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The organization of near-wall turbulence: a comparison between boundary layer SPIV data and channel flow DNS data

Abstract : The vortical structures of near-wall turbulence at moderate Reynolds number are analyzed and compared in datasets obtained from stereoscopic particle image velocimetry (SPIV) in a turbulent boundary layer and from direct numerical simulations (DNS) in a turbulent channel flow. The SPIV data is acquired in the LTRAC water-tunnel at Reτ = δ + = 820 in a streamwise/wall-normal plane, and in the LML wind-tunnel at Reτ = δ + = 2590 in a streamwise/wall-normal plane and in a plane orthogonal to the mean flow. The DNS data is taken from DelAlamo et al [1], at Reτ = 950. The SPIV database is validated through an analysis of its mean velocity profile and power spectra, which are compared to reference profiles. A common detection algorithm is then applied to both the SPIV and DNS datasets in order to retrieve the streamwise and spanwise vortices. The algorithm employed is based on the 2D swirling strength and on a fit of an Oseen vortex model, which allows to retrieve the vortex characteristics, including its radius, vorticity, and position of the center. At all Reynolds numbers, the near-wall region is found to be the most densely populated region, predominantly with streamwise vortices that are on average smaller and more intense than spanwise vortices. In contrast, the logarithmic region is equally constituted of streamwise and spanwise vortices having equivalent characteristics. Two different scalings were employed to analyze the vortex radius and vorticity : the wall-unit scaling and the Kolmogorov scaling. In wall-unit scaling, a good universality in Reynolds numbers of the vortices radius and vorticity is observed in the near-wall and logarithmic region: the vorticity is found to be maximum at the wall, decreasing first rapidly and then slowly with increasing wall-normal distance; the radius is increasing slowly with wall-normal distance in both regions, except for the streamwise vortices for which a sharp increase in radius is observed in the near-wall region. However, the wall-units scaling is found to be deficient in the outer region, where Reynolds number effects are observed. The Kolmogorov scaling appears to be universal both in Reynolds number and wall-normal distance across the three regions investigated, with a mean radius on the order of 8η and a mean vorticity on the order of 1.5τ −1 , but for the SPIV data only. In the DNS dataset, the radius in Kolmogorov scaling slowly decreases with wall-normal distance. This difference between the SPIV and the DNS may be linked to a difference between the boundary layer flow and the channel flow, rather than to the techniques themselves. Finally, the distribution of the vorticity of the detected vortices seem to follow faithfully a log normal distribution, in good agreement with Kolmogorov's theory ([2]).
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Sophie Herpin, M. Stanislas, J. Soria. The organization of near-wall turbulence: a comparison between boundary layer SPIV data and channel flow DNS data. Journal of Turbulence, Taylor & Francis, 2010, 11, pp.N47. ⟨10.1080/14685248.2010.508460⟩. ⟨hal-01826318⟩

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