Oscillatory flow instability in concentrated suspensions

GARCIA, A. A.; ROHT, Y. L.; GAUTHIER, G.; SALIN, D.; HULIN, J.P.; DRAZER, G.; IPPOLITO, I.

Virtual

Congreso; XVI Meeting on Recent Advances in Physics of Fluids and its Applications; 2021

The motion ofindividual particles has been tracked experimentally in a non-Browniannon-colloidal suspension of neutrally buoyant spheres (volume fraction ϕbbetween 15% and 40%) subject to a square-wave oscillatory flow in a Hele-Shawcell with aperture H = 1 mm at low particulate Reynolds numbers (<0.05). We investigated the evolution of the particular velocity field andtrajectories, as a function of ϕb and the oscillationamplitude A0 and period T (2 – 40 s).Initially, theflow is parallel with a blunted velocity profile resulting from the increasedparticle concentration at the center of the channel, consistent with previousresults and shear-induced migration models. The reversibility of the motion ofthe particles from one period to the next is strong in the center region butmuch weaker close to the walls.Then, oneobserves an instability marked by an initially exponential growth of atransverse velocity component vy: the structure of vyis spatially and temporally periodic with a wavelength λ increasing withthe deformation per half period A0/H and with no cleardependence on ϕb. The appearance of the transverse velocityreduces the reversibility of the motion of the particles in the center of thechannel. The ratio T/τ (τ = characteristic growth time) varieslinearly with A0/H above a threshold value ≈ 1 and upto A0/H ≈ 5. The slope of the variation increasessignificantly with ϕb, reflecting a faster development of theinstability. Structuration has been observed up to the highest values of A0/H≈ 25 investigated.