Control of purely-elastic instabilities in cross-slot
geometries
Mahdi Davoodi
Schlumberger Cambridge Research, UK
The cross-slot stagnation point flow is one of
the benchmark problems in fluid mechanics as it allows large strains to develop
and can therefore be used in extensional rheometry.
In such a flow, for purely-elastic cases in creeping flow regimes, elasticity
can break symmetry which is perhaps an unwanted phenomenon if used as a
rheometer and will limit the maximum deformation rate in which these tests can
be performed, or beneficial once used as mixing device. Here, this instability
will be investigated in more detail using a combination of numerical,
experimental and analytical analysis and a series of methods will be proposed
that can potentially be used to delay and control the start point of the
instability.
The first part of this presentation focuses on
the effect of elongational dominated flow on the onset criteria of
symmetry-breaking purely-elastic instability in the cross-slot geometry by
applying a fundamental change on the kinematics of the flowfield
in this region. Here, the standard geometry is modified by adding a cylinder at
the geometric centre and replacing the free
stagnation point by pinned stagnation points at the cylinder walls.
Next, two-phase flows of Newtonian and/or
viscoelastic fluids in a “cross-slot” geometry will be investigated in the
creeping-flow limit. In this part, the effect of injecting two fluids with
different elastic properties from each inlet arm, and effects of interfacial
tension and the viscosity ratio of these flow streams will be investigated.
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