High Resolution Simulations of Porous Fractal Aggregates Settling through Density Gradients

We perform Particle-Resolved Direct Numerical Simulations (PR-DNS) on rigidly-bonded fractal aggregates of spherical particles settling across varying density gradients. These simulations are performed to study the impact of the complex, fractal geometry on the settling of the aggregate, particularly as it passes into a region of high density fluid while retaining less dense fluid in the pore space between individual particles. The study of these aggregates and their behaviors has applications in a broad range of problems in environmental fluid dynamics. We investigate over many simulations the velocity and rotation of the aggregates, varying the number of particles N in an aggregate, the aggregate’s fractal dimension D_f, and the associated fractal prefactor k_f, which together all characterize the overall structure of the cluster. We compare these results with the settling of spheres of comparable volume to the aggregate, to discuss the effect that the shape of the aggregate has on its motion. From these results, we intend to develop general relationships between the size of the aggregate and its fractal parameters to its overall settling velocity during sedimentation.