Turbulent oscillatory flow over rough beds

Sleath, J. F. A.

doi:10.1017/s0022112087002374

Cited by 327 publications

(387 citation statements)

References 18 publications

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“…The turbulence generated during hydraulically rough cases is similarly summarised on Figure 12b. Similar to experimental findings for oscillatory waves (Jensen et al, 1989;Sleath, 1987), under hydraulically rough conditions increasing values of a/k s prompt a reduction in the normalised turbulent kinetic energy density for each of the wave signals considered. Consistent with expectations based on the hydraulically smooth results just discussed, more turbulence is generated during the trailing cycle of the Nwave than in the leading cycle.…”

Section: Rough-turbulent Wave Boundary Layersupporting

confidence: 79%

Numerical simulation of tsunami-scale wave boundary layers

Williams

Fuhrman

2016

Coastal Engineering

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This paper presents a numerical study of the boundary layer flow and properties induced by tsunami-scale waves. For this purpose, an existing onedimensional vertical (1DV) boundary layer model, based on the horizontal component of the incompressible Reynolds-averaged Navier-Stokes (RANS) equations, is newly extended to incorporate a transitional variant of the standard two-equation k-ω turbulence closure. The developed numerical model is successfully validated against recent experimental measurements involving transient solitary wave boundary layers as well as for oscillatory flows, collectively demonstrating the ability to reproduce accurate velocity profiles, turbulence, and bed shear stresses on both smooth and rough beds. The validated model is then employed for the study of transient wave boundary layers at full tsunami scales, covering a wide and realistic geophysical range in terms of the flow duration, bottom roughness, and associated Reynolds numbers. For this purpose, three different "synthetic" (idealised) tsunami wave descriptions are considered i.e. invoking: (1) single wave (solitary-like, but with independent period and wave height), (2) sinusoidal, and (3) N-wave descriptions. The flow, boundary layer thickness, turbulence, and bed shear stresses induced are systematically monitored and parameterised, under both hydraulically smooth and rough bed conditions. The results generally support a notion that the boundary layers induced by * Corresponding author Email addresses: i.a.williams@utwente.nl (Isaac A. Williams ), drf@mek.dtu.dk (David R. Fuhrman ) Preprint submitted to Coastal EngineeringDecember 28, 2015 tsunami-scale waves are both current-like, due to their long durations, but also wave-like in the sense that the boundary layer will not necessarily span the entirety of the water column. The results likewise suggest that there is a continuum connecting wind-wave and tsunami-wave scales, as existing expressions commonly used for characterising boundary layer properties beneath wind waves maintain reasonable accuracy when extrapolated to full tsunami scales. Boundary layers driven by actual field-measured tsunami signals are likewise simulated, stemming from both the 2004 Indian Ocean as well as the 2011 Tohoku events. These results are reconciled with the various synthetic tsunami signals considered, generally confirming their usefulness as idealised tsunami waves.

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Section: Rough-turbulent Wave Boundary Layersupporting

confidence: 79%

Numerical simulation of tsunami-scale wave boundary layers

Williams

Fuhrman

2016

Coastal Engineering

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“…However, such corrections should be applied with caution here because part of the real vertical oxygen flux may be facilitated by wave motions and thus occur at the wave frequency. Wave motions over rough benthic surfaces can give rise to eddies or water parcel ejections at wave frequencies, which expand up into the bottom water, well above the wave boundary layer (Kemp and Simons, 1982;Sleath, 1987;Reidenbach et al, 2007).…”

Section: Discussionmentioning

confidence: 99%

Technical note: Time lag correction of aquatic eddy covariance data measured in the presence of waves

et al. 2015

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Abstract. Extracting benthic oxygen fluxes from eddy covariance time series measured in the presence of surface gravity waves requires careful consideration of the temporal alignment of the vertical velocity and the oxygen concentration. Using a model based on linear wave theory and measured eddy covariance data, we show that a substantial error in flux can arise if these two variables are not aligned correctly in time. We refer to this error in flux as the time lag bias. In one example, produced with the wave model, we found that an offset of 0.25 s between the oxygen and the velocity data produced a 2-fold overestimation of the flux. In another example, relying on nighttime data measured over a seagrass meadow, a similar offset reversed the flux from an uptake of −50 mmol m −2 d −1 to a release of 40 mmol m −2 d −1 . The bias is most acute for data measured at shallow-water sites with short-period waves and low current velocities. At moderate or higher current velocities (> 5-10 cm s −1 ), the bias is usually insignificant. The widely used traditional time shift correction for data measured in unidirectional flows, where the maximum numerical flux is sought, should not be applied in the presence of waves because it tends to maximize the time lag bias or give unrealistic flux estimates. Based on wave model predictions and measured data, we propose a new time lag correction that minimizes the time lag bias. The correction requires that the time series of both vertical velocity and oxygen concentration contain a clear periodic wave signal. Because wave motions are often evident in eddy covariance data measured at shallow-water sites, we encourage more work on identifying new time lag corrections.

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“…13. Histograms of (a) orbital velocity and (bo)acceleration maxima, the associated with on.thore-offshore orbital motion for the field deployment ensemble spectrum which appears in Figure 10 bad Jonsson, 1963;Sleath 1987]. Asymmetries occurring in the laboratory [Flick et al, 19811 or the a field [Hanes & Huntley, 1986] Transpired boundary layers have practical significance in many applications and have been the subject of studies for well over forty years [eg Libby et al, 1952;Antonia et al, 1990].…”

Section: Fig 10 Spectrum Itmentioning

confidence: 99%

Ventilated oscillatory boundary layers

Conley

Inman

1994

J. Fluid Mech.

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Boundary layers arising from flows which oscillate parallel to a permeable bed, and are subject to oscillating percolation of the same frequency as the bed parallel flow, referred to here as ‘ventilated oscillatory boundary layers’, are the subject of this laboratory study. These boundary layers are intended to approximate naturally occurring wave boundary layers over permeable beds. Measurements of boundary-layer velocities, bed stress and turbulent flow properties are presented. It is observed that suction (flow into the bed) enhances the near-bed velocities and bed stress while injection (flow out of the bed) leads to a reduction in these quantities. As the ventilated oscillatory boundary layer experiences both these phenomenon in one full cycle, the result is a net stress and a net boundary-layer velocity in an otherwise symmetric flow. While production of turbulence attributable to injection is enhanced, the finite time required for this to occur leads to a greater vertically averaged turbulence in the suction half-cycle. Turbulence generated in the suction half-cycle is maintained in a compact layer much closer to the bed. These effects appear to hold for$\widetilde{Re}$ranging from 105to 106and for oscillations other than sinusoidal.

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Turbulent oscillatory flow over rough beds

Cited by 327 publications

References 18 publications

Numerical simulation of tsunami-scale wave boundary layers

Numerical simulation of tsunami-scale wave boundary layers

Technical note: Time lag correction of aquatic eddy covariance data measured in the presence of waves

Ventilated oscillatory boundary layers

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