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

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) approximate to 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.

Automated summary

Direct numerical simulations were conducted to study particle-laden turbulent channel flows at friction Reynolds number Re-tau from 600 to 2000. The results showed large-scale particle streaks formed by high-inertia particles at high Re-tau. These particles preferentially accumulated beneath the large-scale low-speed flow streaks observed in the logarithmic region. Different scales of Q(-) structures controlled the formation of near-wall particle streaks.