The quiescent core of turbulent channel flow

Kwon, Yongseok; Philip, Jimmy; Silva, C. M. de; Hutchins, Nicholas; Monty, Jason

doi:10.1017/jfm.2014.295

Cited by 58 publications

(157 citation statements)

References 27 publications

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“…The range of possible velocities in each map was bounded from below by zero and from above by the freestream velocity. In such cases, histograms of the values within each instantaneous streamwise velocity map exhibited welldefined local peaks, which were then unambiguously construed as the nominal velocities within different UMZs [21][22][23].…”

Section: Uniform Momentum Zone Identificationmentioning

confidence: 96%

“…The term Uniform Momentum Zones (UMZs) has been used to describe regions of relatively uniform velocity, which have been observed to appear in the centres of packets of quasi-streamwise hairpin vortices in turbulent boundary layers [1,[21][22][23]28]. Turbulent boundary layers contain a hierarchy of UMZs, with typically two to five UMZs across their thickness at any given time; the size of these zones has been seen to increase with wall-normal distance [23].…”

Section: Uniform Momentum Zone Identificationmentioning

confidence: 99%

“…The turbulence in fully developed USF is nearly homogeneous but strongly anisotropic, and has a largescale structure that is dominated by hairpin-shaped vortices [3], comparable to those found in turbulent boundary layers. This article is an extended version of a paper presented recently at the TSFP-9 conference [20]; we have now included a more detailed description of our techniques, additional examples and discussion of the influence of vortices and updated several figures; most importantly, we have included new analysis of experimental results that document the role of uniform momentum zones that occur in USF as well as in turbulent boundary layers [1,3,9,[21][22][23] and have analyzed the orientation of the scalar flux vector with respect to the orientation of the coherent structures [3]. A description of the experimental apparatus and measurement techniques is provided in section 2.…”

Section: Introductionmentioning

confidence: 99%

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Scalar dispersion by coherent structures in uniformly sheared flow generated in a water tunnel

Vanderwel

Tavoularis

2016

Journal of Turbulence

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In order to investigate the role of coherent structures as mechanisms of scalar dispersion, we studied measurements of a passive scalar plume released in a uniformly sheared turbulent flow generated in a water tunnel. The flow had homogeneous turbulence properties in the measurement domain and contained hairpin vortices similar to those in boundary layers, and so was an ideal test bed to study the effects of coherent structures on turbulent dispersion, free from the effects of inhomogeneities or boundaries. Measurements of the velocity and concentration fields were acquired simultaneously using stereo particle image velocimetry and planar laser induced fluorescence. We found that dye was preferentially located far away from vortices and was less likely to appear in close proximity to vortices, which is attributed to the high dissipation at the periphery of the vortices. However, we also found that dye was not directly correlated with the uniform momentum zones in the flow, suggesting a more complex relationship exists between these zones, the locations of vortices, and dye transport. Considering scalar flux events rather than simply the presence of dye as our condition of interest, a conditional eddy analysis demonstrated that hairpin vortices are responsible for the large scalar flux events as well as the large Reynolds stress events in the flow. The fact that the Reynolds stress was correlated with the scalar flux further confirmed that coherent structures are dominant mechanisms for scalar transport. Furthermore, we found that the scalar flux vector was preferentially inclined by 155 ○ and −25 ○ with respect to the streamwise direction, and was thus approximately orthogonal to the planes of the legs of the most common upright and inverted hairpin structures in the flow. These findings demonstrate that coherent structures play an important and intricate role in turbulent diffusion.

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Section: Uniform Momentum Zone Identificationmentioning

confidence: 96%

Section: Uniform Momentum Zone Identificationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Scalar dispersion by coherent structures in uniformly sheared flow generated in a water tunnel

Vanderwel

Tavoularis

2016

Journal of Turbulence

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“…It is worth highlighting that a similar detection criterion was employed recently to study the uniform momentum core in turbulent channel flows, where it was shown that this region was successfully demarcated by considering the p.d.f.s of the streamwise velocity component (Kwon et al 2014). As a final note, when considering this type of analysis using p.d.f.s of velocity from PIV datasets, any pixel-locking (pixel displacements which are biased to integers, see Adrian & Westerweel 2011) should be carefully dealt with.…”

Section: Detection Of Zones Of Uniform Momentummentioning

confidence: 99%

Uniform momentum zones in turbulent boundary layers

2015

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Structural properties of regions of uniform streamwise momentum in turbulent boundary layers are examined using experimental databases obtained from particle image velocimetry. This investigation employs a large range of Reynolds numbers, spanning more than an order of magnitude (Re τ = 10 3 -10 4 ), enabling us to provide a detailed description of uniform momentum zones as a function of Reynolds number. Our analysis starts by examining the identification criterion used by Adrian et al. (J. Fluid Mech., vol. 422, 2000, pp. 1-54) to report the presence of uniform momentum zones in turbulent boundary layers. This criterion is then applied to show that a zonal-like structural arrangement is prevalent in all datasets examined, emphasising its importance in the structural organisation. Streamwise velocity fluctuations within the zones are observed to be small but they are bounded by distinct step changes in streamwise momentum which indicate that shear layers of intense vorticity separate each zone. A log-linear increase in the number of these zones with increasing Reynolds number is revealed, together with an increase in the thicknesses of zones with increasing distance from the wall. These results support a hierarchical length-scale distribution of coherent structures, which generate zonal-like organisation within turbulent boundary layers. Interpretation of these findings is aided by employing synthetic velocity fields generated using a model based on the attached eddy hypothesis, which is described in the work of Perry and co-workers. Comparisons between the model and experimental results show that a hierarchy of self-similar structures leads to population densities and length-scale distributions of uniform momentum zones that closely adhere to those observed experimentally in this study.

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“…UMZs manifest as peaks in the instantaneous probability density function (PDF) of a velocity flow field, however, so does pixel-locking. Kwon et al (2014) demonstrated in fully developed channel flow that PIV data tended to predict the same UMZ modal velocities in every image. However, direct numerical simulations of the same flow found that there was no bias towards particular velocities for UMZs, demonstrating that the coalesense of UMZ modal velocities in PIV is unphysical.…”

mentioning

confidence: 79%

Quantification and adjustment of pixel-locking in particle image velocimetry

Hearst

Ganapathisubramani

2015

Exp Fluids

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A quantification metric is provided to determine the degree to which a particle image velocimetry data set is pixel-locked. The metric is calculated by integrating the histogram equalization transfer function and normalizing by the worst-case-scenario to return the percentage pixel-locked. When this metric is calculated for each position in the vector field, it is shown that pixel-locking is non-uniform across the field. Hence, pixel-locking adjustments should be made on a vector-by-vector basis rather than uniformly across a field, although the latter is the common practice. A methodology is provided to compensate for the effects of pixel-locking on a vector-by-vector basis. This includes applying a Gaussian filter directly to the images, processing the images with window deformation, ensuring the vector fields are in pixel displacements, applying histogram equalization calculated at each vector coordinate, and mapping the adjusted vector fields to physical space.Pixel-locking is the tendency of a particle image velocimetry (PIV) velocity field to be biased towards integer values of pixel displacements. It is a result of insufficient resolution to accurately determine the sub-pixel displacement. This phenomenon is also frequently referred to as 'peak-locking'. Pixel-locking is an intrinsic error imbedded in an image at acquisition. To a certain degree, the effects of pixel-locking can be mitigated by the sub-pixel estimator during processing, but in severe cases the choice of estimator has no influence (Christensen, 2004).

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The quiescent core of turbulent channel flow

Cited by 58 publications

References 27 publications

Scalar dispersion by coherent structures in uniformly sheared flow generated in a water tunnel

Scalar dispersion by coherent structures in uniformly sheared flow generated in a water tunnel

Uniform momentum zones in turbulent boundary layers

Quantification and adjustment of pixel-locking in particle image velocimetry

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