On the role of the lift force in turbulence simulations of particle deposition

Most calculations of particle deposition in turbulent boundary layers have been performed using an equation of motion in which the form for the lift force is that in a linear shear flow for a particle far from any boundaries, the so-called Saffman formula. Both direct and large eddy simulations of particle deposition in turbulent channel flow have shown that the dependence of the deposition velocity on particle relaxation time is over-predicted using the Saffman force. Since the derivation of the Saffman force there have been more general derivations of the lift on a particle in a shear flow. In this paper an 'optimum' lift force is formulated which represents the most accurate available description of the force acting on a particle in a wall-bounded shear flow. The effect of the force was examined through large eddy simulation (LES) of particle deposition in vertical turbulent channel flow. The optimum force for depositing particles is approximately three times smaller than the lift obtained using the Saffman formula. LES results also show that use of the optimum force yields a dependence of the deposition velocity on particle relaxation time less than that obtained using the Saffman form and in better agreement with experimental measurements. Neglecting the lift force altogether leads to an even smaller dependence of the deposition velocity on particle relaxation time and is in better agreement with empirical relations, although the deposition rates are smaller than experimental measurements for particles with intermediate relaxation times.