RESUMO

Direct numerical simulation was carried out to investigate the spatial features of very large-scale motions (VLSMs) in a turbulent channel flow with Re_τ ~ 930. Using the streak detection algorithm and the population trend, statistics of the spatial extents for both large-scale motions (LSMs) and VLSMs were obtained. This result showed that, at least one VLSM, on average, can be found in the wall-parallel 10δ × δ plane (here δ is the channel half height), which provides a statistical evidence of the presence of VLSMs. Moreover, to clarify one of previous hypotheses regarding the formation of VLSMs, the upstream tracking of individual streaks was utilized for temporal analysis. The present study statistically supported that the connection of the upstream LSMs is mainly contributed to the formation of VLSMs; that is, the concatenation of the small structures to form VLSMs. The downstream tracking of VLSMs indicated that over than 30% of VLSMs survive for 9 wall-unit time. Finally, the spatial organization of LSMs and VLSMs was examined by the conditional averaging.