Settling Velocity and Turbulence Modulation of Bidisperse Inertial Particles in Turbulent Flows

The suspension and settling of fine particles with densities larger than the carrier fluid occur extensively in many natural processes. Examples include sands or pollutants suspended in the atmosphere, sediment transport in rivers, and turbidity currents in coastal waters. The carrier flow is always turbulent and the interactions between the fluid phase and the particle phase are of great significance in these particle-laden flows. Most of the previous experiments and numerical simulations focused on the situation of the diameter of the inertial particles being a fixed single value, which is referred to as monodisperse particles ^{[1- 3]}. In many real-life flows, the inertial particles in turbulence will have different sizes, that is, polydisperse particles ^[4,5]. However, the effects of flow turbulence on the motion of polydisperse inertial particles and the modulation on turbulence induced by the presence of these particles are still not fully understood. Motivated by this, this experimental study aims to investigate the settling velocity of bidisperse inertial particles and the modulation of turbulent characteristics induced by bidisperse inertial particles.

The experiments were carried out in a laboratory flume in which a turbulent open channel flow with different turbulence intensities generated by turbulence grids of different sizes. The bidisperse inertial particles had diameters at and and they were released from above the water surface. Sieves were used to produce dilute particle groups falling in the turbulent water flow. Simultaneous time-resolved particle image velocimetry (PIV) and particle tracking velocimetry (PTV) measurements were made to obtain the turbulent fluid flow fields and the settling velocity fields of the bidisperse inertial particles, respectively. The inertial particles were tagged with fluorescent coating and the PIV and PTV cameras were equipped with optical filters of different colours. This technique efficiently separated the optical signals of the two phases and effectuated the investigation of interaction between flow fluids and inertial particles.

References

[1] Bec J, Homann H, Ray SS (2014) Physical Review Letters, 112(18), 1–5.

[2] Yang TS, Shy SS (2003) Physics of Fluids, 15(4), 868–880.

[3] Good GH, Ireland PJ, Bewley GP, Bodenschatz E, Collins LR, Warhaft Z (2014) Journal of Fluid Mechanics, 759, R3.

[4] Dhariwal R, Bragg AD (2018) Journal of Fluid Mechanics, 839, 594–620.

[5] Saw EW, Shaw RA, Salazar JPLC, Collins LR (2012) New Journal of Physics, 14, 105031.