Abstract
The nature and behaviour of the drag coefficient
$C_D$
of irregularly shaped grains within a wide range of Reynolds numbers
$Re$
is discussed. The morphology of the grains is controlled by their fractal description, and they differ in shape. Using computational fluid dynamics tools, the characteristics of the boundary layer at high
$Re$
has been determined by applying the Reynolds-averaged Navier–Stokes turbulence model. Both grid resolution and mesh size dependence are validated with well-reported previous experimental results applied in flow around isolated smooth spheres. The drag coefficient for irregularly shaped grains is shown to be higher than that for spherical shapes, also showing a strong drop in its value at high
$Re$
. This drag crisis is reported at lower
$Re$
compared to the smooth sphere, but higher critical
$C_D$
, demonstrating that the morphology of the particle accelerates this crisis. Furthermore, the dependence of
$C_D$
on
$Re$
in this type of geometry can be represented qualitatively by four defined zones: subcritical, critical, supercritical and transcritical. The orientational dependence for both particles with respect to the fluid flow is analysed, where our findings show an interesting oscillatory behaviour of
$C_D$
as a function of the angle of incidence, fitting the results to a sine-squared interpolation, predicted for particles within the Stokes laminar regime (
$Re\ll 1$
) and for elongated/flattened spheroids up to
$Re=2000$
. A statistical analysis shows that this system satisfies a Weibullian behaviour of the drag coefficient when random azimuthal and polar rotation angles are considered.
Funder
Australian Research Council
Fondo Nacional de Desarrollo Científico y Tecnológico
Alexander von Humboldt-Stiftung
Publisher
Cambridge University Press (CUP)