Large-scale motions in wall-bounded turbulent flows

Abstract

This study investigated the spatial features of large-scale motions (LSMs) and the influence on turbulence statistics in wall-bounded turbulent flows. Among the LSMs in a broad sense, very-large-scale motions (VLSMs) were also focused. To this end, direct numerical simulations (DNS) for large computational domains were performed to obtain flow fields of zero-pressure-gradient (ZPG) canonical flows, as well as presence of pressure gradient and inhomogeneous fluid property. From channel flow simulation, a streak detection method based on the streamwise velocity fluctuations was used to individually trace the cores of LSMs and VLSMs. The temporal features of the low-speed streaks in the outer region revealed that growing and merging events dominated the LSMs ($1-3\delta$). The VLSMs ($>3\delta$) were primarily created by merging events, and the statistical analysis supported that the merging of large-scale upstream structures contributed to the formation of VLSMs. Comparison of turbulent pipe and channel flows at the same friction Reynolds number showed that population density of LSM is large in the outer region of pipe flow than that of channel flow. The large cross-stream components of turbulence intensity were explained by the population trend. Conditionally-sampled statistics revealed that the VLSMs contributed to formation of the secondary peaks of the streamwise normal stress. Adverse pressure gradient (APG) affected that the streamwise component of the turbulence intensity was significantly increased in the outer region of the axisymmetric internal flow. The swirling motions of the individual hairpins in the outer region were stronger in the APG flow than in the ZPG flow. The current study provides insight into the effects of pressure gradient and local viscosity on the structures as well as analysis of the LSMs and VLSMs in canonical flows. In order to apply the findings in canonical flows to engineering applications, the effect of temperature-dependent v...