Three-dimensional vortex dynamics in oscillatory flow separation

Canals, M.; Pawlak, Geno

doi:10.1017/s0022112011000012

Cited by 8 publications

(7 citation statements)

References 29 publications

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“…Önder and Yuan (2019) added that the appearance of spanwise flow features, "ribs", is not random and the spanwise wavelength ranges from 0.6λ to 0.9λ, comparable to the spacing of the brick-pattern proposed by Ourmières and Chaplin (2004) and Blondeaux et al (2004). Canals and Pawlak (2011) demonstrated that the vortex cores are unstable to the centrifugal instability, which results in the three-dimensional vortex structures. The spanwise wavelength of the ribs is in the range of two to five times the vortex core radius.…”

Section: Three-dimensionality In Flow Structuresmentioning

confidence: 72%

Large Eddy Simulation of three‐dimensional flow structures over wave‐generated ripples

Jin

Coco

Tinoco

et al. 2021

Earth Surf Processes Landf

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We performed a three-dimensional high-resolution Large Eddy Simulation using a sinusoidal ripple-like bedform geometry with spacing and amplitude consistent with equilibrium conditions under the prescribed oscillatory forcing. The simulation results are validated qualitatively and quantitatively with previous laboratory experiments and numerical simulations. Our numerical simulations allow to analyze the presence of "ribs", strong flow rotational features in the spanwise direction developing perpendicular to the ripple crest. We show that ribs tend to appear on the upslope and crest of ripples and display a dominant spacing, ranging from 0.5λ to λ, during wave phases associated to flow acceleration. When the vortex is transported over the downstream ripple during the flow deceleration, ribs appear for the second time over the upslope of the downstream ripple, with the dominant spacing about 1/8 ripple wavelength. The presence of flow three-dimensionality corresponds to a large variability of the shear stress over ripples in the spanwise direction, which could ultimately favor the development of three-dimensional geometry. The threedimensional vortex dynamics are evaluated using the concept of vortex strength. Peak vortex strength is observed at the maximum free stream velocity, but the largest variation of the vortex strength and vortex size in the spanwise direction occurs prior to the flow reversal. The variability of the vortex structures might provide a favorable setting for the development of ribs and might relate to the spacing of ribs.

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Section: Three-dimensionality In Flow Structuresmentioning

confidence: 72%

Large Eddy Simulation of three‐dimensional flow structures over wave‐generated ripples

Jin

Coco

Tinoco

et al. 2021

Earth Surf Processes Landf

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“…However, spatially varying high levels of turbulence generated as oscillatory flows interact with complex boundaries have been observed in numerous laboratory studies, e.g., Liu et al . [] for flows over ripples, and by Canals and Pawlak [] for a flapping plate.…”

Section: Resultsmentioning

confidence: 99%

On the wave and current interaction with a rippled seabed in the coastal ocean bottom boundary layer

Nayak

Kiani

et al. 2015

J. Geophys. Res. Oceans

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Interactions of currents and waves with a rippled seabed in the inner part of the coastal ocean bottom boundary layer are studied using particle image velocimetry, ADV, and bottom roughness measurements. Mean velocity profiles collapse with appropriate scaling in the log layer, but vary substantially in the roughness sublayer. When wave-induced motions are similar or greater than the mean current, the hydrodynamic roughness (z 0 ) determined from velocity profiles is substantially larger than directly measured values. The roughness signature in turbulent energy spectra persists with elevation when its scale falls in the dissipation range, but decays in the log layer for larger roughness elements. Reynolds shear stress profiles peak in the lower parts of the log layer, diminishing below it, and gradually decaying at higher elevations. In contrast, wave shear stresses are negligible within the log layer, but become significant within the roughness sublayer. This phenomenon is caused by an increase in the magnitude and phase lag of the vertical component of wave-induced motion. No single boundary layer length scale collapses the Reynolds stresses, but both the Prandtl mixing length and eddy viscosity profiles agree well with the classical model of linear increase with elevation, especially near the seabed. Within the log region, profiles of shear production and dissipation rates of turbulence converge. Below it, dissipation rapidly increases, peaking near the seabed. Conversely, the shear production decays near the seabed, in agreement with the eddy viscosity model, but in contrast to both laboratory and computational rough wall studies.

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“…The back-and-forth motion generates the pair of counterrotating vortex rings that can be seen downstream. Shedding of such vortex pairs, or dipoles [17], has also been observed in other cases of oscillatory flows past an object [18][19][20][21]. The close-up photograph of 033202-5 Fig.…”

Section: Flow Visualizationmentioning

confidence: 83%

Linear drag law for high-Reynolds-number flow past an oscillating body

et al. 2016

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An object immersed in a fast flow typically experiences fluid forces that increase with the square of speed. Here we explore how this high-Reynolds-number force-speed relationship is affected by unsteady motions of a body. Experiments on disks that are driven to oscillate while progressing through air reveal two distinct regimes: a conventional quadratic relationship for slow oscillations and an anomalous scaling for fast flapping in which the time-averaged drag increases linearly with flow speed. In the linear regime, flow visualization shows that a pair of counterrotating vortices is shed with each oscillation and a model that views a train of such dipoles as a momentum jet reproduces the linearity. We also show that appropriate scaling variables collapse the experimental data from both regimes and for different oscillatory motions into a single drag-speed relationship. These results could provide insight into the aerodynamic resistance incurred by oscillating wings in flight and they suggest that vibrations can be an effective means to actively control the drag on an object.A central goal of fluid dynamics is to relate the shape and motion of a body to the aerodynamic or hydrodynamic forces it experiences. Investigations into such force laws have formed the basis of fluid physics and engineering and have led to far-reaching applications [1], from minimizing drag by streamlining shapes to generating lift and thus enabling heavier-than-air flight [2]. Studies over the past two centuries have revealed fundamental scaling relationships relating forces to speed for the case of steady flow past a body or, equivalently, steady motion of a body through a fluid. For small objects in slow flows, forces vary in direct proportion to the speed, while large objects in fast flows experience forces that increase as the square of speed [1,3]. These linear and quadratic regimes are demarcated by the Reynolds number, which quantifies the relative importance of fluid inertia to viscosity. Large bodies in fast flows correspond to high Reynolds number and the well-known quadratic drag law stems from the separation of flow from the surface and the associated pressure difference across the body [4].Strongly unsteady motions of a body in a flow are likely to significantly modify flow separation and thus alter force-speed scaling relationships. Such time-dependent motions are common in many contexts. For example, the flow-induced fluttering of flexible structures, such as the flapping of a flag, has recently been studied as an archetype of passively excited oscillations [5][6][7]. Animal locomotion provides examples in which oscillations of structures are actively driven, such as in flying or swimming propelled by flapping wings or fins [8-10]. One feature common to all these problems is the superposition of unsteady or ac motions and steady or dc motions. For the case of locomotion, for example, this involves the oscillations or flapping of an appendage superposed on the body translation during steady forward flight or swimming.Here w...

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Three-dimensional vortex dynamics in oscillatory flow separation

Cited by 8 publications

References 29 publications

Large Eddy Simulation of three‐dimensional flow structures over wave‐generated ripples

Large Eddy Simulation of three‐dimensional flow structures over wave‐generated ripples

On the wave and current interaction with a rippled seabed in the coastal ocean bottom boundary layer

Linear drag law for high-Reynolds-number flow past an oscillating body

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