Dynamics of fine scale eddy clusters in turbulent channel flows

Abstract

To investigate dynamics of vortex clusters and large-scale structures in the outer layer of wall turbulence, direct numerical simulations of turbulent channel flows have been conducted up to Re τ = 1270. In the outer layer, the vortex clusters are composed of coherent fine-scale eddies (CFSEs) of which diameter and maximum azimuthal velocity are scaled by the Kolmogorov length and velocity. The large-scale structure in the outer layer is composed of these clusters of the CFSEs, which contributes to the streamwise velocity deficit (i.e. low-momentum region). The CFSE clusters are observed in the low-momentum regions of the outer layer, and the scale of those clusters tends to be enlarged with the increase of a distance from the wall. The dynamics of large-scale structures reveals that the cluster structure generated in the bottom of the logarithmic region moves downstream and its scale increases with the increase of the low-momentum region. The CFSE clusters in the low-momentum regions of u ≤ −u rms consist of the relatively strong CFSEs, which play an important role in the production of the Reynolds shear stress and the dissipation rate of the turbulent kinetic energy. The process of destruction of the CFSE cluster is also clarified in the outer layer.

The intense vorticity structures near the turbulent/non-turbulent interface in a jet

“…Many works have been devoted to the study of these structures, particularly in isotropic turbulence e.g. Siggia (1981), Ashust et al (1987), Vincent & Meneguzzi (1991), Jiménez et al (1993) and Jiménez & Wray (1998), but recently other flows have been used to study the IVSs such as mixing layers (Tanahashi, Iwase & Miyauchi 2001), channel flows (Kang, Tanahashi & Miyauchi 2008) and jets (Ganapathisubramani, Lakshminarasimhan & Clemens 2008). In contrast to the LVSs the characteristics of the IVSs are similar in a variety of different flows, e.g.…”

Characterization of Near-Wall Turbulent Flows by Tomographic Piv

“…This fact seems to be a result of the higher incidence of vortices inside ejection regions. These findings are in agreement with the DNS study of [Kang et al, 2007]. The relative contribution of ejections to the total dissipation rate was, on average, 25% in the present work.…”

“…A slight overestimation of dissipation rate for low-speed regions is verified, what can suggest higher noise in the data close to the wall, where the shear was more intense, or the existence of vortices. A similar behaviour of the dissipation curve was also found by [Kang et al, 2007]. In that direct numerical simulation work, the dissipation rate was attributed to the higher probability of the vortex existence inside lowmomentum regions than inside high-momentum regions.…”

“…It is interesting to observe that the Reynolds stress curve for low-speed regions appears over the curve corresponding to the high-speed region. This finding does not seem to be a result of the threshold value employed, and it was reported by many researchers (e.g., [Talmon et al, 1986, Kang et al, 2007, Dennis and Nickels, 2011b, Dekou et al, 2016). The contribution of the low-speed regions to the total Reynolds stress was 37% (on average), while that from the high-speed regions represented 29%.…”

“…Low-and high-speed region detection and feature extraction Low-and high-speed regions were defined as locations of negative and positive fluctuating streamwise velocities, respectively ( [Lin, 2006, Kang et al, 2007, Dennis and Nickels, 2011b, Dekou et al, 2016). Figure 6.20(a) displays the probability density functions (PDF) of the fluctuating streamwise velocities along wall-normal direction.…”