An experimental study of the migration of dilute suspensions of particles in Poiseuille flow at Reynolds numbers Re = 67-1700 was performed, with a few experiments performed at Re up to 2400. The particles used in the majority of the experiments were neutrally buoyant spheres with diameters d yielding a ratio of pipe to particle diameter in the range D/d = 8-42. The volume fraction of solids was less than 1% in all cases studied. The results of G. Segré & A. Silberberg (J. Fluid Mech. 14, 136, 1962) have been extended to show that the tubular pinch effect in which particles accumulate on a narrow annulus is moved toward the wall as Re increases. A careful comparison with asymptotic theory for Poiseuille flow in a channel was performed. Another inner annulus closer to the centre, and not predicted by this asymptotic theory, was observed at elevated Re. As Re is increased, the distribution of particles over the cross-section of the tube at the measurement location, lying at a distance L. = 310D from the entrance, changes from one centred at the annulus predicted by the theory to one with the particles primarily on the inner annulus. The case of slightly non-neutrally buoyant particles was also investigated. A particle trajectory simulation based on asymptotic theory was performed to facilitate the comparison of theory and the experimental observations.

[1] The phase relations and density of a natural mid-ocean ridge basalt (MORB) were investigated from 28 to 89 GPa and 1600 to 2700 K by in situ X-ray diffraction measurements and chemical analysis of the quenched samples using transmission electron microscopy (TEM). We observed an assemblage of five phases up to 50 GPa, namely an aluminum-bearing magnesium perovskite phase, a calcium perovskite phase, a stishovite phase, the new aluminum-rich (NAL) phase, and a calcium ferrite-type phase. The NAL phase was no longer observed above 50 GPa. The phase proportions were obtained by Rietveld refinement of the in situ X-ray diffraction patterns. After the disappearance of the NAL phase beyond 50 GPa, the proportion of each phase remains constant up to 89 GPa. The density of MORB was calculated using the measured volumes, phase proportions, and chemical compositions of the coexisting phases. The thermoelastic parameters of the MORB sample were estimated from the fit of the measured densities at various pressure and temperature conditions. Resulting MORB density profiles were calculated for different subducting slab temperature profiles. MORB densities are 0.5% to 2% greater than those of the surrounding mantle over the entire lower mantle range, suggesting MORB likely subducts to the core-mantle boundary.

We investigate experimentally the influence of suspended particles on the transition to turbulence. The particles are monodisperse and neutrally-buoyant with the liquid. The role of the particles on the transition depends both upon the pipe to particle diameter ratios and the concentration. For large pipe-to-particle diameter ratios the transition is delayed while it is lowered for small ratios. A scaling is proposed to collapse the departure from the critical Reynolds number for pure fluid as a function of concentration into a single master curve.PACS numbers: 83.80.Hj, 83.50.Ha More than a century after Reynolds' work [1], understanding how turbulent regions grow in a pipe and bring the laminar Poiseuille flow to fully developed turbulence is still not completely achieved. Above a critical Reynolds number, the laminar flow is observed to be unstable, turbulent regions grow and are convected in the pipe. This flow regime is called intermittent. When the flow rate is further increased, the flow becomes fully turbulent [2]. In fact, the transition happens to be subcritical and the flow is linearly stable for all flow rates [3]. A finite amplitude perturbation is needed to trigger the transition and the critical Reynolds depends upon its amplitude. For small perturbations, laminar motion is observed as far as Re ≈ 10 5 but the transition in pure fluid can be reached for Re ≈ 2100 provided that the perturbation is strong enough to allow the growth of turbulent "puffs" [2]. Recent studies have investigated with different kinds of perturbations the nature of the unstable modes, either in the inlet region or in the fully developed flow [4,5].The objective of the present work is to examine how transition to turbulence is affected by the presence of suspended particles in the simplest case of neutral buoyancy. More specifically, we focus upon determining the transition threshold between the laminar and the intermittent regime as a function of the particle volume fraction φ of the suspension. Because the particles are neutrally buoyant and largely drag-free, the present study is related to recent work which has examined global subcritical stability behavior of plane Couette flow forced by the presence of a single spherical bead or a spanwise wire [6,7]. This work also has a practical aspect as it is related to pipeline flow of slurries.Experiments are performed with four sets of spherical polystyrene particles having density ρ = 1.0510 ± 0.001 g.cm −3 and diameters d presented in table I. To obtain neutral buoyancy, the densities of the fluid and of the particles are matched. We choose as a fluid a mixture of 22 % glycerol and 78 % water by mass. The temperature of the mixture is maintained at 25 ± 1 • C by using a thermostated bath as a fluid reservoir in the fluid circulating loop. At this temperature, the viscosity of the mixture is µ = 1.64 ± 0.03 cP. The experimental set-up consists of a straight and horizontal cylindrical glass tube of 2.6 m length mounted on a rigid support structure. Two different tubes havi...