Abstract
We have performed direct numerical simulations of a spatio-temporally intermittent flow in a pipe for Rem = 2250. From previous experiments and simulations of pipe flow, this value has been estimated as a threshold when the average speeds of upstream and downstream fronts of a puff are identical (Barkley et al., Nature 526, 550–553, 2015; Barkley et al., 2015). We investigated the structure of an individual puff by considering three-dimensional snapshots over a long time period. To assimilate the velocity data, we applied a conditional sampling based on the location of the maximum energy of the transverse (turbulent) motion. Specifically, at each time instance, we followed a turbulent puff by a three-dimensional moving window centered at that location. We collected a snapshot-ensemble (10000 time instances, snapshots) of the velocity fields acquired over T = 2000D/U time interval inside the moving window. The cross-plane velocity field inside the puff showed the dynamics of a developing turbulence. In particular, the analysis of the cross-plane radial motion yielded the illustration of the production of turbulent kinetic energy directly from the mean flow. A snapshot-ensemble averaging over 10000 snapshots revealed azimuthally arranged large-scale (coherent) structures indicating near-wall sweep and ejection activity. The localized puff is about 15-17 pipe diameters long and the flow regime upstream of its upstream edge and downstream of its leading edge is almost laminar. In the near-wall region, despite the low Reynolds number, the turbulence statistics, in particular, the distribution of turbulence intensities, Reynolds shear stress, skewness and flatness factors, become similar to a fully-developed turbulent pipe flow in the vicinity of the puff upstream edge. In the puff core, the velocity profile becomes flat and logarithmic. It is shown that this “fully-developed turbulent flash” is very narrow being about two pipe diameters long.
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Notes
Thus, when we say “at the moving window trailing edge S01,” it also implies “slightly downstream of the upstream edge of the puff.”
“Low-speed” (w < 0) and “high-speed” (w > 0) are usually used as relative terms, and refer to deviations from the mean streamwise velocity value at that location.
In this study, we take into account that the near-wall ejection and sweeping correlate and are spatially close. Therefore, the computed range of \(|\text {R}_{u_{r},u_{r}}|>0.2\) leads us to consider these data as well correlated.
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One of the authors (AY) would like to thank Dr. N. Nikitin (Moscow State University) for providing some DNS channel data and helpful comments.
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Yakhot, A., Feldman, Y., Moxey, D. et al. Turbulence in a Localized Puff in a Pipe. Flow Turbulence Combust 103, 1–24 (2019). https://doi.org/10.1007/s10494-018-0002-8
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DOI: https://doi.org/10.1007/s10494-018-0002-8