Pattern recognition analysis of the turbulent flow past a backward facing step

Scarano, Fulvio; Benocci, C.; Riethmuller, M. L.

doi:10.1063/1.870240

Cited by 75 publications

(30 citation statements)

References 8 publications

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“…The peak of the fitted function is taken as the location of the identified vortex structure. The outlined method of feature detection via cross-correlation with a template has similarities with the approaches presented in turbulent wakes by Ferre & Giralt (1989) who applied it on the basis of hot-wire rake measurements or Scarano, Benocci & Riethmuller (1999) who used planar PIV.…”

Section: Vortex Detection and Tracking Methodsmentioning

confidence: 99%

Tracking of vortices in a turbulent boundary layer

et al. 2012

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The motion of spanwise vortical elements and large-scale bulges has been tracked in the outer region between wall-normal distance z/δ = 0.11 and 0.30 of a turbulent boundary layer at Re θ = 2460. The experimental dataset of time-resolved threedimensional velocity fields used has been obtained by tomographic particle image velocimetry. The tracking of these structures yields their respective average trajectories as well as the variations thereof, quantified by the root mean square of the trajectory coordinates as a function of time. It is demonstrated that the variation in convection can be described by a dispersion model for infinitesimal particles in homogeneous turbulence, which suggests that these vortical structures and bulges are transported passively by the external velocity field without significant changes in their topology, at least over the present observation time of 1.2δ/U e . However, this does not mean that the structure's convection velocity is equal to the local flow velocity at each instant. Differences of the order of the Kolmogorov or wall friction velocity have been observed for the spanwise vortical elements. In addition, the simultaneous detection and tracking of multiple structures allows an evaluation of the relative velocity between two spanwise vortex elements, which are approximately aligned along the streamwise direction. The typical streamwise distance between such neighbouring structures is found to be around 0.2δ. Their relative velocities are small, especially the streamwise component, which shows less variation as may be expected based on the relative flow velocity statistics for the same separation distance. This appears consistent with the hairpin packet model, which comprises a set of streamwise aligned hairpins travelling coherently. In exceptional cases, however, the structures approach each other rapidly, forcing an interaction on a time scale of the order of 1δ/U e . It is shown that the measured variation in convection velocity can further be used successfully to predict the temporal development of space-time correlation functions starting from the instantaneous correlation map. In this prediction the structures are assumed to convect without change, following our observations.

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Section: Vortex Detection and Tracking Methodsmentioning

confidence: 99%

Tracking of vortices in a turbulent boundary layer

et al. 2012

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“…As a result, the display of the educed vortex is based on a value of Q in figure 11 that is 60 % less than that of figure 10. The flow pattern induced by the hairpin vortex is illustrated by streamlines relative to the local advection velocity of the flow (Scarano 1999;Adrian et al 2000). The ejection event is induced at the centroid of the vortex lifting up the low-speed streak and near-wall vorticity, resulting in more homogeneous distribution of vorticity (Adrian 2007).…”

Section: Unsteady Flow Organization 41 Turbulent Boundary Layermentioning

confidence: 99%

“…The approach follows that used by Ferré & Giralt (1989) and later modified for PIV measurements by Scarano, Benocci & Riethmuller (1999). The working principle relies on the choice of an indicator function and on the formulation of a spatial template.…”

Section: Pattern Recognition Analysismentioning

confidence: 99%

Counter-hairpin vortices in the turbulent wake of a sharp trailing edge

Ghaemi

Scarano

2011

J. Fluid Mech.

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The unsteady organization and evolution of coherent structures within the turbulent boundary layer and subsequent wake of the sharp symmetric trailing edge of a NACA0012 aerofoil are investigated. The experiments are conducted in an open testsection wind tunnel at Re c = 386 000 based on the aerofoil chord and Re θ = 1300 based on the boundary layer momentum thickness. An initial characterization of the flow field using two-component particle image velocimetry (PIV) is followed by the investigation of the unsteady organization and evolution of coherent structures by time-resolved three-dimensional PIV based on a tomographic approach (Tomo-PIV). The inspection of the turbulent boundary layer prior to the trailing edge in the region between 0.15 and 0.8 δ 99 demonstrated streaks of low-and high-speed flow, while the low-speed streaks are observed to be more coherent along with strong interaction with hairpin-type vortical structures similar to a turbulent boundary layer at zero pressure gradient. The wake region demonstrated gradual deterioration of both the low-and the high-speed streaks with downstream progress. However, the low-speed streaks are observed to lose their coherence at a faster rate relative to the high-speed streaks as the turbulent flow develops towards the far wake. The weakening of the low-speed streaks is due to the disappearance of the viscous sublayer after the trailing edge and gradual mixing through the transport of the remaining low-speed flow towards the free stream. This transport of low-speed flow is performed by the ejection events induced by the hairpin vortices as they also persist into the developing wake. The higher persistence of the high-speed streaks is associated with counter-hairpin vortical activities as they oppose the deterioration of the high-speed streaks by frequently sweeping the high-speed flow towards the wake centreline. These vortical structures are regarded as counter-hairpin vortices as they exhibit opposite characteristics relative to the hairpin vortices of a turbulent boundary layer. They are topologically similar to the hairpins as they appear to be U-shaped but with inverted orientation, as the spanwise portion is in the vicinity of the wake centreline and the legs are inclined at an approximately 60• to the wake axis in the downstream direction demonstrating a strain-dominated topology. The counter-hairpin vortices are partially wrapped around the high-speed streaks and contribute to the wake development by transporting highspeed flow towards the wake centreline. Similar to the hairpin vortices of a turbulent boundary layer, the occurrence of a complete counter-hairpin vortex is occasional while its derivatives (portions of spanwise or quasi-streamwise vortices) are more frequently observed. Therefore, a pattern recognition algorithm is applied to establish characterization based on an ensemble-averaged counter-hairpin vortex. The formation of the counter-hairpin vortices is due to an additional degree of interaction between † Email address for correspondenc...

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“…As mentioned earlier, the most unstable mode is the wave number for which the energy is growing fastest. A more sophisticated way to identify the coherent structures and their evolution from the measurements is by conditional averaging, see, for example, Scarano et al 19 In this study however, we restrict ourselves to the development of the dominant mode, since this property is obtained from the stability analysis as well as from the experiments.…”

Section: Experimental Validationmentioning

confidence: 99%

A linear approach for the evolution of coherent structures in shallow mixing layers

2002

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The development of large coherent structures in a shallow mixing layer is analyzed. The results are validated with experimental data obtained from particle tracking velocimetry. The mean flow field is modeled using the self-similarity of the velocity profiles. The characteristic features of the down-stream development of a shallow mixing layer flow, like the decrease of the velocity difference over the mixing layer, the decreasing growth of the mixing layer width, and the transverse shift of the center of the mixing layer layer are fairly well represented. It turned out that the entrainment coefficient could be taken constant, equal to a value obtained for unbounded mixing layers: ␣ϭ0.085. Linearization of the shallow water equations leads to a modified Orr-Sommerfeld equation, with turbulence viscosity and bottom friction as dissipative terms. Growth rates are obtained for each position downstream, using the model for the mean flow field. For a given energy density spectrum at the inflow boundary, integration of the growth rates along the downstream direction yields the spectra at various downstream positions. These spectra provide a measure for the intensity and the length scale of the coherent structures ͑the dominant mode͒. The length scales found are in good agreement with the measured ones. The length scale of the most unstable mode appears much larger than the length scale of the dominant mode. Obviously, the longevity of the coherent structures plays a significant role. Three growth regimes can be distinguished: in the first regime the dominant mode is growing, in the second regime the dominant mode is dissipating, but other modes are still growing, and in the third regime all modes are dissipating. It is concluded that the development of the coherent structures in a shallow mixing layer can fairly well be described and interpreted by the proposed linear analysis.

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Pattern recognition analysis of the turbulent flow past a backward facing step

Cited by 75 publications

References 8 publications

Tracking of vortices in a turbulent boundary layer

Tracking of vortices in a turbulent boundary layer

Counter-hairpin vortices in the turbulent wake of a sharp trailing edge

A linear approach for the evolution of coherent structures in shallow mixing layers

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