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

Abstract

Stereoscopic particle image velocimetry (PIV) measurements are made in streamwisespanwise and inclined cross-stream planes (inclined at 45 • and 135 • to the principal flow direction) of a turbulent boundary layer at moderate Reynolds number (Re τ ∼ 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 (R uu) and streamwise-wall-normal (R uw) 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 R uu are dominated by slower streamwise structures. Additional analysis of R ww 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. R uu 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.

Similarity of the streamwise velocity component in very-rough-wall channel flow

“…Streamwise velocity correlations in the crossflow plane show clear evidence of large-scale streamwise structure such as those observed by Ganapathisubramani et al (2005) and Hutchins & Marusic (2007) in the log region of boundary layers on smooth walls. The existence of large streamwise structures over fully rough walls has already been noted by Hutchins & Marusic (2007).…”

YIP-Low-Order Representations of Irregular Surface Roughness and Their Impact on Wall Turbulence

“…The streamwise-elongated nature of p uu in smooth-wall turbulence has been previously linked to the low-momentum regions generated by hairpin vortex packets and the negative correlation outboard of the positive region in the spanwise direction represent the imprint of spanwisealternating low-and high-momentum regions. [47,48] In addition, Wu and Christensen [48] recently reported similar characteristics of p uu for flow over irregular roughness near the outer edge of the roughness sublayer, supporting the relative immunity of these vortical structures as well as their coherent organization into larger-scale structural entities to roughness. The present results lend additional support to the contention that hairpin vortex packets exist in rough-wall turbulence as well, including at the wall-normal position of the present measurements (y = 0.0475) which is deep within the roughness sublayer where roughness effects are most dramatic.…”

Investigation of Coherence behind a Single Foamed Tube

“…There exist a number of investigations on the literature about the coherent structures behind circular or finned tubes and also there are other studies investigating the three-dimensionality of structures. Ganapathisuramani et al [21] showed how stream-wise autocorrelation can be used to measure the size of stream-wise structures. Hutchins et al [22,23] used two-point span-wise correlation to obtain statistical information regarding the width and spacing of the coherent structures.…”