Buckling instability causes inertial thrust for spherical swimmers at all scales

Microswimmers, and among them aspirant microrobots, generally have to cope with flows where viscous forces are dominant, characterized by a low Reynolds number ($Re$). This implies constraints on the possible sequences of body motion, which have to be nonreciprocal. Furthermore, the presence of a strong drag limits the range of resulting velocities. Here, we propose a swimming mechanism, which uses the buckling instability triggered by pressure waves to propel a spherical, hollow shell. With a macroscopic experimental model, we show that a net displacement is produced at all $Re$ regimes. An optimal displacement caused by non-trivial history effects is reached at intermediate $Re$. We show that, due to the fast activation induced by the instability, this regime is reachable by microscopic shells. The rapid dynamics would also allow high frequency excitation with standard traveling ultrasonic waves. Scale considerations predict a swimming velocity of order 1 cm/s for a remote-controlled microrobot, a suitable value for biological applications such as drug delivery.Comment: To appear in Phys. Rev. Lett See demonstration movie on https://www.youtube.com/watch?v=cEXMsFwEqs

Study of e+,e− production in elementary and nuclear collisions near the production threshold with HADES

HADES is a second generation experiment designed to study dielectron production in proton, pion, and heavy ion induced reactions at the GSI accelerator facility in Darmstadt. The physics programme of HADES is focused on in-medium properties of the light vector mesons. In this contribution we present status of the HADES experiment, demonstrate its capability to identify rare dielectron signal, show first experimental results obtained from C+C reactions at 2 A GeV and shortly discuss physics programme of up-coming experimental runs. © 2004 Elsevier B.V. All rights reserved. 53 1 49 58 Cited By :1

Effets d'épaisseur et couche limite sur profil portant. Effets de l'épaisseur des profils NACA symétriques sur leurs performances et l'état de la couche limite turbulente.

Le présent travail s'intéresse aux effets de l'épaisseur sur les performances de profils NACA symétriques tels que ceux que l'on rencontre en hydrodynamique navale sur les safrans de gouvernail ou de stabilisation. L'influence de l'épaisseur relative sur les performances est analysée physiquement par l'intermédiaire de mesures de vitesses réalisées dans la couche limite. Le comportement à grande incidence du profil et les conditions de décrochement sont également analysés

Coupled x-ray high-speed imaging and pressure measurements in a cavitating backward facing step flow

International audienceThe purpose of the present experimental study is to get a better understanding of the dynamics of the vapor phase spatiotemporal repartition in a cavitating backward facing step flow. We provide a refined data base of the use of the void fraction transport equation to model such flows. The backward facing step flow provides a well-known test case to compare vortex dynamics between single and two-phase flow. To evidence the vapor phase dynamics, the flow is probed by high-speed x-ray attenuation techniques and by pressure fluctuation measurements at the walls. Long-time dynamics are also visualized using conventional high-speed imaging synchronized with pressure measurements. Large vortex structures, free shear layer instability, wall interaction and reverse flow are observed. The two-phase structures are studied at different cavitation levels corresponding void fractions ranging from 1% to 50%. The topology of the mean and fluctuating void fraction maps is performed, leading to the establishment of three specific areas in the flow. These areas are distinguished by the underlying mechanisms happening within them: vaporization, transport, and condensation. The statistical analysis underlines the existence of extreme events associated with high void fraction levels and wave propagations. While these events are associated with topological changes from a shear layer to a wake mode that also exist in the single-phase case, they are associated with much lower frequency at high cavitation levels

Boiling Front formation in flash depressurization

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Explosive vaporization: Boiling front and droplet formation

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