bedforms, the bed mobility was found to increase with increasing bed slope, and the threshold criteria were compared with previously-reported theoretical models, based on simple force-balance arguments. C 2013 American Institute of Physics. [http://dx

Experimental study of oscillating-grid turbulence interacting with a solid boundary

“…The stroke, S, defined as equal to the amplitude of the grid's motion, was varied by changing the radius of the flywheel; strokes of S = 2.5 cm and 3.0 cm were used. These ranges for f and S are in line with those used in previous OGT studies (Hopfinger & Linden 1982;Thompson & Turner 1975;Hopfinger & Toly 1976;McDougall 1979;Wan Mohtar & Munro 2013).…”

“…Measurements of OGT impinging upon a horizontal boundary made by McDougall (1979) exhibited a monotonic reduction of u at low Reynolds number (Re ≡ u /ν ≈ 40; denotes integral length scale), while Hannoun et al (1988) observed amplification of u at higher Reynolds number (Re ≈ 120). Using OGT, measurements showing amplification of tangential turbulent velocities have also been observed at a sharp density interface between two fluids (Brumley & Jirka 1987;Kit et al 1997) and at a sediment boundary (Wan Mohtar & Munro 2013).…”

Measurements of sediment pickup rate over dune-covered bed

“…Therefore, sediment transport in particular the bedload transport rate can be effectively characterized by the shear velocity. However, experimental measurements and theoretical considerations reveal that over the bedforms where the flow is not uniform and the local turbulence varies significantly with bed position, near-bed turbulence is not necessarily correlated with the local mean flow characteristics and thus sediment entrainment may not be characterized by the local bed shear stress or shear velocity [18,23,27,28,33].…”

“…Application of the sediment pickup rate function introduced by Miwa and Daido [21] requires estimation of the lift and drag force coefficients, the shear velocity and the critical Shields number s *c , which are all difficult to define over a dune. A recent study that is relevant is due to Mohtar and Munro [23] who examined the sediment threshold over a dune in presence of oscillating-grid turbulence. In their experiments with no mean flow, incipient grain motion was only induced as a result of turbulent fluctuations.…”

Threshold Criteria for Incipient Grain Motion with Turbulent Fluctuations on a Horizontal Bed

“…This equation is (only) applicable at Z 2.5M, a region considered to reach the state of quasi-isotropic homogeneous turbulence (Hopfinger & Toly 1976). Several studies have employed oscillating-grid turbulence to investigate the threshold criteria for sediment movement, in particular the work of Bellinsky et al (2005) and Wan Mohtar and Munro (2013). This article is set to qualitatively compare the role played by turbulent fluctuations on the overall trend of θ c0 measured by turbulent fluctuations of grid turbulence and vortex ring.…”