Mapping spheroid rotation modes in turbulent channel flow: effects of shear, turbulence and particle inertia

Abstract

The rotational behaviour of non-spherical particles in turbulent channel flow is studied by Lagrangian tracking of spheroidal point particles in a directly simulated flow. The focus is on the complex rotation modes of the spheroidal particles, in which the back reaction on the flow field is ignored. This study is a sequel to the letter by Zhao et al. (Phys. Rev. Lett., vol. 115, 2015, 244501), in which only selected results in the near-wall buffer region and the almost-isotropic channel centre were presented. Now, particle dynamics all across the channel is explored to provide a complete picture of the orientational and rotational behaviour with consideration of the effects of particle aspect ratio ranging from 0.1 to 10 and particle Stokes number from 0 (inertialess) to 30. The rotational dynamics in the innermost part of the logarithmic wall layer is particularly complex and affected not only by modest mean shear, but also by particle inertia and turbulent vorticity. While inertial disks exhibit modest preferential orientation in either the wall-normal or cross-stream direction, inertial rods show neither preferential tumbling nor spinning. Examination of the co-variances between particle orientation, particle rotation and fluid rotation vectors explains the qualitatively different ‘wall mode’ rotation and ‘centre mode’ rotation. Inertialess spheroids transition between the two modes within a narrow zone ($15<z^{+}<35$) in the buffer region. If the spheroids have inertia, the transition zone between the two modes shifts to the inner part of the logarithmic layer, i.e. $z^{+}\geqslant 40$. We ascribe the transition of inertialess spheroids from the ‘wall mode’ to the ‘centre mode’ rotation to the changeover between the time scales associated with mean shear and small-scale turbulence. Inertial spheroids, however, transition between the two rotational modes when the Kolmogorov time scale becomes comparable to the time scale for particle rotation, i.e. the effective Stokes number is of order unity. The aforementioned findings reveal, in addition to the effects of particle shape and alignment, the importance of the characteristic local time scale of fluid flow for the rotation of both tracer and inertial spheroids in turbulent channel flows.

Turbulence modulations and drag reduction by inertialess spheroids in turbulent channel flow

“…The shapedependence of particle alignment becomes marginal in the core region of the channel. This orientational tendency is in accordance with earlier studies (Challabotla et al 2015;Zhao et al 2015). It is noteworthy that particles exhibit a stronger preference and anisotropy in two-way coupled simulations.…”

“…One-way coupled simulations is also conducted for comparison purposes. We follow the same approach as that adopted in our earlier work (Challabotla et al 2015;Zhao et al 2019). One million tiny particles of each shape are immersed in the same turbulent field without considering the influence of particles on fluid phase.…”

Clustering of inertial spheres in evolving Taylor–Green vortex flow

“…The Kolmogorov scale for time, for instance, increases fivefold from the near-wall region to the channel center at a friction Reynolds number 180 [12]. However, an inertial particle is primarily driven in the direction of the bulk flow, whereas major wall-normal excursions are rare.…”

Alignment and rotation of spheroids in unsteady vortex flow

“…Particle inertia is moreover known to make spheroidal particles orient and rotate completely different from tracer spheroids with the same aspect ratio ; see e.g. Zhao et al [6,14]. The conclusions drawn from the present investigation of inertialess spheroids are therefore not valid for inertial particles, unless the particle inertia is negligibly small, i.e.…”

“…The intricate behavior of non-spherical particles in three-dimensional flow fields has been studied in HIT [5,7,10] as well as in wall turbulence [6,9,14]. These studies have all been carried out in statistically steady flow fields.…”