Results 1 - 10 of 11461
Results 1 - 10 of 11461. Search took: 0.033 seconds
|Sort by: date | relevance|
[en] The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 0.2 to 0.4 times the runner rotational frequency resulting from the so-called vortex breakdown. For low cavitation number, the flow features a cavitation vortex rope animated with precession motion. Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations. For the upper part load range, between 0.7 and 0.85 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession. It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation. This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope. The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines. The selected operating point corresponds to a discharge of 0.83 times the best efficiency discharge. Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone. First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced. Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented. The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit. The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing. Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed.
[en] This paper reviews the results of stability analysis for polygonal configurations of a point vortex system in an annular region depending on the ratio of the inner and outer radii of the annulus. Conditions are found for linear stability of Thomson's configurations for the case N<7. The paper also shows that a system of two vortices between parallel walls is a limiting case of a two-vortex system in an annular region, as the radii of the annulus tend to infinity. (ss 1)
[en] Using a vector vortex coronagraph behind the 1.5 m well-corrected subaperture (WCS) at Palomar, we detected a second object very close to ε Cephei, a δ Scuti F0 IV star. The candidate companion, ∼50 times fainter than ε Cephei, if physically associated, is a late-type K or early M star, and lies at an angular separation of 330 mas, or 1.1 λ/D for the WCS, making it the smallest angle detection ever realized with a coronagraph in terms of λ/D units. The projected separation of the putative companion is ∼8.6 AU, most likely on a highly eccentric orbit. The recently detected near-infrared excess is thus likely not due to hot dust. Moreover, we also show that the previously reported IRAS 60 μm excess was due to source confusion on the galactic plane.
[en] We investigate the influence of columnar defects in layered superconductors on the thermally activated penetration of pancake vortices through the surface barrier. Columnar defects, located near the surface, facilitate penetration of vortices through the surface barrier, by creating ''weak spots,'' through which pancakes can penetrate into the superconductor. Penetration of a pancake mediated by an isolated column, located near the surface, is a two-stage process involving hopping from the surface to the column and the detachment from the column into the bulk; each stage is controlled by its own activation barrier. The resulting effective energy is equal to the maximum of those two barriers. For a given external field there exists an optimum location of the column for which the barriers for the both processes are equal and the reduction of the effective penetration barrier is maximal. At high fields the effective penetration field is approximately 2 times smaller than in unirradiated samples. We also estimate the suppression of the effective penetration field by column clusters. This mechanism provides further reduction of the penetration field at low temperatures
[en] The stability of a cylindrical vortex-sheet under small deformations is studied, in an incompressible perfect fluid. Numerical study of this planar problem concludes in favour of instability: the sheet splits into a number of vortex clusters lesser than 7. This experimental result is confirmed by the study of small motions of the sheet around the unperturbed configuration: the frequency spectrum of vortices set at the vertices of a regular polygon is real only if the number of vortices is lesser than 7
[fr]On examine la stabilite d'une nappe cylindrique de tourbillons relativement a de petites deformations, le fluide dans lequel est placee la nappe etant incompressible et parfait. L'etude numerique de ce probleme plan conclut a l'instabilite: la nappe se scinde en un nombre d'amas tourbillonnaires inferieur a 7. Ce resultat experimental est confirme par l'etude des petits mouvements de la nappe au voisinage de la configuration non deformee: le spectre de frequences de tourbillons disposes aux sommets d'un polygone regulier n'est reel que si les tourbillons sont en nombre inferieur a 7
[en] We examine vortex pinning and dynamics in thin-film superconductors containing logarithmically interacting Pearl vortices moving through square and rectangular pinning arrays for varied vortex densities including densities significantly larger than the pinning density. For both square and rectangular pinning arrays, the critical depinning force shows maxima only at certain integer matching fields where the vortices can form highly ordered lattices. For rectangular arrays the depinning force and commensurability effects are anisotropic, with a much lower depinning threshold for vortex motion in the easy-flow directions. We find evidence for a crossover in pinning behavior in rectangular pinning arrays as the field is increased. We also show analytically, and confirm with simulations, that for B=2Bφ the strongest pinning for one direction of the driving force can be achieved for rectangular pinning arrangements rather than square ones. Under an applied driving force we find a remarkable variety of distinct complex flow phases in both square and rectangular arrays. These flow phases include stable sinusoidal and intricate pinched patterns where vortices from different channels do not mix. As a function of the driving force certain flow states become unstable and transitions between different phases are observed that coincide with changes in the net vortex velocities. In the rectangular arrays the types of flow observed depend on the direction of drive. We also show that two general types of plastic flow occur: stable flows, where vortices always flow along the same paths, and unstable or chaotic flows
[en] We present a systematic experimental study on vortex dynamics in two-dimensional Josephson-junction arrays built of underdamped single junctions in which charging effects can be neglected. Arrays in both square and triangular geometries are measured in small magnetic fields at low temperatures. We find that the whole picture of the spatial dynamics of vortices in two-dimensional arrays is analogous to the dynamics of the phase in a single junction. We study in detail the depinning current, the flux-flow resistance, and the maximum velocity of propagating vortices. Our data show that vortices in underdamped arrays, when driven with a current, experience more damping than can be explained by Ohmic-dissipation alone. A simple semiquantitative model, in which the energy lost to junctions in the wake of the moving vortices is taken into account, explains our data very well. The model shows that vortices will always experience damping no matter how underdamped the single junctions are
[en] We show that we may divide the nuclear current into a sum of an irrotational and a vortical current. The vortical current is a transverse current whose source is the vorticity density. Using specific examples we investigate hor to determine the vorticity density from the experimental data and hoe to calculate it theoretically. (author)
[en] The cleanest way to observe a dynamic Mott insulator-to-metal transition (DMT) without the interference from disorder and other effects inherent to electronic and atomic systems, is to employ the vortex Mott states formed by superconducting vortices in a regular array of pinning sites. Here, we report the critical behavior of the vortex system as it crosses the DMT line, driven by either current or temperature. We find universal scaling with respect to both, expressed by the same scaling function and characterized by a single critical exponent coinciding with the exponent for the thermodynamic Mott transition. We develop a theory for the DMT based on the parity reflection-time reversal (PT) symmetry breaking formalism and find that the nonequilibrium-induced Mott transition has the same critical behavior as the thermal Mott transition. Our findings demonstrate the existence of physical systems in which the effect of a nonequilibrium drive is to generate an effective temperature and hence the transition belonging in the thermal universality class.
[en] Using a nanogram-sized single crystal of Bi2Sr2CaCu2O8-x attached to a microcantilever we demonstrate in a direct way that in magnetic fields nearly parallel to the ab plane the magnetic field penetrates the sample in the form of Josephson vortices rather than in the form of a tilted vortex lattice. We further investigate the relation between the Josephson vortices and the pancake vortices generated by the perpendicular field component