Behavior of Particle Tracers in the Steady and Periodic Wakes of a Circular Cylinder

Shaswat Saincher, Jyotimay Banerjee

Abstract



Tracer particles introduced into the flow domain
for experimental flow visualization often tend to exhibit chaotic
behavior even for the simplest of configurations. These passive
tracers do not exactly follow the regime in general. The spatial
distributions formed by such particles within the flow field and
the probable trajectories followed by them are governed by the
phenomenon of Lagrangian chaos. Since the problem at hand
has it's foundations in the theories of Hamiltonian dynamics and
deterministic chaos, drawing a relation between the Eulerian
flow field and the Lagrangian tracer field is a difficult task.
However, analyzing this behavior would provide useful insight
in interpreting the results of flow visualization experiments
which involve particle tracking. Based on this background,
the phenomenon of chaotic advection for a laminar cross-flow
over a circular cylinder has been investigated numerically. Two
configurations are considered, one involving a steady flow at
Re=20 while the other characterizing a periodic shedding regime
at Re=100. Computational visualization of the flow is carried out
by locally injecting neutrally buoyant particles at zero momentum
and tracking the same by FLUENT's discrete phase model. The
tracks made by the tracers, being equivalent to streaklines, are
generated for different injection locations and the trajectories
have been compared with the streamline patterns. It has been
observed that the steady regime at Re=20 does not exhibit chaotic
advection and the streaklines coincide with the streamtraces. On
the other hand, the shedding regime, being temporally dependent,
depicts a high degree of tracer mixing. The particles tend to
form complex spatial distributions in this case. Further, the
formation of a particular tracer distribution has been observed
to be strongly dependent on the location of the injector within the
domain. Since the tracer particles exhibit stochastic behavior for
an otherwise simple and periodic underlying flow, it is expected
that such patterns can be misleading. A careful study of all
possible tracer distributions is hence advisable for visualization
of such time dependent flows.

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