On the existence and formation of multi-scale particle streaks in turbulent channel flows

Abstract

Direct numerical simulations of particle-laden turbulent channel flows at friction Reynolds number $Re_\tau$ from $600$ to $2000$ have been performed to examine the near-wall particle streaks. Different from the well-observed small-scale particle streaks in near-wall turbulence of low $Re_\tau$ , the present results show large-scale particle streaks through the computational domain formed for relatively high-inertia particles at high $Re_\tau$ . Transferred by large-scale sweep and ejection events ( $Q^-$ ), these high-inertia particles preferentially accumulate in near-wall regions beneath the large-scale low-speed flow streaks observed in the logarithmic region. The corresponding Stokes numbers are associated with the lifetime of large-scale $Q^-$ structures, which increases as the Reynolds number grows. The small-scale particle streaks with a typical Stokes number $St_\nu \approx 30$ are mainly driven by the $Q^-$ structures in the buffer layer, whose lifetime is approximately $30$ in viscous time unit. Therefore, we propose a new structure-based Stokes number normalized by the lifetime of $Q^-$ structures of different scales. The relevant flow scales that control the formation of the large-scale particle streaks are parameterized by the structure-based Stokes number. The small-scale (large-scale) particle streaks are most prominent when the buffer-layer (large-scale) structure-based Stokes number approaches unity. The present findings reveal that formation of near-wall particle streaks is governed by the $Q^-$ structures of different scales, and the particles with different inertia respond efficiently to the $Q^-$ structures of corresponding scales with respect to the particle translational motion.

Direct numerical simulation of particle-laden flow in an open channel at

“…We briefly remark that this domain size is the same as Jie et al. (2022) in DNS of particle-laden channel flow at , but smaller than the DNS of single-phase turbulent channel flow at by Lee & Moser (2015), using . However, the mesh resolution is same as Lee & Moser (2015), i.e.…”

“…This confounds a clear description of whether the qualitative differences seen presently compared with the lower Reynolds numbers of are due simply to a strong Reynolds number dependence, or if the relevant Stokes numbers have not been properly matched between the two (e.g. Jie et al 2022). Future work would benefit from a more complete picture of how these modification statistics behave across a denser range of St and Re, in addition to the dependence on mass fraction.…”

“…Jie et al. 2022). Future work would benefit from a more complete picture of how these modification statistics behave across a denser range of and , in addition to the dependence on mass fraction.…”

“…Although the domain size is smaller than VLSMs in the streamwise direction, which typically scales as λ x = O(10h) (Kim & Adrian 1999), we believe it is large enough for the present purpose. We briefly remark that this domain size is the same as Jie et al (2022) in DNS of particle-laden channel flow at Re τ = 2000, but smaller than the DNS of single-phase turbulent channel flow at Re τ = 5186 by Lee & Moser (2015), using (L x , L y , L z ) = (8πh, 3πh, h). However, the mesh resolution is same as Lee & Moser (2015), i.e.…”

“…Jie et al. (2022) revealed that large-scale particle streaks are formed for relatively high-inertia particles at high (), which are distinct from the typical small-scale particle streaks observed in low channel flow. To the best of the authors’ knowledge, this is the highest-Reynolds-number particle-laden channel flow case simulated with DNS, presumably due to the rapidly increasing computational cost with higher Reynolds numbers.…”