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Kursa M.,The Interdisciplinary Center | Bajer K.,University of Warsaw | Lipniacki T.,Polish Institute of Fundamental Technological Research
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We demonstrate that a single reconnection of two quantum vortices can lead to the creation of a cascade of vortex rings. Our analysis involves localized induction approximation, high-resolution Biot-Savart and Gross-Pitaevskii simulations. The latter showed that the rings cascade starts on the atomic scale, with rings diameters orders of magnitude smaller than the characteristic line spacing in the tangle. Vortex rings created in the cascades may penetrate the tangle and annihilate on the boundaries. This provides an efficient decay mechanism for sparse or moderately dense vortex tangle at very low temperatures. © 2011 American Physical Society.

Jarzabek D.M.,Polish Institute of Fundamental Technological Research
Review of Scientific Instruments | Year: 2015

A direct method for the evaluation of the torsional spring constants of the atomic force microscope cantilevers is presented in this paper. The method uses a nanoindenter to apply forces at the long axis of the cantilever and in the certain distance from it. The torque vs torsion relation is then evaluated by the comparison of the results of the indentations experiments at different positions on the cantilever. Next, this relation is used for the precise determination of the torsional spring constant of the cantilever. The statistical analysis shows that the standard deviation of the calibration measurements is equal to approximately 1%. Furthermore, a simple method for calibration of the photodetector's lateral response is proposed. The overall procedure of the lateral calibration constant determination has the accuracy approximately equal to 10%. © 2014 AIP Publishing LLC.

Zielinski T.G.,Polish Institute of Fundamental Technological Research
Journal of Vibration and Acoustics, Transactions of the ASME | Year: 2010

This paper presents a fully coupled multiphysics modeling and experimental validation of the problem of active reduction of noise generated by a thin plate under forced vibration. The plate is excited in order to generate a significant low-frequency noise, which is then reduced by actuators in the form of piezoelectric patches glued to the plate with epoxy resin in locations singled out earlier during finite element (FE) analyses. To this end, a fully coupled FE system relevant for the problem is derived. The modeling is very accurate: The piezoelectric patches are modeled according to the electromechanical theory of piezoelectricity, the layers of epoxy resin are thoroughly considered, and the acoustic-structure interaction involves modeling of a surrounding sphere of air with the nonreflective boundary conditions applied in order to simulate the conditions found in anechoic chamber. The FE simulation is compared with many experimental results. The sound pressure levels computed in points at different distances from the plate agree excellently with the noise measured in these points. Similarly, the computed voltage amplitudes of controlling signal turn out to be very good estimations. © 2010 American Society of Mechanical Engineers.

Hoffman J.,Polish Institute of Fundamental Technological Research
Journal of Physics D: Applied Physics | Year: 2015

Our experiments with the ablation of graphite by a nanosecond laser pulse showed the formation of craters with a depth of upto 60 μm. The creation of such deep craters is hard to explain solely by evaporation. Existing models should be supplemented by an additional mass removal process that ensures penetration of the material. The recoil pressure at the surface of the target generates a compression wave propagating deep into the material. Possible mechanisms of fracture by the longitudinal compression wave are discussed. A phenomenological model of material fragmentation is proposed. Modelling results are in good agreement with the experiment. The model may be used for polycrystalline graphite as well as other brittle materials treated by the nanosecond laser pulse. © 2015 IOP Publishing Ltd.

Stupkiewicz S.,Polish Institute of Fundamental Technological Research
Computer Methods in Applied Mechanics and Engineering | Year: 2013

A fully coupled implicit scheme is developed for quasi-steady-state wear problems. The formulation admits finite configuration changes due to both deformation and wear. The unconditionally stable implicit backward-Euler scheme is used for time integration of the shape evolution problem. Thus, the solution may proceed with large time increments, contrary to the commonly used explicit forward-Euler scheme, in which the time increment is restricted by the stability condition. This comes at the cost that the shape transformation mapping constitutes an additional unknown. As a result, a kind of an arbitrary Lagrangian-Eulerian (ALE) formulation is obtained in which the problem is solved simultaneously for the nodal positions and displacements. The incremental coupled problem is solved using the Newton method which leads to a highly efficient computational scheme, as illustrated by two- and three-dimensional numerical examples. © 2013 Elsevier B.V.

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