Vortex generation in oscillatory canopy flow

Ghisalberti, Marco; Schlosser, Tamara

doi:10.1002/jgrc.20073

Cited by 26 publications

(28 citation statements)

References 49 publications

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“…This then suggests that a simple N-box model would be inaccurate and 1-or 2-equation turbulence models that account for canopy-scale and wake-scale turbulence are required, as in Li & Yan (2007). Contrary to this, recent laboratory studies have suggested that in oscillatory flow, the Reynolds stress is only significant in extreme cases (Lowe et al 2008;Ghisalberti & Schlosser 2013). Proper validation of this idea requires phase-dependent Reynolds stress measurements, which were not presented by either study.…”

Section: Aquatic Canopy Flow Modellingmentioning

confidence: 78%

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A simple and practical model for combined wave–current canopy flows

et al. 2015

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Laboratory experiments were used to evaluate and improve modelling of combined wave-current flow through submerged aquatic canopies. Horizontal in-canopy particle image velocimetry (PIV) and wavemaker-measurement synchronization allowed direct volume averaging and ensemble averaging by wave phase, which were used to fully resolve the volume-averaged unsteady momentum budget. Parameterizations for the drag, Reynolds stress, vertical advection, wake production and shear production were tested against the laboratory measurements. The drag was found to have small errors due to unsteadiness and the finite aspect ratio of the canopy elements. The Smagorinsky model for the Reynolds stress showed much better agreement with the measurements than the quadratic friction parameterization used in the literature. A proposed parameterization for the vertical advection based on linear wave theory was also found to be effective and is much more computationally efficient than solving the pressure Poisson equation. A simple 1D 0-equation Reynolds-averaged Navier-Stokes (RANS) model was developed to use these parameterizations. The basic framework of the model is an extrapolation from previous 2-and 3-box models to N boxes. While the resolution of the model is flexible, the filter length for the Smagorinsky parameterization has to be chosen appropriately. With the proper filter length, the N-box model demonstrated good agreement with the measurements at both low and high resolution. Scaling analysis was used to establish a region of parameter space where the N-box model is expected to be effective. The following conditions define this region: the wave-induced velocity is of similar or greater magnitude than the background current, the drag to shear length ratio is small enough to produce canopy behaviour, the wave orbital excursion is not much larger than the drag length, the Froude number is small and the canopy is under shallow submergence, yet far from emergent. Under these assumptions, the dominant balance is between pressure and unsteadiness, the drag is secondary, and the other terms are small. The simple Reynolds stress parameterization in the N-box model is appropriate under these conditions because the Reynolds stress is unlikely to be the dominant source of error. This finding is important because the Reynolds stress is typically one of the dominant drivers of computational cost and model complexity. Based on these findings, the N-box model is expected to be a practical tool for a wide range of combined wave-current canopy flows because of its simplicity and computational efficiency.

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Section: Aquatic Canopy Flow Modellingmentioning

confidence: 78%

“…excellent example is provided by the artificial canopy flows inGhisalberti & Schlosser (2013), which had h = 3 cm and A w up to 1.2 m. However, β is small over a very large region of parameter space that is relevant to natural aquatic canopies in pure oscillatory flow.…”

mentioning

confidence: 99%

A simple and practical model for combined wave–current canopy flows

et al. 2015

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“…As discussed earlier, shear layer vortices are generated only under certain conditions in oscillatory flow: namely, when the wave period is long enough to allow the shear‐driven instability to be generated before flow reversal (i.e., when KC

≳

5), and when the vortex instabilities are strong enough to overcome the stabilising effects of viscosity, (i.e., when Re

≳

1000) (Ghisalberti and Schlosser ). The runs of this study cover KC below and above this threshold while the values of Re were always above 1000 (Table ).…”

Section: Resultsmentioning

confidence: 99%

“…The second is the Reynolds number ( Re ):

R e = \frac{U_{\infty} A_{\infty}}{ν},

defined here using the particle excursion amplitude far above the canopy ( A ∞ ) as the characteristic length scale (ν is the kinematic fluid viscosity). At sufficiently high Re and KC (namely, when KC

≳

5 and Re

≳

1000), KH‐vortices will be generated (Ghisalberti and Schlosser ). Marine canopies typically have drag length scales of O (1–100 cm) (Luhar et al ) and are commonly exposed to waves with T ∼ O (10 s) and U ∞ ∼ O (1–100 cm/s).…”

Section: Methodsmentioning

confidence: 99%

Vertical mixing in coastal canopies

Abdolahpour

Ghisalberti

Lavery

et al. 2016

Limnology & Oceanography

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The spatial extent over which meadows of submerged aquatic vegetation, such as seagrass, have an ecological and environmental influence is tightly limited by the exchange of water across canopy boundaries. In coastal environments, the process of vertical mixing can govern this material exchange, particularly when mean currents are weak. Despite a recently improved understanding of vertical mixing in steady canopy flows, a framework that can predict mixing in wave-dominated canopy flows is still lacking. Accordingly, an extensive laboratory investigation was conducted to characterize the rate of vertical mixing in wave-dominated flows through measurement of the vertical turbulent diffusivity (Dt,z) of an injected dye sheet. A simple model of coastal canopies, an array of wooden dowels of variable packing density, was subjected to waves with a wide and realistic range of height and period. Vertical mixing across the top of a submerged canopy is shown to be driven by both the shear layer that forms at the top of the canopy and wake turbulence generated by canopy stems. By allowing for an additive contribution from these two processes, we present a predictive formulation for the rate of vertical mixing in coastal canopies across a range of wave and canopy conditions. The rate of vertical mixing, and the dominant mixing mechanism, is highly dependent upon a Keulegan–Carpenter number (KC) that represents the ratio of the particle excursion length to the length scale that defines the canopy drag. This study enables a significantly enhanced predictive capability for the residence time of ecologically and environmentally significant species within coastal canopies

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“…Waves can also generate coherent vortices at the canopy‐water interface (Fig. ) (Ghisalberti and Schlossser ) and considerably modify the near‐bed turbulent flow structure (Pujol et al ). Laboratory flume studies have recently investigated combined current‐wave flows through aquatic canopies (Li and Yan ; Paul et al ; Hu et al ; Zeller et al ).…”

Section: Overview Of the Seagrass‐sediment‐light Feedbackmentioning

confidence: 99%

Feedback between sediment and light for seagrass: Where is it important?

et al. 2016

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A feedback between seagrass presence, suspended sediment and benthic light can induce bistability between two ecosystem states: one where the presence of seagrass reduces suspended sediment concentrations to increase benthic light availability thereby favoring growth, and another where seagrass absence increases turbidity thereby reducing growth. This literature review identifies (1) how the environmental and seagrass meadow characteristics influence the strength and direction (stabilizing or destabilizing) of the seagrass-sediment-light feedback, and (2) how this feedback has been incorporated in ecosystem models proposed to support environmental decision making. Large, dense seagrass meadows in shallow subtidal, noneutrophic systems, growing in sediments of mixed grain size and subject to higher velocity flows, have the greatest potential to generate bistability via the seagrass-sediment-light feedback. Conversely, seagrass meadows of low density, area and height can enhance turbulent flows that interact with the seabed, causing water clarity to decline. Using a published field experiment as a case study, we show that the seagrass-sedimentlight feedback can induce bistability only if the suspended sediment has sufficient light attenuation properties. The seagrass-sediment-light feedback has been considered in very few ecosystem models. These models have the potential to identify areas where bistability occurs, which is information that can assist in spatial prioritization of conservation and restoration efforts. In areas where seagrass is present and bistability is predicted, recovery may be difficult once this seagrass is lost. Conversely, bare areas where seagrass presence is predicted (without bistability) may be better targets for seagrass restoration than bare areas where bistability is predicted.

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Vortex generation in oscillatory canopy flow

Cited by 26 publications

References 49 publications

A simple and practical model for combined wave–current canopy flows

A simple and practical model for combined wave–current canopy flows

Vertical mixing in coastal canopies

Feedback between sediment and light for seagrass: Where is it important?

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