Effects of aerodynamic interactions on the motion of heavy particles in a bidisperse suspension

Abstract

The effect of local aerodynamic interactions on the motion of heavy particles in a bidisperse suspension in both quiescent and turbulent air is studied by a hybrid simulation and a theoretical treatment. The particles are assumed to be small in size compared to the Kolmogorov length of the carrier air turbulence, and Stokes disturbance flows are used to represent the effect of particles. We first consider the case of no background air turbulence to validate the numerical and theoretical approaches by comparing with previous results in suspension mechanics. In a bidisperse suspension with background air turbulence, in addition to the previously known increase due to preferential sweeping, aerodynamic interactions contribute to a second augmentation in mean settling rate which depends on the flow dissipation rate. This additional increase in settling rate due to local aerodynamic interactions is also coupled with preferential concentration, in agreement with the experimental observations of Aliseda et al. (2002, Journal of Fluid Mechanics, 468, 77–105). In all cases, the numerical results are explained by the theoretical approach.

Keywords::

• Particle/fluid flows
• Suspensions
• Turbulent flows
• Sediment transport

Acknowledgements

This study has been supported by the National Science Foundation through grants ATM-0114100 and ATM-0527140, and by the National Center for Atmospheric Research (NCAR). NCAR is sponsored by the National Science Foundation. The support of NCAR Faculty Fellowship to LPW is gratefully acknowledged. LPW thanks Professor Martin Maxey of Brown University for exposing him the literature of suspension mechanics and for providing insightful comments. Most of the simulations were conducted using the SGI Origin 3800/2100 at NCAR. OA is grateful to the additional computing resources provided by the Scientific Computing Division at NCAR.

Notes

¹The terms ‘particles’ and ‘droplets’ are used interchangeably in this paper. Cloud droplets are small and behave like solid particles as far as the viscous drag is concerned [48]. “Particles” is a more general term for other applications to which the current study may also be relevant.

²The term ‘aerodynamic interaction’ and ‘hydrodynamic interaction’ are used interchangeably in this paper. Traditionally, the term ‘hydrodynamic interaction’ is used in the suspension mechanics to represent mutual interactions of finite-size solid particles suspended in a viscous liquid. Here in our application the fluid medium is air, therefore, “aerodynamic interaction” would be a better term. The physics of the two are identical, regardless if the carrier medium is a liquid or a gas.