Oscillatory boundary layer close to a rough wall

Fornarelli, Francesco; Vittori, Giovanna

doi:10.1016/j.euromechflu.2008.06.002

Cited by 27 publications

(44 citation statements)

References 20 publications

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“…1 In most practical situations wave boundary layers occur over rough beds (e.g., beds covered with sand or gravel). Turbulence properties in wave boundary layers over rough beds have been the focus in a few studies, e.g., Keiller and Sleath, 16 Sleath, 17 Jensen, Sumer, and Fredsøe, 15 and Fornarelli and Vittori 18 (see also, e.g., Grass 19 for a description of the similarity of structural features of turbulent flow over smooth and rough boundaries in steady currents). Yet the laminar-turbulent transition is not fully understood and to the authors' knowledge, no study is yet available investigating turbulent spots over rough boundaries in oscillatory flows.…”

Section: Introductionmentioning

confidence: 99%

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

2012

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This study is a continuation of the investigation of turbulent spots in wave boundary layers over a smooth wall reported by Carstensen et al. [J. Fluid Mech. 646, 169–206 (2010)10.1017/S0022112009992825]. The present paper summarises the results of an experimental investigation of turbulent spots in wave boundary layers over a rough wall. Two kinds of experiments were made: flow visualization tests and velocity profile measurements. The experiments show that turbulent spots as observed in the smooth-wall wave boundary layers are present irrespective of wall roughness conditions. As in the smooth-wall case the turbulent spots over the rough wall appear locally in time and space and grow in size as time progresses.

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Section: Introductionmentioning

confidence: 99%

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

2012

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“…There are extensive studies of periodic flows around a non-zero mean, and oscillatory flows around zero mean flows. Readers interested in those topics are referred to recent studies of Scotti & Piomelli (2001), Tardu & Da Costa (2005), He & Jackson (2009) and Manna, Vacca & Verzicco (2012) for pulsating flows and Fornarelli & Vittori (2009) and Van der A et al (2011) for oscillatory flows.…”

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confidence: 99%

Transition of transient channel flow after a change in Reynolds number

Seddighi

2015

J. Fluid Mech.

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It has previously been shown that the transient flow in a channel following a step increase of Reynolds number from 2800 to 7400 (based on channel half-height and bulk velocity) is effectively a laminar-turbulent bypass transition even though the initial flow is turbulent (He & Seddighi, J. Fluid Mech., vol. 715, 2013, pp. 60-102). In this paper, it is shown that the transient flow structures exhibit strong contrasting characteristics in large and small flow perturbation scenarios. When the increase of Reynolds number is large, the flow is characterized by strong elongated streaks during the initial period, followed by the occurrence and spreading of isolated turbulent spots, as shown before. By contrast, the flow appears to evolve progressively and the turbulence regeneration process remains largely unchanged during the flow transient when the Reynolds number ratio is low, and streaks do not appear to play a significant role. Despite the major apparent differences in flow structures, the transient flow under all conditions considered is unambiguously characterized by laminar-turbulent transition, which exhibits itself clearly in various flow statistics. During the pre-transition period, the time-developing boundary layers in all the cases show a strong similarity to each other and follow closely the Stokes solution for a transient laminar boundary layer. The streamwise fluctuating velocity also shows good similarity in the various cases, irrespective of the appearance of elongated streaks or not, and the maximum energy growth exhibits a linear rate similar to that in a spatially developing boundary layer. The onset of transition is clearly definable in all cases using the minimum friction factor, and the critical time thus defined is strongly correlated with the free-stream turbulence in a power-law form.

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“…The first experimental work of Keiller and Sleath (1976) focused on regular rough surfaces using hemispheres. This was later reproduced numerically by Fornarelli and Vittori (2009) who also compared their results to another experimental work carried out using square roughness elements (Krstic & Fernando, 2002). Sleath (1987) and Jensen et al (1989) revealed the existence of jets of fluid emanating from irregular sediment beds, indicating the possibility of coherent structures created close to the bed and ejected outward during the oscillatory motion.…”

Section: Rough Wallmentioning

confidence: 74%

“…8a. In other studies, including Fornarelli and Vittori (2009), with regular rough walls, vortices were formed behind all hemispheres during the wall flow reversal. Herein, there exists a preferential formation of vortices from specific grain features owing to the irregularity of the bed roughness considered.…”

Section: Vorticesmentioning

confidence: 99%

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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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Oscillatory boundary layer close to a rough wall

Cited by 27 publications

References 20 publications

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

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

Transition of transient channel flow after a change in Reynolds number

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

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