Particles In Turbulent Channel Flow Research Articles

Clustering, rotation, and swirl of inertial particles in turbulent channel flow

Clustering dynamics of inertial particles in turbulent channel flow are studied via tessellation-based analysis of high-fidelity simulation data at Reτ≈230 with various values of mass loading (10%−100%) and the Stokes number (St+=[1−60]). We then characterise the solenoidal, rotational, and swirling motions of clusters by computing the probability density functions (PDFs) of the divergence, curl, and helicity of the particle velocity, as well as their dependence on wall-normal distance, using the methods of Oujia et al. (2020); Maurel–Oujia et al. (2023). Particle inertia gives heavier tails to the PDFs of divergence and curl, suggesting enhanced intermittency in the convergence/divergence of clusters, and in their rotational motions. The fluctuations of the divergence and curl are most intense in the buffer layer, due to the stronger fluctuations of fluid velocity there. Similarities are identified between the cluster dynamics in the logarithmic region and those in homogeneous isotropic turbulence, including the dependence of divergence, curl, and helicity on Stokes number. The effect of increasing mass loading on cluster dynamics is relatively small except in the viscous sublayer, where attenuation of clustering, rotation, and swirling motions are observed. The effect of increasing Stokes number on the viscous sublayer is different, resulting in more intense convergence/divergence and rotation of particle clusters, as the particles become more independent of the carrier fluid.

On the Preferential Concentration of Particles in Turbulent Channel Flow: The Effect of the Added-Mass Factor

Preferential concentration, observed in turbulent flows when particle response times are of the same order of the flow’s characteristic timescales, manifests as non-uniform particle distributions in space. Unraveling its governing mechanisms holds crucial implications for both natural and industrial processes reliant on particle-laden flows. Focusing on particles with small inertia, this study employs Direct Numerical Simulations coupled with Lagrangian particle tracking to investigate the influence of the added-mass factor on the preferential concentration of particles denser than the fluid in the one-way coupling regime. It is shown how the added-mass factor β affects particle distribution within the channel through the statistical correlations between particle concentration and typical descriptors of the flow topology. The results suggest that increasing values of β (corresponding to lighter particles) significantly reduce the effectiveness of turbophoresis in producing particle accumulation in the near-wall region. Resulting in a gradual decorrelation between particle concentration and both the strain-rate and the vorticity tensors, higher values of β lead to a more uniform particle distribution, regardless of the Stokes number.

Effect of fluid inertial torque on the rotational and orientational dynamics of tiny spheroidal particles in turbulent channel flow

Rotation and orientation of non-spherical particles in a fluid flow depend on the hydrodynamic torque they experience. However, little is known about the effect of the fluid inertial torque on the dynamics of tiny inertial spheroids in turbulent channel flows, as only Jeffery torque has been considered in previous studies by point-particle direct numerical simulations. In this study, we investigate the rotation and orientation of tiny spheroids with both fluid inertial torque and Jeffery torque in a turbulent channel flow. By comparing with the case in the absence of fluid inertial torque, we find that the rotational and orientational dynamics of spheroids is significantly affected by the fluid inertial torque when the Stokes number, which is non-dimensionalized by fluid viscous time scale, is larger than the critical value $St_c\approx 2$ , indicating that the fluid inertial torque is non-negligible for most particle cases considered in earlier studies. In contrast to the earlier findings considering only Jeffery torque (Challabotla et al., J. Fluid Mech., vol. 776, 2015, p. R2), we find that prolate (oblate) spheroids with a large Stokes number tend to tumble (spin) in the streamwise–wall-normal plane in a thinner region near the wall due to the presence of the fluid inertial torque. Approaching the channel centre, the flow shear gradually vanishes, but the velocity difference between local fluid and particles is still pronounced and increasing as particle inertia grows. As a result, in the core region, fluid inertial torque is dominant and drives the particles to align with its broad side normal to the streamwise direction rather than a random orientation observed in earlier studies without fluid inertial torque. Meanwhile, the presence of fluid inertial torque enhances the tumbling rates of spheroids in the core region. In addition, the effect of fluid inertial force on the dynamics of spheroids is also examined in this study, but the results indicate the effect of fluid inertial force is weak. Our findings imply the importance of fluid inertial torque in modelling the dynamics of inertial non-spherical particles in turbulent channel flows.

Mechanisms of drag reduction by semidilute inertial particles in turbulent channel flow

Mechanisms of drag reduction by semidilute inertial particles in turbulent channel flow

Multiscale clustering of heavy and light small particles in turbulent channel flow at high Reynolds numbers

To understand the fundamental mechanism of the clustering of heavy and light small particles in high-Reynolds-number wall turbulence, we conduct direct numerical simulations of particle dispersions in turbulent channel flow at the friction Reynolds number Reτ=2000. Heavy and light particles form clusters with different shapes, and the cluster size is larger for particles with longer relaxation time. To reveal the origin of these multiscale clusters of particles, we investigate particle distributions around multiscale vortices. Vortices tend to sweep out heavy particles and tend to attract light ones with a relaxation time comparable with their turnover time. These clustering phenomena due to coherent vortices can occur irrespective of their size, existing height and the friction Reynolds number. We can therefore explain the multiscale nature of particle clustering in terms of the time scale matching between particles and vortices.

Role of the hierarchy of coherent structures in the transport of heavy small particles in turbulent channel flow

To investigate the transport of heavy small particles (inertial particles) in high-Reynolds- number wall turbulence, we conduct direct numerical simulations of inertial particles in turbulent channel flow at the friction Reynolds number $Re_\tau =1000$ . In the statistically steady state, particles distribute inhomogeneously; particles with different relaxation times form voids with different sizes in the bulk of the flow, whereas they form streak-like clusters with different widths along the streamwise direction. To explore the origin of the multiscale voids and clusters of particles, we identify objectively the axes of tubular vortices and the spines of streaks with different sizes. These identifications enable us to show quantitatively that (i) vortices sweep out the particles of the relaxation time comparable with their turnover time, irrespective of their size and existing height, and that, among these particles, (ii) those swept out by wall-detached tubular vortices form clusters isotropically around them, whereas (iii) those swept out by wall-attached vortices are attracted by a nearby low-speed streak. These explain the reason why the multiscale clusterings are well described in terms of the local Stokes number defined by the turnover time of multiscale vortices. Furthermore, these descriptions of the particle transport give us a clear view to understand velocity statistics and wall-deposition mechanism of inertial particles in high-Reynolds-number wall turbulence.

Effect of roughness on elongated particles in turbulent channel flow

In this paper, we examine the deposition and resuspension of rigid elongated particles transported by turbulence in a channel bounded by two-dimensional roughness. To analyze these processes, we use an Euler–Lagrange approach based on Direct Numerical Simulation of the carrier phase and Lagrangian Particle Tracking of the dispersed phase. Four different channel configurations are considered: One is the classical channel flow bounded by smooth flat walls, whereas the other three correspond to a channel with walls of increasing roughness. The roughness shape is obtained by superimposition of sinusoidal functions with different amplitude k and is characterized by the mean absolute value of the amplitude, k̄= 0.012, 0.024 and 0.050 (k̄= 0 for a smooth wall). The friction Reynolds number is Reτ=150 for all cases. Particles are modeled as prolate ellipsoids and classified according to their aspect ratio λ. Three different particles sets are considered: λ=1, corresponding to the reference case of spheres, λ=3, corresponding to slightly elongated particles, and λ=10, corresponding to long fiber-like particles. The particle response time is St+=5 for all sets. In turbulent flow bounded by smooth walls, particles are known to accumulate preferentially in the near-wall region, leaving the central region of the channel scarcely populated. Wall roughness produces a completely different scenario: Particles exhibit a more homogeneous distribution along the wall-normal direction. We show that the aspect ratio does not affect the preferential distribution and the velocity statistics of the particles. The effect of elongation, however, becomes important for their preferential orientation, which is much weaker than in the smooth-walls case, in the near-wall region, while recovering the smooth-walls case in the outer region of the channel. This finding supports the validity of Townsend’s similarity hypothesis, namely that the bulk flow dynamics are unaffected by the roughening of the bounding walls.

Spatial distribution of particles in turbulent channel flow of dilute suspensions

We experimentally investigated the kinematics and suspension of particles in the near-wall region of a horizontal turbulent channel flow of water. The particles were glass beads with a diameter of 0.014H, where H is the full channel height. The experiments involved dilute particle suspensions at bulk volumetric concentrations (ϕ) of 0.05, 0.12, and 0.27%, and at Reynolds numbers (Re) of 20,200, 40,400, and 60,500. Measurement of Lagrangian position and velocity of the particles was carried out using 3D particle tracking velocimetry. At the lowest Re, the results showed a large near-wall accumulation of particles due to gravitational settling at all three bulk concentrations. The analysis of turbulence statistics suggested negligible effect due to inter-particle collisions even when the local volumetric particle concentration reached 2%. With increasing Re to 40,400 and 60,500, different concentration profiles were observed for ϕ = 0.05% with respect to the profiles of ϕ = 0.12 and 0.27%. A bi-model particle concentration distribution, with an inner and outer peak, was present for ϕ = 0.05% at Re = 40,400 and 60,500. In contrast, the particles at ϕ = 0.12 and 0.27% had a wall-peaking profile. The autocorrelation function of the streamwise Lagrangian velocity and wall-normal dispersion of the particles showed greater stability of particle motions in the near-wall region for ϕ = 0.12 and 0.27% with respect to ϕ = 0.05% at the two higher Re. The observations are associated with a stronger particle-wall lubrication at the higher volumetric concentrations of ϕ = 0.12 and 0.27%.

Interface-resolved simulations of small inertial particles in turbulent channel flow – CORRIGENDUM

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Mitigation of turbophoresis in particle-laden turbulent channel flows by using incident electric fields

The influence of electric forces on inertial particles in turbulent channel flows is explored using direct numerical simulations. It is found that the near-wall buildup of particle concentration due to turbophoresis can be reduced up to 2 orders of magnitude by imposing an ac electric field.

Interface-resolved simulations of small inertial particles in turbulent channel flow

We present a direct comparison between interface-resolved and one-way-coupled point-particle direct numerical simulations (DNS) of gravity-free turbulent channel flow laden with small inertial part ...

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

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.

Influence of the quiescent core on tracer spheroidal particle dynamics in turbulent channel flow

Numerical studies into the dynamics of non-spherical particles in turbulent channel flow have, until now, been mostly confined to low Reynolds numbers. In this paper, we investigate the dynamics of tracer non-spherical particles in a channel flow at for the first time. Tracer spheroidal particles suspended in a turbulent channel flow are computed by means of direct numerical simulation coupled with a Lagrangian point-particle approach. We examine the rotational dynamics and alignment of tracer spheroids of different aspect ratio in both the near-wall region and, in particular, the quiescent core which is not observed at lower Reynolds numbers. In the near-wall region, the effect of Reynolds number on preferential alignment is negligible with particle rotation exhibiting a weak dependence for all aspect ratios investigated. However, the particle rotation is strongly damped in the quiescent core demonstrating that the motion of tracer spheroids becomes quiescent. The striking difference in spheroids' rotational dynamics observed inside and outside of the quiescent core is correlated with the local Kolmogorov time scale in the core and outside of it.

Acceleration statistics of prolate spheroidal particles in turbulent channel flow

ABSTRACTComputation of a dilute suspension of prolate spheroidal particles in a turbulent channel flow is undertaken to study the influence of inertia and shape on acceleration statistics. A pseudo-spectral direct numerical simulation is coupled with Lagrangian tracking under the one-way coupling assumption. Simulations are carried out at friction Reynolds number , for three aspect ratios , 3 and 10, and two Stokes numbers St=5 and 30. Results indicate that, as a consequence of the filtering effect of inertia, particle acceleration RMS decreases with increasing inertia. In addition, the normalised streamwise acceleration PDFs depart from that of the conditional fluid and their tails become narrower as inertia is increased. Furthermore, acceleration statistics show that particle elongation has a significant effect on the mean drag and on the particle selective sampling. Moreover, the classification of elongated particle behaviour in turbulent channel flow based on a global Stokes number is questioned. The zero-crossing acceleration autocorrelation timescales presented points out the need for a local dimensionless number estimation adapted to the case of prolate spheroids.

Clustering of Inertial Particles in Turbulent Channel Flow

Particle clustering phenomenon is relevant to many industrial and natural systems. Particles denser than the carrier fluid tend to accumulate in the high-strain low-vorticity regions, which leads to inhomogeneous distribution of particles. This phenomenon is known as “preferential concentration”. The deposition and entrainment of particles in low-Reynolds number turbulent channel flow is well understood. However, particles clustering under the influence of multi-scale vortices in high-Reynolds-number turbulent channel flow is not fully investigated. Present study aims to understand this situation through the DNS of particle-laden turbulent channel flow at Reτ = 500 and 1000.

Application limits of Jeffery’s theory for elongated particle torques in turbulence: a DNS assessment

Non-spherical particles suspended in fluid flows are subject to hydrodynamic torques generated by fluid velocity gradients. For small axisymmetric particles, the most popular formulation of hydrodynamic torques is that given by Jeffery (Proc R Soc Lond A 102:161–179, 1922), which is valid for uniform shear flow in the viscous Stokes regime. In the lack of simple alternative formulations outside the Stokes regime, the Jeffery formulation has been widely applied to inertial particles in turbulent flows, where it is bound to produce inaccurate results. In this paper we quantify the statistical error incurred when the Jeffery formulation is used to study the motion of elongated axisymmetric particles under nonlinear shear flow conditions. Considering the archetypical case of prolate ellipsoidal particles in turbulent channel flow, we show that error for ellipsoids of the same length, l, as the Kolmogorov scale of the flow, $$\eta _K$$ , is indeed small (order $$1\%$$ ) but increases exponentially up to $$l \simeq 10 \eta _K$$ before becoming almost independent of elongation.

Orientation and rotation dynamics of triaxial ellipsoidal tracers in wall turbulence

The rotational dynamics of triaxial ellipsoidal particles in turbulent channel flow have been explored. The non-inertial particles were tracked in a Lagrangian approach in an Eulerian flow field that resulted from a direct numerical simulation. Although the tracer particles translated along with the local fluid, they did not adhere to the local fluid rotation. The triaxial ellipsoids were characterized by two independent shape parameters that both were varied from 0.1 to 10. In spite of the anisotropic velocity field in the channel center, the tumbling of the particles closely resembled earlier results in homogeneous isotropic turbulence. The orientation of the particles varied substantially from the center to the near-wall region where triaxial particles tended to align their major axis in the mean flow direction and their minor axis in the wall-normal direction. These preferential alignments caused the ellipsoid to tumble as a rod about the major axis and like a disk about the minor axis. These observations show the dual nature of triaxial ellipsoids embedded in real turbulence.

Finite-size particles in turbulent channel flow: quadrant analysis and acceleration statistics

ABSTRACTThe interaction between finite-size particles and turbulent channel flow in the absence of gravity is studied by direct numerical simulations (DNS). The study is motivated by DNS observations of a turbulent channel flow with high-density, pointwise particles, that cluster in regions of high streamwise root-mean-square (RMS) acceleration close to the wall, contrary to what is observed in homogeneous isotropic turbulence. The aim of the present study is to explore if this is still the case when size effects are taken into account in the DNS. Based on the analysis of the velocity and acceleration statistics, the present DNS shows that, close to the wall, particles with ρp/ρf ranging from 2 to 4 are surrounded by regions with low streamwise RMS velocity but high streamwise RMS acceleration. According to the normalised particle acceleration probability density functions (PDFs), size effects become important in the near-wall region. As particle inertia increases, the normalised PDFs of particle acceleration tend to a Gaussian distribution. The tails of the normalised PDFs of the fluid conditioned by the presence of particles are higher than that of the unconditioned fluid close to the wall.

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.

Acceleration Statistics of Finite-Size Particles in Turbulent Channel Flow in the Absence of Gravity

The interaction between finite-size particles and turbulent channel flow in the absence of gravity is studied here by direct numerical simulation. The turbulent flow is resolved by an efficient Fourier Chebyshev method, coupled with a direct forcing method for particle tracking. For the volume fraction and particle size we studied here, the presence of particles decreases slightly the streamwise mean velocity in the near wall region. The velocity fluctuations in the vicinity of particles are dissipated more than generated by the presence of the wakes. According to the statistics of the fluid in the vicinity of particles, particles segregate in regions of low streamwise and spanwise r.m.s. velocity close to the wall. Moreover, between the region of maximum particle concentration and the wall, particles are swept by flow presenting high acceleration fluctuations.