Particle-laden turbulent flow is a ubiquitous type of multiphase flow in numerous natural and industrial processes. It depicts a large number of dispersed particles, which could be solid particles, liquid droplets, or gas bubbles with density different from the carrier fluid, suspending or settling in turbulence. Many of our activities and living quality are related to particle-laden turbulent flow, such as weather prediction, pollutant transportation in the atmosphere and ocean, fuel combustion in engines, CO₂ sequestration for mitigation of greenhouse effect, etc. Usually, the carrier fluid is a turbulent flow and due to the multi-scale property of turbulence the interaction between dispersed particles and turbulence is not solely limited to the small scale eddies but also include eff...[ Read more ]

Particle-laden turbulent flow is a ubiquitous type of multiphase flow in numerous natural and industrial processes. It depicts a large number of dispersed particles, which could be solid particles, liquid droplets, or gas bubbles with density different from the carrier fluid, suspending or settling in turbulence. Many of our activities and living quality are related to particle-laden turbulent flow, such as weather prediction, pollutant transportation in the atmosphere and ocean, fuel combustion in engines, CO₂ sequestration for mitigation of greenhouse effect, etc. Usually, the carrier fluid is a turbulent flow and due to the multi-scale property of turbulence the interaction between dispersed particles and turbulence is not solely limited to the small scale eddies but also include effect of the large scales. Furthermore, the multiplicity of physical properties of the dispersed particles should also be considered in order to fully understand the mechanisms of their interaction occurring in various natural and industrial processes.

In this study, experimental and numerical techniques are developed to investigate the clustering behaviour and settling velocity of inertial particles in both isotropic and anisotropic turbulence. In the experimental investigation, an optical technique making of different light wavelengths emitted from seeding and inertial particles is adopted to separate the fluid and particle phases. Simultaneous particle image velocimetry (PIV) and particle tracking velocimetry (PTV) measurement technique is used to obtain instantaneous velocities of the two phases on a measurement plane. In the numerical investigation, direct numerical simulation (DNS) is used to solve the Navier-Stokes equation for fluid phase and resolve all turbulent scales down to Kolmogorov scales. For the particle phase, the Maxey-Riley equation is used in the Lagrangian framework.

The experimental results show that the reduction of settling velocity of inertial particles in turbulence is a joint consequence of effects of small and large turbulence scales and gravity. The Stokes number representing solely the small scale effect can only partially determine whether settling velocity of inertial particles is enhanced or reduced. From the experimental results, a multiscale parameter Sv_ηRe_L^1/2 including all the effect of small and large turbulence scales and gravity is proposed to represent the multiscale interaction between the two phases. In addition, the results also show that the clustering of bidisperse inertial particles is weaker than the corresponding monodisperse inertial particles and the normalized settling velocity of the mixed situation is larger.

The simulation results show that clustering and settling velocity of bidisperse inertial particles in a turbulent channel flow is determined by the effective volume fraction ratio of small and large inertial particles when the interaction between two-phase is included (i.e. two-way coupling simulation). When increasing the initial particle number ratio, i.e., decreasing the effective volume fraction ratio, clustering of small and large particles in the bidisperse cases are both enhanced, while their settling velocities are both reduced. Regardless of the different effective volume fraction ratio, weaker clustering of bidisperse inertial particles is still observed in the logarithmic layer, which is consistent with experimental results even though the Stokes number in simulations is higher. Using three different vortex identification methods, it is confirmed that Rortex is a better indicator of vortices in turbulence to represent the mechanism of preferential concentration.