Monte Carlo simulation of hydrodynamic drag and thermophoresis of fractal aggregates of spheres in the free-molecule flow regime

The hydrodynamic drag and thermophoretic properties of fractal-like aggregates of spheres in the free-molecular flow regime are examined using a Monte Carlo method. A modification of the sampling procedure that is used to assign velocities to the computational molecules is developed, and allows for a relatively fast and accurate determination of the particle transport properties for small-Mach number (i.e., slow-moving) conditions. Aggregate targets in the simulation are generated using an algorithm which mimics cluster–cluster aggregation, and simulations are performed on aggregates with a range of fractal dimensions and structure factors. The results indicate that for fractal dimensions in the range of Df=1.7Df=1.7–2.0, the hydrodynamic radius of the aggregate scales as am∼aNS0.47, in which aa is the sphere radius and NSNS is the number of spheres. Results also show that the thermophoretic velocity of a randomly-oriented aggregate will be slightly larger than that of the isolated spheres. The enhancement in velocity increases with NSNS, yet for relatively large-scale aggregates with NS=3000NS=3000 the increase is only around a factor of 1.08.