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.

Buenos Aires

Jornada; Meeting of the IRP: INGÉNIERIES VERTES PAR LA MÉCANIQUE DES FLUIDES (IVMF); 2022

IRP-IVMF

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 7.6 mPa.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 aperture H = 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.