3 research outputs found
Clustering of vertically constrained passive particles in homogeneous, isotropic turbulence
We analyze the dynamics of small particles vertically confined, by means of a
linear restoring force, to move within a horizontal fluid slab in a
three-dimensional (3D) homogeneous isotropic turbulent velocity field. The
model that we introduce and study is possibly the simplest description for the
dynamics of small aquatic organisms that, due to swimming, active regulation of
their buoyancy, or any other mechanism, maintain themselves in a shallow
horizontal layer below the free surface of oceans or lakes. By varying the
strength of the restoring force, we are able to control the thickness of the
fluid slab in which the particles can move. This allows us to analyze the
statistical features of the system over a wide range of conditions going from a
fully 3D incompressible flow (corresponding to the case of no confinement) to
the extremely confined case corresponding to a two-dimensional slice. The
background 3D turbulent velocity field is evolved by means of fully resolved
direct numerical simulations. Whenever some level of vertical confinement is
present, the particle trajectories deviate from that of fluid tracers and the
particles experience an effectively compressible velocity field. Here, we have
quantified the compressibility, the preferential concentration of the
particles, and the correlation dimension by changing the strength of the
restoring force. The main result is that there exists a particular value of the
force constant, corresponding to a mean slab depth approximately equal to a few
times the Kolmogorov length scale, that maximizes the clustering of the
particles