Structural dependent drag force and orientation prediction for small fractal aggregates

Aggregates settle in viscous fluids in specific directions depending on their morphology. We investigated the settling behavior of fractal aggregates with D-f = 1.85 in fluids using the Accelerated Stokesian Dynamics method. The results have been compared with experimental data. A simple equation is proposed which gives the drag force in dependence oil the radius of gyration r(gyr), the projection area A(p) and the aggregate orientation during settling. This equation reproduces the data from ASD within +/- 10%. for Re < 1. Additionally, a new algorithm has been developed which allows a prediction of the orientation of such fractal aggregates provided that they have time enough to orient in the fluid. Moreover, the drag force acting on them can be predicted in a narrow range (maximum error 15,.). This algorithm allows a very fast evaluation of the orienting behavior with small deviations. (C) 2008 Elsevier Inc. All rights reserved.