Lihao Zhao

Turbulence modulation and drag reduction by spherical particles

This letter reports on the pronounced turbulence modulations and the accompanying drag reduction observed in a two-way coupled simulation of particle-laden channel flow. The present results support the view that drag reduction can be achieved not only by means of polymeric or fiber additives but also with spherical particles.

Stokes number effects on particle slip velocity in wall-bounded turbulence and implications for dispersion models

The particle slip velocity is adopted as an indicator of the behavior of heavy particles in turbulent channel flow. The statistical moments of the slip velocity are evaluated considering particles with Stokes number, defined as the ratio between the particle response time and the viscous time scale of the flow, in the range 1 < St < 100. The slip velocity fluctuations exhibit a monotonic increase with increasing particle inertia, whereas the fluid-particle velocity covariance is gradually reduced for St ⩾ 5. Even if this covariance equals the particle turbulence intensity, a substantial amount of particle slip may occur. Relevant to two-fluid modeling of particle-laden flows is the finding that the standard deviation of the slip velocity fluctuations is significantly larger than the corresponding mean slip velocity.

Slip velocity of rigid fibers in turbulent channel flow

In this study, the slip velocity between rigid fibers and a viscous carrier fluid is investigated for the reference case of turbulent channel flow. The statistical moments of the slip velocity are evaluated modelling fibers as prolate spheroids with Stokes number, St, ranging from 1 to 100 and aspect ratio, λ, ranging from 3 to 50. Statistics are compared one-to-one with those obtained for spherical particles (λ = 1) to highlight effects due to fiber elongation. Comparison is also made at different Reynolds numbers (Reτ =150, 180, and 300 based on the fluid shear velocity) to discuss effects due to an increase of turbulent fluctuations. Results show that elongation has a quantitative effect on slip velocity statistics, particularly evident for fibers with small St. As St increases, differences due to the aspect ratio tend to vanish and the relative translational motion between individual fibers and surrounding fluid is controlled by fiber inertia through preferential concentration. A clear manifestation o...

Shape effects on dynamics of inertia-free spheroids in wall turbulence

The rotational motion of inertia-free spheroids has been studied in a numerically simulated turbulent channel flow. Although inertia-free spheroids were translated as tracers with the flow, neither the disk-like nor the rod-like particles adapted to the fluid rotation. The flattest disks preferentially aligned their symmetry axes normal to the wall, whereas the longest rods were parallel with the wall. The shape-dependence of the particle orientations carried over to the particle rotation such that the mean spin was reduced with increasing departure from sphericity. The streamwise spin fluctuations were enhanced due to asphericity, but substantially more for prolate than for oblate spheroids.

Rotation of Nonspherical Particles in Turbulent Channel Flow.

The effects of particle inertia, particle shape, and fluid shear on particle rotation are examined using direct numerical simulation of turbulent channel flow. Particles at the channel center (nearly isotropic turbulence) and near the wall (highly sheared flow) show different rotation patterns and surprisingly different effects of particle inertia. Oblate particles at the center tend to rotate orthogonally to their symmetry axes, whereas prolate particles rotate around their symmetry axes. This trend is weakened by increasing inertia so that highly inertial oblate spheroids rotate nearly isotropically about their principle axes at the channel center. Near the walls, inertia does not move the rotation of spheroids towards isotropy but, rather, reverses the trend, causing oblate spheroids to rotate strongly about their symmetry axes and prolate spheroids to rotate normal to their symmetry axes. The observed phenomena are mostly ascribed to preferential orientations of the spheroids.

On particle spin in two-way coupled turbulent channel flow simulations

The rotational motion of spherical particles suspended in a turbulent flow field may not necessarily adapt to the fluid rotation, i.e., the particle spin may be different from the fluid vorticity. ...

On the relative rotational motion between rigid fibers and fluid in turbulent channel flow

In this study, the rotation of small rigid fibers relative to the surrounding fluid in wall-bounded turbulence is examined by means of direct numerical simulations coupled with Lagrangian tracking. Statistics of the relative (fiber-to-fluid) angular velocity, referred to as slip spin in the present study, are evaluated by modelling fibers as prolate spheroidal particles with Stokes number, St, ranging from 1 to 100 and aspect ratio, λ, ranging from 3 to 50. Results are compared one-to-one with those obtained for spherical particles (λ = 1) to highlight effects due to fiber length. The statistical moments of the slip spin show that differences in the rotation rate of fibers and fluid are influenced by inertia, but depend strongly also on fiber length: Departures from the spherical shape, even when small, are associated with an increase of rotational inertia and prevent fibers from passively following the surrounding fluid. An increase of fiber length, in addition, decouples the rotational dynamics of a fiber from its translational dynamics suggesting that the two motions can be modelled independently only for long enough fibers (e.g., for aspect ratios of order ten or higher in the present simulations).

A Voronoï analysis of preferential concentration in a vertical channel flow

We use three-dimensional Voronoi analysis and results from a direct numerical simulation to study the preferential concentration of inertial particles in a vertical channel flow at Reynolds number 395. By comparing results in upward and downward flows with results from a channel flow without gravity, we are able to determine how gravity affects the particle clustering. Gravity increases the drift of particles towards the walls in an upward flow, while in the downward flow more particles are transported to the centre of the channel. For particles with Stokes number 100, the mean wall-normal particle velocity is positive in the entire core region. A significant increase in variance of the Voronoi probability distribution in the core region is observed in downward flow for Stokes numbers 30 and 100, indicating stronger particle clustering than in upward flow or flow without gravity. The increased clustering in the downward flow is believed to be partly caused by the reversed wall-normal drift assisting in br...

Turbulent Couette–Poiseuille flow with zero wall shear

A particular pressure-driven flow in a plane channel is considered, in which one of the walls moves with a constant speed that makes the mean shear rate and the friction at the moving wall vanish. The Reynolds number considered based on the friction velocity at the stationary wall (uτ,S) and half the channel height (h) is Reτ,S = 180. The resulting mean velocity increases monotonically from the stationary to the moving wall and exhibits a substantial logarithmic region. Conventional near-wall streaks are observed only near the stationary wall, whereas the turbulence in the vicinity of the shear-free moving wall is qualitatively different from typical near-wall turbulence. Large-scale-structures (LSS) dominate in the center region and their spanwise spacing increases almost linearly from about 2.3 to 4.2 channel half-heights at this Reτ,S. The presence of LSS adds to the transport of turbulent kinetic energy from the core region towards the moving wall where the energy production is negligible. Energy is supplied to this particular flow only by the driving pressure gradient and the wall motion enhances this energy input from the mean flow. About half of the supplied mechanical energy is directly lost by viscous dissipation whereas the other half is first converted from mean-flow energy to turbulent kinetic energy and thereafter dissipated.

How to Discriminate Between Light and Heavy Particles in Turbulence

Light and heavy particles behave completely different in a turbulent flow field. In the present communication we discuss how one can discriminate between light and heavy particle behavior and how the important class of intermediate particles is identified. This latter class of particles exhibit challenging dynamics since they are affected both by inertia and Stokes drag, i.e. \( {{d{\bf v}} / {dt}} = St^{ - 1} \left( {{\bf u} - {\bf v}} \right)\), where u and v are the Eulerian fluid and Lagrangian particle velocities. This Lagrangian equation is expressed in non-dimensional form where t and St denote time and particle relaxation time \( \tau = 2/9\left( {\rho _p /\rho _f } \right)a^2 /v_f \) normalized by an arbitrarily chosen time scale. This expression for τ is applicable for spherical particles with radius a smaller than the smallest length scales of the flow and provided that the particle Reynolds number is below unity. However, for the dimensionless particle relaxation time St, i.e. the socalled Stokes number, to play a distinguishing role in the particle dynamics, the choice of time scale becomes essential. With a properly selected time scale, the particle motion is dominated by Stokes drag if St >1.