Experimental study of turbulent oscillatory boundary layers in an oscillating water tunnel

Yuan, Jinliang; Madsen, Ole Secher

doi:10.1016/j.coastaleng.2014.03.007

Cited by 56 publications

(52 citation statements)

References 26 publications

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“…The maximum flow velocity and acceleration in the test section are about 2 m/s and 2 m/s 2 , which are sufficiently high to create sheet-flow conditions. Yuan and Madsen [2014] showed that the system can very precisely generate the specified piston motion and the cross-sectional average velocity predicted from the piston velocity is in excellent agreement with the actual free-stream velocity measured in the test section. The overall inaccuracy in generating the intended free-stream oscillatory flow in the WCS is assessed to be less than 1 cm/s, which is immaterial compared to the amplitudes of oscillatory flows in this study (O(100 cm/s)), so it is not necessary to verify the free-stream flow with velocity measurements.…”

Section: Wave-current-sediment Facilitysupporting

confidence: 55%

“…The laser line on a digital image is a red band (20-40 pixels wide) with the digital redness value (from 0 to 255 with 0 being black and 255 being red) across the red band decaying toward the edges in a manner resembling a normal distribution. Following Yuan and Madsen [2014], we perform normal-distribution fitting to the cross-band redness variation for each X-location and take the fitted peak as the position of the laser line Z(X) (in pixels). Based on some targets with known dimensions drawn on the front sidewall of the WCS, we first obtain the horizontal and vertical calibration parameters for the vertical plane of the WCS' front sidewall.…”

Section: Wave-current-sediment Facilitymentioning

confidence: 99%

“…As waves propagate into shallow waters, their nonlinear features become significant and the associated oscillatory boundary layer flows exhibit asymmetry and skewness between the two half-periods. There are many studies focused on the effects of flow asymmetry and skewness on boundary layer flows [e.g., Gonzalez-Rodriguez and Madsen, 2011;Yuan and Madsen, 2014] and sediment transport [e.g., Ribberink and Al-Salem, 1995;Gonzalez-Rodriguez and Madsen, 2007;van der A et al, 2010]. Generally speaking, wave nonlinearity makes the flow during the onshore half-period stronger than that during the offshore half-period, so a net onshore CSST is produced.…”

Section: Introductionmentioning

confidence: 99%

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Bottom‐slope‐induced net sheet‐flow sediment transport rate under sinusoidal oscillatory flows

Yuan

Madsen

2017

JGR Oceans

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It is generally believed that the slope of beaches can lead to a net downslope (usually offshore) sediment transport rate under shoaling waves, but very few high‐quality measurements have been reported for a quantitative understanding of this phenomenon. In this study, full‐scale (1:1) experiments of bottom‐slope‐induced net sheet‐flow sediment transport rate under sinusoidal oscillatory flows are conducted using a tilting oscillatory water tunnel. The tests cover a variety of flow‐sediment conditions on bottom slopes up to 2.6°. A laser‐based bottom profiler system is developed for measuring net transport rate based on the principle of mass conservation. Experimental results suggest that for a given flow‐sediment condition the net transport rate is in the downslope direction and increases linearly with bottom slope. A conceptual model is presented based on the idea that gravity helps bottom shear stress drive bedload transport and consequently enhances (reduces) bedload transport and suspension when the flow is in the downslope (up‐slope) direction. The model predicts both the measured net sediment transport rates and the experimental linear relationship between net transport rates and bottom slope with an accuracy generally better than a factor of 2. Some measured net transport rates in this study are comparable to those due to flow skewness obtained in similar sheet‐flow studies, despite that our maximum slope could be milder than the actual bottom slope in surf zones, where sheet‐flow conditions usually occur. This shows that the slope effect may be as important as wave nonlinearity in producing net cross‐shore sheet‐flow sediment transport.

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Section: Wave-current-sediment Facilitysupporting

confidence: 55%

Section: Wave-current-sediment Facilitymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Bottom‐slope‐induced net sheet‐flow sediment transport rate under sinusoidal oscillatory flows

Yuan

Madsen

2017

JGR Oceans

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“…The whole facility is filled with water, so a vertical oscillation of the piston is translated into a horizontal oscillatory flow in the test channel, which is up to 2 m/s and 2 m/s

^{2}

in velocity and acceleration, respectively. A few previous studies have demonstrated that the facility can accurately produce a target oscillatory flow with a variety of wave shape (see Yuan & Madsen, ). Thus, the facility can provide full‐scale approximation of wave‐driven near‐bed flows.…”

Section: Methodsmentioning

confidence: 99%

An Experimental Investigation of Acceleration‐Skewed Oscillatory Flow Over Vortex Ripples

Yuan

Wang

2019

JGR Oceans

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A full-scale experimental study of acceleration-skewed oscillatory flow over vortex ripples, which is an approximation of wave-driven flow over a rippled seabed, was conducted in an oscillatory water tunnel. Three acceleration-skewed-flow tests and a reference sinusoidal-flow tests were involved in this study. In each test, two-dimensional vortex ripples were generated from a coarse-sand movable bed, and the flow field was measured with a particle image velocimetry. The dominant feature of phase-averaged flow is the generation of onshore (offshore) coherent vortices during the onshore (offshore) half-cycle. The acceleration skewness expedites the offshore-onshore flow reversal and therefore speeds up the ejection of offshore vortex, while the opposite occurs for the onshore vortex. The ripple-averaged flow is similar to the acceleration-skewed turbulent oscillatory flow over a flat bed in terms of leading Fourier harmonics, boundary layer thickness, and cycle-averaged flow, suggesting a possible analogy between the two. From the aspect of flow turbulence, acceleration skewness elongates the offshore accelerating quarter, so the turbulence embedded in the offshore vortex can be better developed. This, together with a quick ejection process, allows the offshore vortex to carry a significant amount of residual turbulence to the onshore-side ripple flank, which may even enhance the production of turbulence within the onshore half-cycle. Some results on pressure field and form drag are also presented in this paper. Plain Language SummaryIn shallow coastal regions, shoaling waves produce oscillatory bottom flows, which create sand ripples on a sandy seabed. The acceleration skewness of the oscillatory flow is a result of the nonlinear feature of the overlying water waves, and it can differentiate the two half-cycles of the oscillatory flow and therefore leading to a net (cycle-averaged) sediment transport. In this study, we use an oscillatory water tunnel and particle image velocimetry to investigate the oscillatory boundary layer flow over vortex ripples with a focus on the effect of acceleration skewness. Our measurements provide detailed descriptions of the formation-ejection process of coherent vortices, which is produced by the flow separation behind the ripple crest. Some useful observations on flow turbulence are also provided. The results showed that acceleration skewness can substantially differentiate the two coherent vortices developed within the two half-cycles. The offshore-side vortex can carry more turbulence over the ripple crest than the onshore-side vortex, which possibly leads to some net onshore sediment transport rate. We also showed that the ripple-averaged flow is fairly close to that over a flat bed, so flat-bed boundary layer model can be possibly used for rippled bed with some parameterizations.

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“…Common bottom roughness formulations for turbulent conditions assume a logarithmic velocity profile near the bed [Grant and Madsen, 1982;Fredsoe, 1984;Madsen, 1994], an approach that has been verified in laboratory studies [van der A et al, 2011;Yuan and Madsen, 2014]. The calculations of bottom shear stress using the bottom roughness and near-bed velocities include wave-current formulations [Madsen, 1994;Soulsby and Clarke, 2005], as well as terms to account for a fixed rippled bed [Grant and Madsen, 1979] or a movable bed [Harris and Wiberg, 2001].…”

Section: Methods A) Observationsmentioning

confidence: 99%

Field observations and numerical model simulations of a migrating inlet system

Hopkins¹

2017

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Waves, currents, and bathymetric change observed along 11 km of the southern shoreline of Martha's Vineyard include storm events, strong tidal flows (> 2 m/s), and an inlet migrating 2.5 km in ~7 years. A field-verified Delft3D numerical model developed for this system is used to examine the hydrodynamics in the nearshore and their effect on the migrating inlet. An initial numerical experiment showed that the observed 70⁰ tidal modulation of wave direction in the nearshore was owing to interactions with tidal currents, and not to depth-induced refraction as waves propagated over complex shallow bathymetry. A second set of simulations focused on the separation of tidal currents from the southeast corner of Martha's Vineyard, showing the positive correlation between flow separation and sediment transport around a curved shoreline. Observations of waves, currents, and bathymetric change during hurricanes were reproduced in a third numerical experiment examining the competition between storm waves, which enhance inlet migration, and strong tidal currents, which scour the inlet and reduce migration rates. The combined field observations and simulations examined here demonstrate the importance of wave and tidal current forcings on morphological evolution at timescales of days to months.

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Experimental study of turbulent oscillatory boundary layers in an oscillating water tunnel

Cited by 56 publications

References 26 publications

Bottom‐slope‐induced net sheet‐flow sediment transport rate under sinusoidal oscillatory flows

Bottom‐slope‐induced net sheet‐flow sediment transport rate under sinusoidal oscillatory flows

An Experimental Investigation of Acceleration‐Skewed Oscillatory Flow Over Vortex Ripples

Field observations and numerical model simulations of a migrating inlet system

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