Spectral analysis of turbulent flow and suspended sediment transport over fixed dunes

Venditti, Jeremy G.; Bennett, Sean J.

doi:10.1029/2000jc900094

Cited by 151 publications

(155 citation statements)

References 35 publications

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“…The time dependency between the horizontal shear stress and sediment concentration also varies with elevation (Venditti and Bennett, 2000). Given the existence of an undertow in this case (~ 0.2 m/s offshore), vertical variations are expected in spilling breakers (erosive runs)…”

Section: Spectral Analysis Of Turbulence and Suspensionmentioning

confidence: 99%

Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach

Kassem

Thompson

Amos

et al. 2015

Continental Shelf Research

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Abstract:The suspension of sediments by oscillatory flows is a complex case of fluid-particle interaction. The aim of this study is to provide insight into the spatial (time) and scale (frequency) relationships between wave-generated boundary layer turbulence and eventdriven sediment transport beneath irregular shoaling and breaking waves in the nearshore of a prototype sandy barrier beach, using data collected through the Barrier Dynamics Experiment II (BARDEX II). Statistical, quadrant and spectral analyses reveal the anisotropic and intermittent nature of Reynolds' stresses (momentum exchange) in the wave boundary layer, in all three orthogonal planes of motion. The fractional contribution of coherent turbulence structures appears to be dictated by the structural form of eddies beneath plunging and spilling breakers, which in turn define the net sediment mobilisation towards or away from the barrier, and hence ensuing erosion and accretion trends. A standing transverse wave is also observed in the flume, contributing to the substantial skewness of spanwise turbulence. Observed low frequency suspensions are closely linked to the mean flow (wave) properties. Wavelet analysis reveals that the entrainment and maintenance of sediment in suspension through a cluster of bursting sequence is associated with the passage of intermittent slowly-evolving large structures, which can modulate the frequency of smaller motions. Outside the boundary layer, small scale, higher frequency turbulence drives the suspension. The extent to which these spatially varied perturbation clusters persist is associated with suspension events in the high frequency scales, decaying as the turbulent motion ceases to supply momentum, with an observed hysteresis effect. The suspension of sediment in turbulent flows is a complex case of fluid-particle interaction, governed by shear stresses (momentum exchanges) at the bed and within the benthic boundary layer (BBL). Defining the physical processes which dictate the resuspension of sediments in coastal and estuarine settings is fundamental for accurate predictions of bed morphology evolution (Van Rijn et al., 2007), and has profound implications for the biogeochemical processes that shape their local ecology (Thompson et al., 2011). It is also a prerequisite to quantifying erosion and deposition trends, and hence guiding engineering applications such as beach nourishment, defence schemes against erosion and flooding, maintenance of marine infrastructure and waterways, and aggregate dredging. There is a genuine need for better, robust models of suspended sediment transport in the coastal zone (Aagaard and Jensen, 2013). In a vision paper on future research needs in coastal dynamics, Van Rijn et al. (2013) highlighted the pressing need for research to support such models, focusing in particular on sand transport in the shoreface (non-breaking waves), surf and swash zones; employing field and controlled laboratory experiments.The mobilisation of sediments in the nearshore and shoreface is dominated b...

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Section: Spectral Analysis Of Turbulence and Suspensionmentioning

confidence: 99%

Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach

Kassem

Thompson

Amos

et al. 2015

Continental Shelf Research

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“…At a higher deceleration, the shear layer over the lobe becomes broader and is lifted toward the top surface by the upward moving of the mean flow. An internal boundary layer is developed after the reattachment on the stoss side of dunes (Kostaschuk & Church 1993;Best et al 2001;Nelson et al 1993;Kadota & Nezu 1999;Bennett & Best 1995;Venditti & Bennett 2000;Balachandar et al 2007), which results in a near-wall peak of the normal stresses in Figure 19(a,c). The internal boundary layer has higher streamwise stressess in the saddle plane, since the local Reynolds number is higher; however, the spanwise stresses (Figure 19(c)) near the bed have a larger peak in the lobe plane due to the spanwise motion of the internal boundary layer.…”

Section: Reynolds Stressesmentioning

confidence: 99%

“…Although flow over dunes has been extensively studied in field experiments (Matthes 1947;Jackson 1976;Flemming 1978;Gabel 1993;Kostaschuk & Church 1993;Bennett & Best 1995;Babakaiff & Hickin 1996;Kostaschuk & Villard 1996;Roden 1998;Carling et al 2000;Kostaschuk 2000;Best et al 2001), fixed 2D dunes have been the focus of the studies in laboratory experiments (Müller & Gyr 1986;Nelson et al 1993;Kadota & Nezu 1999;Schmeeckle et al 1999;Venditti & Bennett 2000;Hyun et al 2003;Balachandar et al 2007;Balachandar & Patel 2008), numerical simulations (Yoon & Patel 1996;Schmeeckle et al 1999;Yue et al 2005;Polatel et al 2006;Yue et al 2006;Stoesser et al 2008;Grigoriadis et al 2009;Omidyeganeh & Piomelli 2011), as well as theoretical studies (McLean & Smith 1986;Nelson & Smith 1989;McLean et al 1999). A comprehensive review of literature on dunes is given by Best (2005).…”

Section: Introductionmentioning

confidence: 99%

Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 1. Turbulence statistics

Omidyeganeh

Piomelli

2013

J. Fluid Mech.

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This is the accepted version of the paper.This version of the publication may differ from the final published version. We performed large-eddy simulations of flow over a series of three-dimensional dunes at laboratory scale (Reynolds number based on the average channel depth and streamwise velocity was 18,900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The bedform three-dimensionality was imposed by shifting a standard two-dimensional dune shape in the streamwise direction according to a sine wave. The statistics of the flow are discussed in ten cases with in-phase and staggered crestlines, different deformation amplitudes and wavelengths. The results are validated qualitatively against experiments. The three-dimensional separation of flow at the crestline alters the distribution of wall pressure, which in turn may cause secondary flow across the stream, which directs lowmomentum fluid, near the bed, toward the lobe (the most downstream point on the crestline) and high-momentum fluid, near the top surface, toward the saddle (the most upstream point on the crestline). The mean flow is characterized by a pair of counterrotating streamwise vortices, with core radius of the order of the flow depth. However, for wavelengths smaller than the flow depth, the secondary flow exists only near the bed and the mean flow away from the bed resembles the two-dimensional case. Staggering the crestlines alters the secondary motion; the fastest flow occurs between the lobe and the saddle planes, and two pairs of streamwise vortices appear (a strong one, centred about the lobe, and a weaker one, coming from the previous dune, centred around the saddle). The distribution of the wall stress and the focal points of separation and attachment on the bed are discussed. The sensitivity of the average reattachment length, depends on the induced secondary flow, the streamwise and spanwise components of the channel resistance (the skin friction and the form drag), and the contribution of the form drag to the total resistance are also studied. Three-dimensionality of the bed increases the drag in the channel; the form drag contributes more than in the two-dimensional case to the resistance, except for the staggered-crest case. Turbulent-kinetic energy is increased in the separated-shear layer by the introduction of three-dimensionality, but its value normalized by the plane-averaged wall stress is lower than in the corresponding twodimensional dunes. The upward flow on the stoss side and higher deceleration of flow on the lee side over the lobe plane lift and broaden the separated-shear layer, respectively, affecting the turbulent kinetic energy. Permanent repository link

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“…The importance of these coherent structures in the transport of sediment has been discussed in the literature (Schmeeckle et al 1999;Venditti & Bennett 2000;Best 2005a). However the instantaneous characteristics of these eddies have not been studied yet.…”

Section: Separated Shear-layer Turbulencementioning

confidence: 99%

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

Omidyeganeh

Piomelli

2013

J. Fluid Mech.

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This is the accepted version of the paper.This version of the publication may differ from the final published version. We performed large-eddy simulations of the flow over a series of three-dimensional (3D) dunes at laboratory scale. The bedform three-dimensionality was imposed by shifting a standard two-dimensional (2D) dune shape in the streamwise direction according to a sine wave. The turbulence statistics were discussed in Part 1 of this article [M. Omidyeganeh, U. Piomelli, J. Fluid Mech. 721, pp. 454-483, (2013)]. Coherent flow structures and their statistics are discussed concentrating on two cases with the same crestline amplitudes and wavelengths but di↵erent crestline alignments: in-phase and staggered. The present paper shows that the induced large-scale mean streamwise vortices are the primary factor that alters the features of the instantaneous flow structures. Wall turbulence is insensitive to the crestline alignment; alternating high-and low-speed streaks appear in the internal boundary layer developing on the stoss side, whereas over the node plane (the plane normal to the spanwise direction at the node of the crestline), they are inclined towards the lobe plane (the plane normal to the spanwise direction at the most downstream point of the crestline) due to the mean spanwise pressure gradient. Spanwise vortices (rollers) generated by Kelvin-Helmholtz instability in the separated shear-layer appear regularly over the lobe with much larger length scale than those over the saddle (the plane normal to the spanwise direction at the most upstream point of the crestline). Rollers over the lobe may extend to the saddle plane and a↵ect the reattachment features; their shedding is more frequent than in 2D geometries. Vortices shed from the separated shear-layer in the lobe plane undergo a three-dimensional instability while advected downstream, and rise toward the free surface. They develop into a horseshoe shape (similar to the 2D case) and a↵ect the whole channel depth, whereas those generated near the saddle are advected downstream and toward the bed. When the tip of such a horseshoe reaches the free surface, the ejection of flow at the surface causes "boils" (upwelling events on the surface). Strong boil events are observed on the surface of the lobe planes of 3D dunes more frequently than in the saddle planes. They also appear more frequently than in the corresponding 2D geometry. The crestline alignment of the dune alters the dynamics of the flow structures, in that they appear in the lobe plane and are advected towards the saddle plane of the next dune, where they are dissipated. Boil events occur at a higher frequency in the staggered alignment, but with less intensity than in the in-phase alignment. Permanent repository link

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Spectral analysis of turbulent flow and suspended sediment transport over fixed dunes

Cited by 151 publications

References 35 publications

Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach

Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach

Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 1. Turbulence statistics

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

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