POSTER

T

he steady streaming flow induced by vibrations of a cylinder immersed in a fluid is investigated initialy for the silicon oil (µ=9.3×10

⁻³

Pa.s). Two pairs of counter-rotating vortices within a viscous boundary layer is generated, and by transfer of momentum and vorticity, larger eddies can be generated outside the boundary layer (outer streaming). We reached regimes of large streaming Reynolds number Re

_s

≫ 1, for which not only the typical flow velocity reaches up to a few cm/s, but also the outer vortices generated around the cylinder significantly thicken and stretch along the vibration axis up to 8 times the size they have at Re

_s

Fluid used: Viscoelsatic

Extensional relaxation time = 0.08 s Extensional viscosity = 4×10

²

Pa.s Solution viscosity= 23×10

^-3

Pa.s

(HPAM 0.1% + 1% NaCl)

Bahrani et al., Exp. Fluids, 61, 91 (2020)

A viscoelastic fluid (HPAM 0.1% + 1% NaCl) is chosen such that the relaxation time of its extensional viscosity is of the same order as the period of oscillations.

First, when elasticity is significant enough, we observe that the inner boundary layer vortices are much larger than for a Newtonian fluid of the same viscosity. This is generally associated to the disappearance of outer vortices. We propose that extensional viscosity is involved in the enlargement of inner vortices. Second, for strong enough forcing, the streaming flow moves away from the usual the four- vortices pattern. Indeed, a more complex structure can appear where each initial vortex splits into two smaller ones, showing a steady multiple vortices structure. To

the best of our knowledge, these phenomena were unobserved so far.

< 1.

Transion to a complex flow structure, for f = 20 Hz,

with Re

_s

-increasing