300 research outputs found
Mixing layer instability and vorticity amplification in a creeping viscoelastic flow
We report quantitative evidence of mixing-layer elastic instability in a
viscoelastic fluid flow between two widely spaced obstacles hindering a channel
flow at and . Two mixing layers with nonuniform shear velocity
profiles are formed in the region between the obstacles. The mixing-layer
instability arises in the vicinity of an inflection point on the shear velocity
profile with a steep variation in the elastic stress. The instability results
in an intermittent appearance of small vortices in the mixing layers and an
amplification of spatio-temporal averaged vorticity in the elastic turbulence
regime. The latter is characterized through scaling of friction factor with
, and both pressure and velocity spectra. Furthermore, the observations
reported provide improved understanding of the stability of the mixing layer in
a viscoelastic fluid at large elasticity, i.e. and , and
oppose the current view of suppression of vorticity solely by polymer
additives.Comment: 6 pages, 7 figure
On the role of initial velocities in pair dispersion in a microfluidic chaotic flow
Chaotic flows drive mixing and efficient transport in fluids, as well as the
associated beautiful complex patterns familiar to us from our every day life
experience. Generating such flows at small scales where viscosity takes over is
highly challenging from both the theoretical and engineering perspectives. This
can be overcome by introducing a minuscule amount of long flexible polymers,
resulting in a chaotic flow dubbed \textit{elastic turbulence}. At the basis of
the theoretical frameworks for its study lie the assumptions of a spatially
smooth and random-in-time velocity field. Previous measurements of elastic
turbulence have been limited to two-dimensions. Using a novel three-dimensional
particle tracking method, we conduct a microfluidic experiment, allowing us to
explore elastic turbulence from the perspective of particles moving with the
flow. Our findings show that the smoothness assumption breaks already at scales
smaller than a tenth of the system size. Moreover, we provide conclusive
experimental evidence that \textit{ballistic} separation prevails in the
dynamics of pairs of tracers over long times and distances, exhibiting a memory
of the initial separation velocities. The ballistic dispersion is universal,
yet it has been overlooked so far in the context of small scales chaotic flows.Comment: 28 pages (Main Article: 17 pages ; Supplementary Information: 11
pages), 5 Main Figures, 6 Supplementary Figures, 3 Supplementary Notes,
Supplementary Reference
Drag enhancement and drag reduction in viscoelastic flow
Creeping flow of polymeric fluid without inertia exhibits elastic
instabilities and elastic turbulence accompanied by drag enhancement due to
elastic stress produced by flow-stretched polymers. However, in
inertia-dominated flow at high \mbox{Re} and low fluid elasticity , a
reduction in turbulent frictional drag is caused by an intricate competition
between inertial and elastic stresses. Here, we explore the effect of inertia
on the stability of viscoelastic flow in a broad range of control parameters
and (\mbox{Re}, \mbox{Wi}). We present the stability diagram of observed
flow regimes in \mbox{Wi}-\mbox{Re} coordinates and find that instabilities'
onsets show unexpectedly non-monotonic dependence on . Further, three
distinct regions in the diagram are identified based on . Strikingly, for
high elasticity fluids we discover a complete relaminarization of flow at
Reynolds number of the order of unity, different from a well-known turbulent
drag reduction. These counterintuitive effects may be explained by a finite
polymer extensibility and a suppression of vorticity at high \mbox{Wi}. Our
results call for further theoretical and numerical development to uncover the
role of inertial effect on elastic turbulence in a viscoelastic flow.Comment: 8 pages, 6 figure
Stokes flow analogous to viscous electron current in graphene
Electron transport in two-dimensional conducting materials such as graphene,
with dominant electron-electron interaction, exhibits unusual vortex flow that
leads to a nonlocal current-field relation (negative resistance), distinct from
the classical Ohm's law. The transport behavior of these materials is best
described by low Reynolds number hydrodynamics, where the constitutive
pressure-speed relation is Stoke's law. Here we report evidence of such
vortices observed in a viscous flow of Newtonian fluid in a microfluidic device
consisting of a rectangular cavityanalogous to the electronic system. We
extend our experimental observations to elliptic cavities of different
eccentricities, and validate them by numerically solving bi-harmonic equation
obtained for the viscous flow with no-slip boundary conditions. We verify the
existence of a predicted threshold at which vortices appear. Strikingly, we
find that a two-dimensional theoretical model captures the essential features
of three-dimensional Stokes flow in experiments.Comment: 6 pages, 6 figure
Fluid Vesicles in Flow
We review the dynamical behavior of giant fluid vesicles in various types of
external hydrodynamic flow. The interplay between stresses arising from
membrane elasticity, hydrodynamic flows, and the ever present thermal
fluctuations leads to a rich phenomenology. In linear flows with both
rotational and elongational components, the properties of the tank-treading and
tumbling motions are now well described by theoretical and numerical models. At
the transition between these two regimes, strong shape deformations and
amplification of thermal fluctuations generate a new regime called trembling.
In this regime, the vesicle orientation oscillates quasi-periodically around
the flow direction while asymmetric deformations occur. For strong enough
flows, small-wavelength deformations like wrinkles are observed, similar to
what happens in a suddenly reversed elongational flow. In steady elongational
flow, vesicles with large excess areas deform into dumbbells at large flow
rates and pearling occurs for even stronger flows. In capillary flows with
parabolic flow profile, single vesicles migrate towards the center of the
channel, where they adopt symmetric shapes, for two reasons. First, walls exert
a hydrodynamic lift force which pushes them away. Second, shear stresses are
minimal at the tip of the flow. However, symmetry is broken for vesicles with
large excess areas, which flow off-center and deform asymmetrically. In
suspensions, hydrodynamic interactions between vesicles add up to these two
effects, making it challenging to deduce rheological properties from the
dynamics of individual vesicles. Further investigations of vesicles and similar
objects and their suspensions in steady or time-dependent flow will shed light
on phenomena such as blood flow.Comment: 13 pages, 13 figures. Adv. Colloid Interface Sci., 201
Vesicle dynamics in elongation flow: Wrinkling instability and bud formation
We present experimental results on the relaxation dynamics of vesicles
subjected to a time-dependent elongation flow. We observed and characterized a
new instability, which results in the formation of higher order modes of the
vesicle shape (wrinkles), after a switch in the direction of the gradient of
the velocity. This surprising generation of membrane wrinkles can be explained
by the appearance of a negative surface tension during the vesicle deflation,
due to compression in a sign-switching transient. Moreover, the formation of
buds in the vesicle membrane has been observed in the vicinity of the dynamical
transition point.Comment: 4 pages, 4 figure
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