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Time evolution of uniform momentum zones in a turbulent boundary layer. (English) Zbl 1419.76318
Summary: Time-resolved planar particle image velocimetry was used to analyse the structuring of a turbulent boundary layer into uniform momentum zones (UMZs). The instantaneous peak-detection method employed by R. J. Adrian et al. [ibid. 422, 1–54 (2000; Zbl 0959.76503)] and C. M. De Silva et al. [ibid. 786, 309–331 (2016; Zbl 1381.76106)] is extended to account for temporal coherence of UMZs. The resulting number of zones detected appears to follow a normal distribution at any given instant. However, the extreme cases in which the number of zones is either very high or very low, are shown to be linked with two distinct flow states. A higher than average number of zones is associated with a large-scale $$Q2$$ event in the log region which creates increased small-scale activity within that region. Conversely, a low number of zones corresponds to a large-scale $$Q4$$ event in the log region and decreased turbulent activity away from the wall. The residence times, within the measurement plane, of zones belonging to the latter scenario are shown to be on average four times larger than those of zones present during higher than average zone structuring states. For both cases, greater residence times are observed for zones of higher momentum that are generally closer to the free stream.

##### MSC:
 76F40 Turbulent boundary layers
##### Keywords:
boundary layer structure; turbulent flows
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