1,721,178 research outputs found
Investigation of vortex dynamics downstream of moving leaflets using robust image velocimetry
No full textThe interaction of a sudden flow through a rectangular slot with moving leaflets, hinged at its border, was investigated experimentally in a Plexiglas vessel. This configuration resembles schematically some key features of many biological flows, e.g. in sea-animal propulsion, where the moving flaps control the flow, optimizing thrust, or in heart valves, where leaflets prevent backflow. Therefore, the comprehension of the basic mechanisms of the flow-structure interaction and of the features of the flow is of interest in a wide range of applications. Although some detail of the phenomenon could depend on the specific leaflet design, material and forcing, the objective of the present work is to investigate the overall dependence of the flow field on the leaflet arrangement. Specifically, three leaflet configurations have been tested at Reynolds number Re = 2,000 and Strouhal number St = 0.2: two symmetrical leaflets, two non-symmetrical leaflets, one being twice as wide as the other, and a single leaflet. Velocity fields were obtained using Robust Image Velocimetry in order to accurately resolve the structure of the vorticity field. The dynamics of the opening leaflets, the vorticity fields and the features of the vortices generated during the leaflet opening were investigated and compared in the different leaflet configurations. Advantages in the opening time, maximum aperture and closing time were observed in the two-leaflet non-symmetrical case in comparison to the other configurations
Robust evaluation of the dissimilarity between interrogation windows in image velocimetry
No full textImage velocimetry techniques, which extract motion information by comparison of image regions, typically make use of cross-correlation to measure the degree of matching. In this work, a novel measure of the dissimilarity between interrogation windows is proposed which is based on a more robust estimator than cross-correlation. The method is validated on synthetic images and on two experimental data sets obtained from a periodically pulsed jet and a backward-facing step. The former is a basically laminar flow, whereas the latter is fully turbulent. Both of them are characterized by regions of high velocity gradients. The efficiency of the robust image velocimetry (RIV) is compared with a cross-correlation algorithm (PIV). The analysis of results shows that the RIV is less sensitive to the appearance and disappearance of particles, and to high velocity gradients and, in general, to noise, generating less spurious velocity vectors. As a consequence RIV resolves better the vorticity peaks at the center of the vortex rings generated by the pulsed jet, obtaining, for a given interrogation window size, a higher spatial resolution. Moreover, in the analysis of the flow field generated by the backward-facing step, the RIV performs better in the shear layer at the border of the recirculation region, leading to a more reliable estimation of Reynolds shear stress and horizontal velocity component
Investigation on vortex dynamics downstream moving leaflets by means of Robust Image Velocimetry
No full textA Preliminary Study of a Conformal Electromagnetic Metasurface for Brain Tumor Detection
No full textWe introduce a radiofrequency system for brain tumor detection, where a conformal metasurface is employed, by opportunely modifying the classical planar structure. The proposed system, working at 3 MHz, consists in a 5×5 matrix whose unit-cells are made of 8-turn spirals, excited by an active RF coil placed in its near-field region. The detection of the tumors is carried out by observing the amplitude variation and the frequency shift of the driving coil input impedance. By performing accurate full-wave simulations, we demonstrated that, by designing the metasurface in order to produce a 5-spot magnetic field distribution, it is possible to detect and spatially identify brain tumor malignancies, despite the extremely low frequency herein adopted. The findings, obtained from accurate numerical simulations, suggest that the conformal near-field focused metasurface is able to detect inclusions as small as 1 mm, demonstrating excellent sensitivity and spatial resolution. Future studies and developments will be aimed at exploiting a real-time reconfigurability to sequentially scan the tissue or region of interest
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