Laurence Bergougnoux

Influence of small inertia on Jeffery orbits – CORRIGENDUM - Volume 1022

The anisotropy of the ultrasonic backscatter coefficient (BSC) was investigated in sheared suspensions of porcine red blood cells (RBCs). Experiments were conducted in Couette and tubular flow devices using a focused mono-element transducer (15 or 32 MHz) at various insonification angles relative to the flow direction. For suspensions of disaggregated RBCs, the BSC was lowest at 90°, when the ultrasound beam was perpendicular to the flow. This angular dependence is attributed to a shear-induced anisotropic microstructure, particularly a pair-depleted region aligned with the velocity direction. A similar angular trend was observed in suspensions of aggregated RBCs, where the minimum BSC occurred around 90°–95°, likely due to regions depleted in aggregate pairs acting as effective scatterers. Finally, the impact of BSC anisotropy on RBC aggregate size estimation was evaluated using model-based inverse methods. Among tested models, the effective medium theory combined with the polydisperse structure factor model provided the most reliable estimates. These findings highlight the role of shear-induced microstructure in shaping BSC anisotropy and its implications for ultrasound-based quantification of RBC aggregation.

We experimentally investigate the rotational dynamics of neutrally buoyant flat bodies of revolution (spheroids, disks and rings with different cross-sectional shapes) in shear flows. In the Stokes regime, the axis of revolution of these rigid particles moves in one of a family of closed periodic Jeffery orbits. Inertia is able to lift the orbit degeneracy and induces drift among several rotations towards limiting stable orbits. Furthermore, permanent alignment can be achieved for disks and rings with triangular cross-sectional shapes, provided the inertia is sufficiently high. The bifurcations between the different dynamics are compared with those predicted by small-inertia asymptotic theories and numerical simulations.

We experimentally investigate the rotational dynamics of neutrally buoyant axisymmetric particles in a simple shear flow. A custom-built shearing cell and a multi-view shape-reconstruction method are used to obtain direct measurements of the orientation and period of rotation of particles having oblate and prolate shapes (such as spheroids and cylinders) of varying aspect ratios. By systematically changing the viscosity of the fluid, we examine the effect of inertia (which may be originated from either phase) on the dynamical behaviour of these suspended particles up to a particle Reynolds number of approximately one. While no significant effect on the period of rotation is found in this small-inertia regime, a systematic drift among several rotations toward limiting stable orbits is observed. Prolate particles are seen to drift towards the tumbling orbit in the plane of shear, whereas oblate particles are driven either to the tumbling or to the vorticity-aligned spinning orbits, depending on their initial orientation. These results are compared with recent small-inertia asymptotic theories, assessing their range of validity, as well as to numerical simulations in the small-inertia regime for both prolate and oblate particles.

The sedimentation dynamics of a dilute suspension of non-Brownian spheres is experimentally examined at small particle Reynolds numbers but at Reynolds numbers based on the container size extending up to the small-but-finite inertial regime. While the long-time velocity fluctuations are independent of the Reynolds number in the Stokes regime, they are seen to decrease with increasing Reynolds number above a critical container Reynolds number of approximately of the Reynolds number. The microstructure of the suspension is also seen to evolve with increasing Reynolds number and to depart from random positioning as it becomes more sub-homogeneous and disordered.

Fiber orientation in oscillatory shearing flow was studied both experimentally and numerically. Optical measurements were made using a custom flow cell containing rigid, noncolloidal fibers suspended at high concentration in a Newtonian fluid. Simulations that account for hydrodynamic drag and excluded volume predict fiber alignment in the vorticity direction for some conditions, in agreement with the measurements. Vorticity alignment was found to be a complex function of strain amplitude and fiber concentration, confinement, and aspect ratio.

The coupling between particle–particle and particle–fluid interactions is examined by studying the sedimentation of clouds of spheres in a model cellular flow at a small but finite Reynolds number. The model flow consists of counter-rotating vortices and is aimed at capturing key features of the vortical effects on particles. The dynamics of clouds settling in this vortical flow is investigated through a comparison between experiments and point-particle simulations.

Particle trapping and deposition around an obstacle occur in many natural and industrial situations and in particular in the nuclear industry. In the steam generator of a nuclear power plant, the progressive obstruction of the flow due to particle deposition reduces the efficiency and can induce tube cracking leading to breaking and damage. The steam generator then loses its role as a safety barrier of the nuclear power plant. From a fundamental standpoint, dilute and concentrated particulate flows have received a growing attention in the last decade. In this study, we investigate the transport of solid particles around obstacles in a confined flow. Experiments were performed in a simplified configuration by considering a laminar flow in a vertical tube. An obstacle was inserted at the middle height of the tube and neutrally-buoyant particles were injected at different locations along the tube. We have investigated first the trajectories of individual particles using particle tracking (PT). Then, the particle trajectories were modeled by using the Boussinesq-Basset-Oseen equation with a flow velocity field either measured using particle image velocimetry (PIV) or calculated by the Code_Saturne software in order to account for the three-dimensional (3D) character of the obstacle wake. This paper presents a comparison between the experimental observations and the predictions of the modeling for an obstacle consisting of a rectangular step at a Reynolds number of ≈100 and evidences the importance of accounting for the 3D complex nature of the flow.

The thickness of the diffuse front between a sedimenting dilute suspension and the clear fluid above grows linearly in time due to polydispersity in the size of the particles and due to a hydrodynamic effect in which randomly heavy clusters fall out of the front leaving it depleted. Experiments and simplified point-particle numerical simulations agree that these two effects are not simply linearly additive.