Abstract
The preferential organisation of coherent vortices in a turbulent boundary layer in relation to local large-scale streamwise velocity features was investigated. Coherent vortices were identified in the wake region using the Triple Decomposition Method (originally proposed by Kolář) from 2D particle image velocimetry (PIV) data of a canonical turbulent boundary layer. Two different approaches, based on conditional averaging and quantitative statistical analysis, were used to analyze the data. The large-scale streamwise velocity field was first conditionally averaged on the height of the detected coherent vortices and a change in the sign of the average large scale streamwise fluctuating velocity was seen depending on the height of the vortex core. A correlation coefficient was then defined to quantify this relationship between the height of coherent vortices and local large-scale streamwise fluctuating velocity. Both of these results indicated a strong negative correlation in the wake region of the boundary layer between vortex height and large-scale velocity. The relationship between vortex height and full large-scale velocity isocontours was also studied and a conceptual model based on the findings of the study was proposed. The results served to relate the hairpin vortex model of Adrian et al. to the scale interaction results reported by Mathis et al., and Chung and McKeon.
Highlights
Groups of vortices in the logarithmic and wake regions of wall-bounded turbulent flows have been associated with low-momentum regions and ejection motions, including in the attached eddy model of Perry and Chong [6], the hairpin packet model of Adrian et al [7], and the vortex clusters observed by Lozano-Durán et al [8]
To further study correlation between the large-scale structures and the spatial location of the coherent vortices, the vortex heights were binned into four different ranges, (a) 0.15 < yv /δ < 0.3, (b) 0.3 < yv /δ < 0.5, (c) 0.5 < yv /δ < 0.7, and, (d) yv /δ > 0.7
The location of coherent vortex structures was shown to be correlated to the local behavior of large-scale velocity structures
Summary
Coherent motions in wall turbulence have been treated in detail in various reviews [1,2].Coherent vortex structures or ‘eddies’, including horseshoe or hairpin vortices were first proposed as a key turbulent feature in 1952 [3] and were later experimentally observed [4].The structural organisation of vortex structures and their position relative to large-scale velocity structures in wall-bounded turbulence has been a topic of interest for decades.Falco [5] identified vortices at the upstream border of large-scale bulges in the velocity field.Groups of vortices in the logarithmic and wake regions of wall-bounded turbulent flows have been associated with low-momentum regions and ejection motions, including in the attached eddy model of Perry and Chong [6], the hairpin packet model of Adrian et al [7], and the vortex clusters observed by Lozano-Durán et al [8]. Coherent motions in wall turbulence have been treated in detail in various reviews [1,2]. Coherent vortex structures or ‘eddies’, including horseshoe or hairpin vortices were first proposed as a key turbulent feature in 1952 [3] and were later experimentally observed [4]. The structural organisation of vortex structures and their position relative to large-scale velocity structures in wall-bounded turbulence has been a topic of interest for decades. Groups of vortices in the logarithmic and wake regions of wall-bounded turbulent flows have been associated with low-momentum regions and ejection motions, including in the attached eddy model of Perry and Chong [6], the hairpin packet model of Adrian et al [7], and the vortex clusters observed by Lozano-Durán et al [8].
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