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A viscous drop in a planar linear flow: the role of deformation on streamline topology
Published online by Cambridge University Press: 07 April 2025
Abstract
Planar linear flows are a one-parameter family, with the parameter $\hat {\alpha }\in [-1,1]$ being a measure of the relative magnitudes of extension and vorticity;
$\hat {\alpha } = -1$,
$0$ and
$1$ correspond to solid-body rotation, simple shear flow and planar extension, respectively. For a neutrally buoyant spherical drop in a hyperbolic planar linear flow with
$\hat {\alpha }\in (0,1]$, the near-field streamlines are closed for
$\lambda \gt \lambda _c = 2 \hat {\alpha } / (1 - \hat {\alpha })$,
$\lambda$ being the drop-to-medium viscosity ratio; all streamlines are closed for an ambient elliptic linear flow with
$\hat {\alpha }\in [-1,0)$. We use both analytical and numerical tools to show that drop deformation, as characterized by a non-zero capillary number (
$Ca$), destroys the aforementioned closed-streamline topology. While inertia has previously been shown to transform closed Stokesian streamlines into open spiralling ones that run from upstream to downstream infinity, the streamline topology around a deformed drop, for small but finite
$Ca$, is more complicated. Only a subset of the original closed streamlines transforms to open spiralling ones, while the remaining ones densely wind around a configuration of nested invariant tori. Our results contradict previous efforts pointing to the persistence of the closed streamline topology exterior to a deformed drop, and have important implications for transport and mixing.
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- © The Author(s), 2025. Published by Cambridge University Press