A note on turbulent spots over a rough bed in wave boundary layers

Carstensen, Stefan; Sumer, B. Mutlu; Fredsøe, Jørgen

doi:10.1063/1.4767536

Cited by 16 publications

(41 citation statements)

References 23 publications

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“…Among the experimental contributions to the study of oscillatory boundary layers over a naturally rough wall, it is worthwhile to mention the contributions of Sleath (1987), Jensen, Sumer & Fredsoe (1989) and recently that of Carstensen, Sumer & Fredsoe (2012). All of these contributions consider a natural rough wall.…”

Section: Introductionmentioning

confidence: 98%

Transition to turbulence in an oscillatory flow over a rough wall

Mazzuoli

Vittori

2016

J. Fluid Mech.

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A study of the oscillatory incompressible flow close to a wall covered with fixed rigid spheres is carried out by numerical means to provide information on unsteady flows over a rough wall. The simulations are carried out for two bottom configurations, characterized by different values of the diameter of the spheres and different values of the Reynolds number for a total of 10 cases. Three different flow regimes are identified as functions of both the Reynolds number and the diameter of the spheres. The force exerted by the flow on the spheres is discussed also in relation to the different flow regimes. 2016 Cambridge University Press

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Section: Introductionmentioning

confidence: 98%

Transition to turbulence in an oscillatory flow over a rough wall

Mazzuoli

Vittori

2016

J. Fluid Mech.

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“…Cao (1997), Wu and Yang (2004), Wu and Jiang (2007)). Considerable recent work therefore focusses on the structural form of these features of flow, their role in fluid and sediment entrainment, bed shear stress generation, energy transfer and velocity asymmetry; and the influence of the space-time structure of the flow, with emphasis on oscillatory flows, and different bed roughnesses (Adrian and Marusic, 2012;Carstensen et al, 2010;Carstensen et al (2012); Grigoriadis et al, 2013;Hardy et al, 2010;Hare et al, 2014;Okamoto et al, 2007). Advanced visualisation and data analysis techniques reveal complex interactions between passing coherent energetic structures and sediments in suspension, such as particle response to turbulent fluctuations in the frequency domain (e.g.…”

Section: Introductionmentioning

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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“…Hladík, Jonáš, and Uruba (2011) presented PIV measurements that offered a detailed description of turbulent spots in both side and plan views. As with the present smooth-wall conditions, the present rough-wall conditions do not fall within the limits described by Carstensen et al (2012) for the formation of turbulent spots. Moreover, the lack of plan information in the current work does not allow us to confirm the existence of turbulent spots which have a well-defined character in plain view.…”

Section: Rough Wallmentioning

confidence: 48%

“…The recent experimental study of Carstensen, Sumer, and Fredsøe (2012) describes the existence of turbulent spots in the range 5 × 10 4 < R < 10 5 motivated by their original work with smooth-wall oscillatory boundary layers (Carstensen et al, 2010). Turbulent spots were first described for unidirectional flow by Emmons (1951) as structures that are formed after strong fluctuations randomly positioned in space.…”

Section: Rough Wallmentioning

confidence: 99%

Coherent structures in oscillatory flows within the laminar-to-turbulent transition regime for smooth and rough walls

Mujal-Colilles¹,

Christensen

Bateman

et al. 2016

Journal of Hydraulic Research

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We investigate coherent structures present in oscillatory boundary layers over smooth and rough beds for Reynolds numbers between 10 3 and 10 4 , in the transition to turbulence regime. A two-camera 2D-particle image velocimetry (PIV) system was used to visualize coherent structures in an oscillatory-flow tunnel. The obtained results show that smooth-bed flow is populated by vortex-tube structures that are formed due to the KelvinHelmholtz instability. Three types of coherent structures are observed in the rough-wall experiments: vortices randomly distributed in space; turbulent bursts -spatial structures without a clear shape compared to vortices but with a longer life than the former; and shear layers of vortices originating due to flow separation from some of the grains on the bed. The reported study contributes to the description of coherent structures in oscillatory flows that are captured with the PIV technique, particularly a new structure in flows over rough beds in the transition to turbulence regime.

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A note on turbulent spots over a rough bed in wave boundary layers

Cited by 16 publications

References 23 publications

Transition to turbulence in an oscillatory flow over a rough wall

Transition to turbulence in an oscillatory flow over a rough wall

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

Coherent structures in oscillatory flows within the laminar-to-turbulent transition regime for smooth and rough walls

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