Stability of Transverse Shear Flows in Shallow Open Channels

Chu, Vincent H.; Wu, Jiafeng; Khayat, Roger E.

doi:10.1061/(asce)0733-9429(1991)117:10(1370)

Cited by 117 publications

(110 citation statements)

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“…This is also apparent in the accretive runs, as well as what seems to be higher order harmonics. Chu et al (1991) have reported on unstable transverse shear flows leading to large scale turbulent motions across wide and shallow open channels (horizontal length scales significantly larger than water depth), induced by the growth of small scale disturbances induced by bed friction (critical value of 0.12-0.145) or depth variation. However, neither of these appear to be applicable in this case, given the relatively uniform cross section of the flume, and the lower wave-induced bed friction factors reported in Table 1, and some alternative mechanism must be at work.…”

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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“…The substitution of the disturbance into the shallow-water momentum equation (3.7) yields a modified Rayleigh equation (MRE) (Chu et al 1991;Chen & Jirka 1997;Socolofsky & Jirka 2004), 9) where the eigenfunction, φ ≡ φ v , satisfies the boundary conditions, φ → 0 as y → ±∞, (9.10) c = ω/k is the wave speed, and H (y) is a Heaviside step function, given by…”

Section: Linear Stability Analysismentioning

confidence: 99%

“…As a check of the numerical method, we first present results for uniform bottom friction, (γ = 0) (figure 20), but for which a velocity difference (R = 1) is imposed, as in Chu et al (1991). The frictionless, S = 0, case is identical to the results of Monkewitz & Huerre (1982) for a free shear layer.…”

Section: Linear Stability Analysismentioning

confidence: 99%

Shear instability and coherent structures in shallow flow adjacent to a porous layer

2007

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Results are presented from an experimental study of shallow flow in a channel partially obstructed by an array of circular cylinders. The cylinder array is a model for emergent vegetation in an open channel, but also represents a simple sparse porous medium. A shear layer with regular vortex structures forms at the edge of the array, evolving downstream to an equilibrium width and vortex size. The vortices induce nearly periodic oscillations with a frequency that matches the most unstable linear mode for a parallel shear flow. The shear layer is asymmetric about the array interface and has a two-layer structure. An inner region of maximum shear near the interface contains a velocity inflection point and establishes the penetration of momentum into the array. An outer region, resembling a boundary layer, forms in the main channel, and establishes the scale of the vortices. The vortex structure, educed by conditional sampling, shows strong crossflows with sweeps from the main channel and ejections from the array, which create significant momentum and mass fluxes across the interface. The sweeps maintain the coherent structures by enhancing shear and energy production at the interface. A linear stability analysis is consistent with the experimental results and demonstrates that the instability is excited by the differential drag between the channel and the array.

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“…Linear stability of such flows is usually analyzed under the assumption that the base flow is symmetric with respect to the transverse coordinate [1][2][3][4]. Experimental data [5] show that symmetry of the base flow can be distorted by the presence of the non-uniform friction force acting in the transverse direction.…”

Section: Introductionmentioning

confidence: 99%

Stability of shallow mixing layers with asymmetric base flow profile

Kremenetsky¹,

Kolyshkin²

2017

Engineering for Rural Development

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Abstract. Linear stability of shallow flows is usually analyzed in the literature under the assumption that the base flow profile is symmetric with respect to the transverse coordinate. Another widely used assumption is the rigid-lid assumption where perturbations at the upper surface are not considered. Experimental data show that the symmetry of the base flow can be distorted by a non-uniform friction in the transverse direction of the flow. Such a situation occurs in applications in case of the presence of aquatic vegetation (for example, in case of floods). In this case there is a sharp change of the resistance force at the interface. Experiments show that nonuniform resistance force plays an important role in development of the mixing layer. In the present paper linear stability analysis of shallow mixing layers with non-uniform friction is investigated. Both previously mentioned assumptions are removed and the problem is solved for asymmetric base flow profile for arbitrary Froude numbers. The friction coefficient is assumed to be a function of the transverse coordinate. Experimentally measured asymmetric base flow profile is used in the paper. The linear stability problem is solved numerically by a collocation method based on Chebyshev polynomials using different values of the parameters of the problem.Keywords: shallow flows, linear stability, asymmetric base flow. IntroductionShallow flows often occur in applications (examples include flows at river junctions or in compound channels). Linear stability of such flows is usually analyzed under the assumption that the base flow is symmetric with respect to the transverse coordinate [1][2][3][4]. Experimental data [5] show that symmetry of the base flow can be distorted by the presence of the non-uniform friction force acting in the transverse direction. Such a situation often occurs during floods where moving fluid is in contact with aquatic vegetation.Linear stability of shallow mixing layers for a symmetric base flow (a hyperbolic tangent velocity profile) is analyzed in [6; 7], where it is shown that a non-uniform friction stabilizes the flow. In the present paper linear stability of shallow mixing layers with non-uniform friction is investigated under the assumption that the base flow is asymmetric with respect to the transverse coordinate. The base flow profile is obtained from experimental data [5] where semi-analytical formulas for the velocity distribution in the transverse direction are also obtained. Linear stability is analyzed for several experimental cases quoted in [5]. Spline interpolation is used to smooth experimental velocity profiles. As it is shown in [5], the velocity distribution has a two-layer structure where sharp change of the velocity occurs in a shear layer at the boundary between aquatic vegetation and the main channel. The presence of an inflection point in the velocity profile indicates possible hydrodynamic instabilities. Linear stability analysis is used in the paper to calculate the critical values of the friction coefficie...

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Stability of Transverse Shear Flows in Shallow Open Channels

Cited by 117 publications

References 11 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

Shear instability and coherent structures in shallow flow adjacent to a porous layer

Stability of shallow mixing layers with asymmetric base flow profile

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