Buoyant Finite-size Particles Research Articles

How do the finite-size particles modify the drag in Taylor–Couette turbulent flow

We experimentally investigate the drag modification by neutrally buoyant finite-size particles with various aspect ratios in a Taylor–Couette (TC) turbulent flow. The current Reynolds number, $Re$ , ranges from $6.5\times 10^3$ to $2.6\times 10^4$ , and the particle volume fraction, $\varPhi$ , is up to $10\,\%$ . Particles with three kinds of aspect ratio, $\lambda$ , are used: $\lambda =1/3$ (oblate), $\lambda =1$ (spherical) and $\lambda =3$ (prolate). Unlike the case of bubbly TC flow (van Gils et al., J. Fluid Mech., vol. 722, 2013, pp. 317–347; Verschoof et al., Phys. Rev. Lett., vol. 117, issue 10, 2016, p. 104502), we find that the suspended finite-size particles increase the drag of the TC system regardless of their aspect ratios. The overall drag of the system increases with increasing $Re$ , which is consistent with the literature. In addition, the normalized friction coefficient, $c_{f,\varPhi }/c_{f,\varPhi =0}$ , decreases with increasing $Re$ , the reason could be that in the current low volume fractions the turbulent stress becomes dominant at higher $Re$ . The particle distributions along the radial direction of the system are obtained by performing optical measurements at $\varPhi =0.5\,\%$ and $\varPhi = 2\,\%$ . As $Re$ increases, the particles distribute more evenly in the entire system, which results from both the greater turbulence intensity and the more pronounced finite-size effects of the particles at higher $Re$ . Moreover, it is found that the variation of the particle aspect ratios leads to different particle collective effects. The suspended spherical particles, which tend to cluster near the walls and form a particle layer, significantly affect the boundary layer and result in maximum drag modification. The minimal drag modification is found in the oblate case, where the particles preferentially cluster in the bulk region, and, thus, the particle layer is absent. Based on the optical measurement results, it can be concluded that, in the low volume fraction ranges ( $\varPhi =0.5\,\%$ and $\varPhi = 2\,\%$ here), the larger drag modification is connected to the near-wall particle clustering. The present findings suggest that the particle shape plays a significant role in drag modification, and the collective behaviours of rigid particles provide clues to understand the bubbly drag reduction.

Modulation of turbulence by finite-size particles in statistically steady-state homogeneous shear turbulence

Abstract

Direct numerical simulation of wall-to-liquid heat transfer in turbulent particle-laden channel flow

Direct numerical simulation on the turbulent flow between two parallel walls under constant heat flux reveals that, adding neutrally buoyant finite-size particles at volume fractions of 0.1 and 0.3 and bulk Reynolds numbers of 3800 and 5600 in general reduces the Nusselt number on average but enhances it near the walls. The enhancement is contributed by the near-wall streamwise vortex intensification, which is more significant for spheres than for prolates and oblates. These findings are helpful to intensify the wall-to-liquid heat transfer in particle-laden turbulent flow.

Experimental study of negatively buoyant finite-size particles in a turbulent boundary layer up to dense regimes

We experimentally investigate the two-phase interplay in an open-channel turbulent boundary layer laden with finite-size particles at global volume fractions between 4 and 25 %. The working fluid (water) and the dispersed phase (hydrogel spheres) have closely matched refractive indices, allowing us to measure the properties of both phases using particle image velocimetry and particle tracking velocimetry, respectively. The particles have a diameter of approximately 9 % of the channel depth and are slightly denser than the fluid. The negative buoyancy causes a strong vertical concentration gradient, characterized by discrete and closely spaced particle layers parallel to the wall. Even at the lowest considered volume fractions, the near-wall fluid velocity and velocity gradients are strongly reduced, with large mean shear throughout most of the channel height. This indicates that the local effective viscosity of the suspension is greatly increased due to the friction between particle layers sliding over one another. The particles consistently lag the fluid and leave their footprint on its mean and fluctuating velocity profiles. The turbulent activity is damped near the wall, where the nearly packed particles disrupt and suppress large-scale turbulent fluctuations and redistribute some of the kinetic energy to smaller scales. On the other hand, in the outer region of the flow where the local particle concentration is low, the mean shear produces strong Reynolds stresses, with enhanced sweeps and ejections and frequent swirling events.

Buoyant finite-size particles in turbulent duct flow

Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) have been employed to investigate the dynamics of finite-size spherical particles, slightly heavier than the carrier fluid, in a horizontal turbulent square duct flow. Interface resolved Direct Numerical Simulations (DNS) have also been performed with the Immersed Boundary Method (IBM) at the same experimental conditions, bulk Reynolds number $Re_{2H}$ = 5600, duct height to particle size ratio $2H/d_p$ = 14.5, particle volume fraction $\Phi$ = 1% and particle to fluid density ratio $\rho_p/\rho_f$ = 1.0035. A good agreement has been observed between experiments and simulations in terms of the overall pressure drop, concentration distribution and turbulent statistics of the two phases. Additional experimental results considering two particle sizes, $2H/d_p$ = 14.5 and 9 and multiple $\Phi$ = 1, 2, 3, 4 and 5% are reported at the same $Re_{2H}$. The pressure drop monotonically increases with the volume fraction, almost linearly and nearly independently of the particle size for the above parameters. However, despite the similar pressure drop, the microscopic picture, the fluid velocity statistics, differs significantly with the particle size. This one-to-one comparison between simulations and experiments extends the validity of interface resolved DNS in complex turbulent multiphase flows and highlights the ability of experiments to investigate such flows in considerable details, even in regions where the local volume fraction is relatively high.

Modification of turbulence and stratification of stably stratified turbulent channel flows by finite-size particles

Neutrally buoyant finite-size particles laden in stratified turbulent channel flows are found to enhance internal gravity waves, thus blocking heat transfer. In the near-wall region, these particles are preferentially located in the high-speed streaks.

Modulation of the regeneration cycle by neutrally buoyant finite-size particles

Direct numerical simulations of turbulent suspension flows are carried out with the force-coupling method in plane Couette and pressure-driven channel configurations. Dilute to moderately concentrated suspensions of neutrally buoyant finite-size spherical particles are considered when the Reynolds number is slightly above the laminar–turbulent transition. Tests performed with synthetic streaks, in both turbulent channel and Couette flows, show clearly that particles trigger the instability in channel flow whereas the plane Couette flow becomes laminar. Moreover, we have shown that particles have a pronounced impact on pressure-driven flow through a detailed temporal and spatial analysis whereas they have no significant impact on the plane Couette flow configuration. The substantial difference between the two flow configurations is related to the spatial preferential distribution of particles in the large-scale rolls (inactive motion) in Couette flow, whereas they are accumulated in the ejection (active motion) regions in pressure-driven flow. Through investigation of particle modification in two distinct flow configurations, we were able to show the specific response of turbulent structures and the modulation of the fundamental mechanisms composing the regeneration cycle in the buffer layer of the near-wall turbulence. Especially for pressure-driven flow, the particles enhance the lift-up and let it act continuously whereas the particles do not significantly alter the streak breakdown process. The reinforcement of the streamwise vortices is attributed to the vorticity stretching term by the wavy streaks. The smaller and more numerous wavy streaks enhance the vorticity stretching and consequently strengthen the circulation of large-scale streamwise vortices in suspension flow.

Dissipative inertial transport patterns near coherent Lagrangian eddies in the ocean.

Recent developments in dynamical systems theory have revealed long-lived and coherent Lagrangian (i.e., material) eddies in incompressible, satellite-derived surface ocean velocity fields. Paradoxically, observed drifting buoys and floating matter tend to create dissipative-looking patterns near oceanic eddies, which appear to be inconsistent with the conservative fluid particle patterns created by coherent Lagrangian eddies. Here, we show that inclusion of inertial effects (i.e., those produced by the buoyancy and size finiteness of an object) in a rotating two-dimensional incompressible flow context resolves this paradox. Specifically, we obtain that anticyclonic coherent Lagrangian eddies attract (repel) negatively (positively) buoyant finite-size particles, while cyclonic coherent Lagrangian eddies attract (repel) positively (negatively) buoyant finite-size particles. We show how these results explain dissipative-looking satellite-tracked surface drifter and subsurface float trajectories, as well as satellite-derived Sargassum distributions.

The effect of neutrally buoyant finite-size particles on channel flows in the laminar-turbulent transition regime

The presence of finite-size particles in a channel flow close to the laminar-turbulent transition is simulated with the Force Coupling Method which allows two-way coupling with the flow dynamics. Spherical particles with channel height-to-particle diameter ratio of 16 are initially randomly seeded in a fluctuating flow above the critical Reynolds number corresponding to single phase flow relaminarization. When steady-state is reached, the particle volume fraction is homogeneously distributed in the channel cross-section (ϕ ≅ 5%) except in the near-wall region where it is larger due to inertia-driven migration. Turbulence statistics (intensity of velocity fluctuations, small-scale vortical structures, wall shear stress) calculated in the fully coupled two-phase flow simulations are compared to single-phase flow data in the transition regime. It is observed that particles increase the transverse r.m.s. flow velocity fluctuations and they break down the flow coherent structures into smaller, more numerous and sustained eddies, preventing the flow to relaminarize at the single-phase critical Reynolds number. When the Reynolds number is further decreased and the suspension flow becomes laminar, the wall friction coefficient recovers the evolution of the laminar single-phase law provided that the suspension viscosity is used in the Reynolds number definition. The residual velocity fluctuations in the suspension correspond to a regime of particulate shear-induced agitation.