Study of the circularity effect on drag of disk-like particles

Abstract

This paper presents a study of the terminal fall velocity, drag coefficient and descent style of ‘wavy-edge’ flat particles. Being highly non-spherical and with a size of up to a few centimetres, these particles show strong self-induced motions that lead to various falling styles that result in distinct drag coefficients. This study is based on experimental measurements of the instantaneous 3D velocity and particle trajectory settling in water. A disk of D=30mm, t=1.5mm and ρ=1.38g/cm3 is manufactured as a reference particle. The disk was initially designed to lie within the Galileo number - dimensionless moment of inertia (G−I*) domain corresponding to the fluttering regime. A total of 35 other particles with the same frontal area and material properties were manufactured. These are manufactured to have different amplitudes (a) of the sinusoidal wave on the edge and number of cycles (N) around the entire perimeter. Thus, 5 sets of particles are manufactured with different relative wave amplitudes; i.e. a/D=0.03, 0.05, 0.1, 0.15, 0.2. Each set consisting of 7 particles from N=4 to N=10. The isoperimetric quotient is used as a measure of the particle circularity and is also linked with different characteristic settling behaviors. Disks and other planar particles with small a/D ratio were found to descent preferably with ‘Planar zig-zag’ behavior with events of high tilted angle. In contrast, particles with high a/D ratio were found to follow a more uniform descent with low tilted angle to the vertical motion. The differences in projected frontal area were shown not to be sufficient to compensate the differences in descent velocity, leading to unequal drag coefficients. Therefore, we believe that the falling styles of these irregular particles go hand by hand with characteristic wake structures, as shown for disks with various dimensionless moment of inertia, that enhance the descent of particles with low circularity.