title: Boundary effect on the drag force on a nonhomogeneous floc.
abstract: The boundary effect on the drag force acting on a spherical floc having a nonhomogeneous structure is examined by considering a spherical floc at the centerline of a cylindrical tube. The floc is simulated by an entity having a two-layer structure, and its porous nature mimicked by varying the relative magnitude of the permeability of its inner and outer layers. The results of numerical simulation reveal that the tube wall has the effect of compressing the streamlines and vorticity contours. Also, as in the case of a rigid entity, the wake in the rear region of a floc, which arises from the convective motion of the fluid, is depressed. For fixed volume-averaged permeability, the influence of the tube wall on the behavior of a heterogeneous floc is more significant than that on the behavior of a homogeneous floc, and the influence varies with the structure of the former. The heterogeneous structure of a floc leads to a deviation in the modified drag coefficient-Reynolds number relation from a Stokes-law-like correlation. The smaller the average permeability of a floc the greater the deviation, but the presence of the tube wall has the effect of reducing the deviation.
authors.affiliation: Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China. jphsu@ccms.ntu.edu.tw
authors.forename: Jyh-Ping
authors.initials: JP
authors.lastname: Hsu
authors.forename: Yu-Heng
authors.initials: YH
authors.lastname: Hsieh