Clusters and coherent voids in particle-laden wake flow

Inertial point particles suspended in a two-dimensional unsteady circular cylinder flow at Re = 100 are studied by one-way coupled three-dimensional numerical simulations. The striking clustering pattern in the near-wake is strongly correlated with the periodically shed Kármán vortex cells. The particles are expelled from the vortex cores due to the centrifugal mechanism and coherent voids encompassing the local Kármán cells are therefore observed. The particle clustering at the upstream side of each void hole form a smooth edge, where the particle velocity magnitude is consistently lower than at the downstream edge of the voids. The trajectories of these particles originate from the side of the cylinder where the sign of vorticity is opposite to that of the vortex encompassed by the corresponding void hole. The particles are seen to decelerate along a substantial part of their trajectories. Particle inertia is parameterized by means of a Stokes number Sk and smooth edges around the void holes still exist when Sk is increased, although their formation is delayed due to larger inertia. Increasing inertia contributes to a decoupling of the particle acceleration from the slip velocity, which almost coincided at Sk = 1publishedVersio

A novel preparation method for ZnO/γ-Al_2O_3 nanofibers with enhanced absorbability and improved photocatalytic water-treatment performance by Ag nanoparticles

A novel method for synthesizing ZnO/γ-Al_2O_3 nanofibers by electrospinning and subsequent calcination is reported. The prepared nanofibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The ZnO/γ-Al_2O_3 nanofibers exhibited excellent capacity for adsorbing organics with a negative zeta potential such as methyl orange (95.8%) and heavy metal ions such as Cr(VI) in aqueous solution. The mechanism of adsorption was investigated, and the adsorption results were fitted using the Langmuir and Freundlich models. Once silver nanoparticles (Ag NPs) were decorated on the surface of the nanofibers by photoreduction, the Ag/ZnO/γ-Al_2O_3 nanofibers manifested efficient photocatalytic degradation of methyl orange under UV-light illumination. Results confirmed that our Ag/ZnO/γ-Al_2O_3 nanofibers are a promising adsorbent for the removal of methyl orange and Cr(VI) ions and the adsorbent can be sustainably reused

A novel preparation method for ZnO/γ-Al_2O_3 nanofibers with enhanced absorbability and improved photocatalytic water-treatment performance by Ag nanoparticles

A novel method for synthesizing ZnO/γ-Al_2O_3 nanofibers by electrospinning and subsequent calcination is reported. The prepared nanofibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The ZnO/γ-Al_2O_3 nanofibers exhibited excellent capacity for adsorbing organics with a negative zeta potential such as methyl orange (95.8%) and heavy metal ions such as Cr(VI) in aqueous solution. The mechanism of adsorption was investigated, and the adsorption results were fitted using the Langmuir and Freundlich models. Once silver nanoparticles (Ag NPs) were decorated on the surface of the nanofibers by photoreduction, the Ag/ZnO/γ-Al_2O_3 nanofibers manifested efficient photocatalytic degradation of methyl orange under UV-light illumination. Results confirmed that our Ag/ZnO/γ-Al_2O_3 nanofibers are a promising adsorbent for the removal of methyl orange and Cr(VI) ions and the adsorbent can be sustainably reused

Inertial torque on a small spheroid in a stationary uniform flow

How anisotropic particles rotate and orient in a flow depends on the hydrodynamic torque they experience. Here we compute the torque acting on a small spheroid in a uniform flow by numerically solving the Navier-Stokes equations. Particle shape is varied from oblate (aspect ratio λ = 1 / 6 ) to prolate ( λ = 6 ) , and we consider low and moderate particle Reynolds numbers ( Re ≤ 50 ) . We demonstrate that the angular dependence of the torque, predicted theoretically for small particle Reynolds numbers, remains qualitatively correct for Reynolds numbers up to Re ∼ 10 . The amplitude of the torque, however, is smaller than the theoretical prediction, the more so as Re increases. For Re larger than 10, the flow past oblate spheroids acquires a more complicated structure, resulting in systematic deviations from the theoretical predictions. Overall, our numerical results provide a justification of recent theories for the orientation statistics of ice crystals settling in a turbulent flow.publishedVersio

Particles in wall turbulence

The overall objective of this doctoral thesis is to investigate how tiny particles are dispersed in a turbulent channel flow and how the fluid turbulence is modulated due to the presence of particles. Both spherical and prolate spheroidal particles are considered in present work and their translational and rotational motions are described in a Lagrangian framework using point-particle approximation. The turbulence of the fluid phase is obtained by means of direct numerical simulation (DNS). Accounting for the feed-backs from the particles onto the flow field, a novel scheme of torque-coupling has been developed and implemented together with the more conventional force-coupling in current program. In the case of spherical particle Reynolds number effect on particle velocity statistics and Stokes number effect on the particle slip velocity have been explored in dilute suspension flow with one-way coupling approach. The study on turbulence modulations by spherical particles has been carried out in two-way coupled simulations. The effects of Stokes number and particle volume fraction have been mainly investigated and the mechanisms of particle-turbulence interaction were elucidated by kinetic energy transfer and particle induced dissipation between particle phase and local fluid phase. In the case of prolate spheroidal particle both torque-coupling and force-coupling has been applied in the simulations of particle suspension turbulent shear flow in which the turbulence modulations and particle orientations and spins have been examined. Some results on the comparison of Lagrangian approach and statistical approach in simulating the particle suspensions were presented

Bow Shock Clustering in Particle-laden Wetted Cylinder Flow

The inhomogeneous distributions of inertial particles in the unsteady flow around a wetted circular cylinder at Re=100 were investigated in one-way coupled three-dimensional numerical simulations. The Kármán vortex cells determine the local patterns of particle clustering. In the vicinity of the cylinder, a bow shock-like clustering was explored via Voronoï diagrams. The shape of the bow shock in terms of width and thickness strongly depends on Stokes number, Sk. For high-Sk particles, the bow shock was wider and less affected by the vortex shedding in the near wake and thus extended further downstream. We tracked low-Sk particles in a steady potential flow aimed at mimicking the viscous flow field close to the cylindrical surface. The converging tendency of the particle trajectories at the upstream of the cylinder supported the appearance of a bow shock. Furthermore, we analysed the trajectory of a Sk=0.1 particle in viscous flow, wherein the direction of the perpendicular component of the Stokes drag force was altered from outwards to inwards at the inflection point of the trajectory. This observation suggests that the centrifugal mechanism makes particle trajectories converge and thereby contributes to the formation of the bow shock.publishedVersio