Particle velocity field structuration in oscillatory channel flow of a suspension

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

Workshop; IUTAM Symposium: From Stokesian suspension dynamics to particulate flows in turbulence; 2022

The motion of individual particles has been tracked experimentally in a non-Brownian suspension of neutrally buoyantspheres in an iso-index, iso-density fluid of viscosity 7.6mPa.s (volume fraction ϕ between 0.15 and 0.4). The suspensionis subject to a square-wave oscillatory flow in a Hele-Shaw cell of apertureH = 1 mm at low particulate Reynolds numbers(< 0.05). We investigated the evolution with time of the velocity field of the particles and of their trajectories as a functionof ϕ and of the amplitude A and period T of the oscillating flow.Initially, the flow is parallel with a blunted velocity profile resulting from an increased particle concentration at thecenter of the channel, consistent with previous results and shear-induced migration models.Then, one observes an instability marked by the appearance and growth of a transverse velocity component vy. Thestructure of vy is spatially and temporally periodic with a wavelength λ increasing with the deformation A/H and withno clear dependence on ϕ or on T. The growth rate of the instability is characterized by the ratio T/τ (τ = time forreaching half the peak amplitude of the instability): T/τ varies linearly with A/H above a threshold ≈ 1 and levels offabove A/H ≈ 5. The slope of the variation increases strongly with ϕ, reflecting a faster development of the instability.Structuration has been observed only for ϕ ≥ 15% and up to the highest values of A/H ≈ 25 investigated. A possiblerelation between dependence on ϕ of the characteristic growth rate T/τ and the shear induced particle diffusion coefficientwill be discussed.