Investigation of large-scale coherence in a turbulent boundary layer using two-point correlations

B. Ganapathisubramani, N. Hutchins, W. T. Hambleton, E. K. Longmire, I. Marusic

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Abstract

Stereoscopic particle image velocimetry (PIV) measurements are made in streamwise-spanwise and inclined cross-stream planes (inclined at 45° and 135° to the principal flow direction) of a turbulent boundary layer at moderate Reynolds number (Reτ ∼ 1100). Two-point spatial velocity correlations computed using the PIV data reveal results that are consistent with an earlier study in which packets of hairpin vortices were identified by a feature-detection algorithm in the log region, but not in the outer wake region. Both streamwise-streamwise (Ruu) and streamwise-wall-normal (Ruw) correlations are significant for streamwise displacements of more than 1500 wall units. Zero crossing data for the streamwise fluctuating component u reveal that streamwise strips between zero crossings of 1500 wall units or longer occur more frequently for negative u than positive u, suggesting that long streamwise correlations in Ruu are dominated by slower streamwise structures. Additional analysis of Rww correlations suggests that the long streamwise slow-moving regions contain discrete zones of strong upwash over extended streamwise distances, as might occur within packets of angled hairpin vortices. At a wall-normal location outside of the log region (z/ δ = 0.5), the correlations are shorter in the streamwise direction and broader in the spanwise direction. Correlations in the inclined cross-stream plane data reveal good agreement with the streamwise-spanwise plane. Ruu in the 45° plane is more elongated along the in-plane wall-normal direction than in the 135° plane, which is consistent with the presence of hairpin packets with a low-speed region lifting away from the wall.

Original languageEnglish (US)
Pages (from-to)57-80
Number of pages24
JournalJournal of Fluid Mechanics
Volume524
DOIs
StatePublished - Feb 10 2005

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