Self-adaptation and viscous selection in concentrated two-dimensional vortex dipoles

International audienceIn this Letter we deal with 2D direct numerical simulations of concentrated vortex dipoles. We show that various initial dipolar vorticity distributions evolve towards a specific family of dipoles parametrized by the dipole aspect ratio a/b, where a is the radius of the vortices based on the vorticity polar moment in half a plane and b is the separation between the vortex centroids. This convergence is achieved through viscous effects. The considered Reynolds numbers Re= G/v are Re= 3000 and Re= 15000. Moreover, all the dipoles of this family are quasi-steady solutions of the Euler equations. Their scatter plots and drift velocities are given for a/b<03. © 2000 American Institute of Physics

Spatio-temporal development of the long and short-wave vortex-pair instabilities

International audienceWe consider the spatio-temporal development of the long-wave and short-wave instabilities in a pair of counter-rotating vortices in the presence of a uniform axial advection velocity. The stability properties depend upon the aspect ratio a/b of the vortex pair, where a is the core radius of the vortices and b their separation, and upon W0/U0 the ratio between the self-induced velocity of the pair and the axial advection velocity. For sufficiently small W0/U0, the instabilities are convective, but an increase of W0/U0 may lead to an absolute instability. Near the absolute instability threshold, spatial growth rates are larger than those predicted by temporal stability theory. Considering aeronautical applications, it is shown that instabilities of the type considered in this communication cannot become absolute in farfield wakes of high aspect ratio wings. © 2000 American Institute of Physics

Direct and adjoint global modes of a recirculation bubble: Lift-up and convective non-normalities

International audienceThe stability of the recirculation bubble behind a smoothed backward-facing step is numerically computed. Destabilization occurs first through a stationary three-dimensional mode. Analysis of the direct global mode shows that the instability corresponds to a deformation of the recirculation bubble in which streamwise vortices induce low- and high-speed streaks as in the classical lift-up mechanism. Formulation of the adjoint problem and computation of the adjoint global mode show that both the lift-up mechanism associated with the transport of the base flow by the perturbation and the convective non-normality associated with the transport of the perturbation by the base flow explain the properties of the flow. The lift-up non-normality differentiates the direct and adjoint modes by their component: the direct is dominated by the streamwise component and the adjoint by the cross-stream component. The convective non-normality results in a different localization of the direct and adjoint global modes, respectively downstream and upstream. The implications of these properties for the control problem are considered. Passive control, to be most efficient, should modify the flow inside the recirculation bubble where direct and adjoint global modes overlap, whereas active control, by for example blowing and suction at the wall, should be placed just upstream of the separation point where the pressure of the adjoint global mode is maximum. © 2009 Cambridge University Press

Amplifier and resonator dynamics of a low-Reynolds-number recirculation bubble in a global framework

International audienceThe stability behaviour of a low-Reynolds-number recirculation flow developing in a curved channel is investigated using a global formulation of hydrodynamic stability theory. Both the resonator and amplifier dynamics are investigated. The resonator dynamics, which results from the ability of the flow to self-sustain perturbations, is studied through a modal stability analysis. In agreement with the literature, the flow becomes globally unstable via a three-dimensional stationary mode. The amplifier dynamics, which is characterized by the ability of the flow to exhibit large transient amplifications of initial perturbations, is studied by looking for the two- and three-dimensional initial perturbations that maximize the energy gain over a given time horizon. The optimal initial two-dimensional perturbations have the form of wave packets localized in the upstream part of the recirculation bubble. It is shown that they are first amplified while travelling downstream along the shear layer of the recirculation bubble and then decay when leaving the recirculation bubble. Maximal energy gain is thus achieved for a time horizon approximately corresponding to the propagation of the wave packet along the whole recirculation bubble. The resonator and amplifier dynamics are associated with different types of structures in the flow: three-dimensional steady structures for the resonator dynamics and nearly two-dimensional unsteady structures for the amplifier dynamics. A comparison of the strength of the two dynamics is proposed. The transient energetic growth of the two-dimensional unsteady perturbations is large at moderate time, compared to the very weak exponential growth of the three-dimensional stationary mode. This suggests that, as soon as there is noise in the system, the amplifier dynamics dominates the resonator dynamics, thus explaining the appearance of unsteadiness rather than the emergence of stationary structures in similar experimental flows. © 2008 Cambridge University Press

Pan-Africanism: a contorted delirium or a pseudonationalist paradigm? Revivalist critique

This essaic-article goes against established conventions that there is anything ethno-cultural (and hence national) about the so-called African tribes. Drawing largely from the culture history of precolonial/prepolitical Africans—that is, the Bantu/Cushitic-Ethiopians (Azanians)—the author has demonstrated vividly that far from being distinct ethno-culture national communities, the so-called tribes of African states are better considered subculture groups, whose regional culture practices erstwhile paid tribute to their nation’s main culture center in Karnak. For example, using the culture symbols and practices of some local groups and linking them to the predynastic and dynastic Pharaonic periods, I argued that there is compelling evidence against qualifying Africa’s tribes as distinct ethno-culture national entities. In genuine culture context, I stressed that the Ritual of Resurrection and its twin culture process of the mummification of deceased indigenous Pharaohs tend to suggest that the object of the Bantu/Cushitic-Ethiopians national culture was life (in its eternal manifestation) and then resurrection later, and that there are recurring (culturally sanctioned) ethical examples among the culture custodians of these subculture groups that generally pay tribute to the overarching culture norm. Furthermore, the fact that the Ritual of Resurrection began in the Delta region and ended at the Sources of the Nile, where the spirit of the deceased indigenous Pharaohs was introduced into the spiritual world of their ancestors, contradicts conventional perceptions that ancient Egypt was a distinct national community isolated from precolonial/prepolitical Africa/Azania

Reaction ⁶Li(p, Δ⁺⁺)⁶He At 1.04 GeV And The Δ−N Interaction

The reaction ⁶Li(p, Δ⁺⁺)⁶He has been studied at 1.04 GeV for transferred momenta ranging from 0.11 to 0.35 (GeV/c)2. An exponential decrease of the cross section is observed. A Glauber-type calculation is presented. The possibility of extracting information on σ(ΔN) and α(ΔN) is discussed

Phase Behavior of Polyelectrolyte Block Copolymers in Mixed Solvents

We have studied the phase behavior of the poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymer in a mixture of two miscible solvents, water and tetrahydrofuran (THF). The techniques used to examine the different polymers, structures and phases formed in mixed solvents were static and dynamic light scattering, small-angle neutron scattering, nuclear magnetic resonance and fluorescence microscopy. By lowering the water/THF mixing ratio X, the sequence unimers, micron-sized droplets, polymeric micelles was observed. The transition between unimers and the micron-sized droplets occurred at X = 0.75, whereas the microstructuration into core-shell polymeric micelles was effective below X = 0.4. At intermediate mixing ratios, a coexistence between the micron-sized droplets and the polymeric micelles was observed. Combining the different aforementioned techniques, it was concluded that the droplet dispersion resulted from a solvent partitioning that was induced by the hydrophobic blocks. Comparison of poly(n-butyl acrylate) homopolymers and poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymers suggested that the droplets were rich in THF and concentrated in copolymers and that they were stabilized by the hydrophilic poly(acrylic acid) moieties.Comment: 11 pages, 12 figures, to appear in Macromolecule

Interactions between Magnetic Nanowires and Living Cells : Uptake, Toxicity and Degradation

We report on the uptake, toxicity and degradation of magnetic nanowires by NIH/3T3 mouse fibroblasts. Magnetic nanowires of diameters 200 nm and lengths comprised between 1 {\mu}m and 40 {\mu}m are fabricated by controlled assembly of iron oxide ({\gamma}-Fe2O3) nanoparticles. Using optical and electron microscopy, we show that after 24 h incubation the wires are internalized by the cells and located either in membrane-bound compartments or dispersed in the cytosol. Using fluorescence microscopy, the membrane-bound compartments were identified as late endosomal/lysosomal endosomes labeled with lysosomal associated membrane protein (Lamp1). Toxicity assays evaluating the mitochondrial activity, cell proliferation and production of reactive oxygen species show that the wires do not display acute short-term (< 100 h) toxicity towards the cells. Interestingly, the cells are able to degrade the wires and to transform them into smaller aggregates, even in short time periods (days). This degradation is likely to occur as a consequence of the internal structure of the wires, which is that of a non-covalently bound aggregate. We anticipate that this degradation should prevent long-term asbestos-like toxicity effects related to high aspect ratio morphologies and that these wires represent a promising class of nanomaterials for cell manipulation and microrheology.Comment: 21 pages 12 figure

Modelling of friction stir welding of DH36 steel

A 3-D computational fluid dynamics (CFD) model was developed to simulate the friction stir welding of 6-mm plates of DH36 steel in an Eulerian steady-state framework. The viscosity of steel plate was represented as a non- Newtonian fluid using a flow stress function. The PCBN-WRe hybrid tool was modelled in a fully sticking condition with the cooling system effectively represented as a negative heat flux. The model predicted the temperature distribution in the stirred zone (SZ) for six welding speeds including low, intermediate and high welding speeds. The results showed higher asymmetry in temperature for high welding speeds. Thermocouple data for the high welding speed sample showed good agreement with the CFD model result. The CFD model results were also validated and compared against previous work carried out on the same steel grade. The CFD model also predicted defects such as wormholes and voids which occurred mainly on the advancing side and are originated due to the local pressure distribution between the advancing and retreating sides. These defects were found to be mainly coming from the lack in material flow which resulted from a stagnant zone formation especially at high tra- verse speeds. Shear stress on the tool surface was found to in- crease with increasing tool traverse speed. To produce a “sound” weld, the model showed that the welding speed should remain between 100 and 350 mm/min. Moreover, to prevent local melt- ing, the maximum tool’s rotational speed should not exceed 550 RPM