1,720,968 research outputs found
Sviluppo di tecniche di analisi d'immagine per la descrizione lagrangiana del campo di moto
Full text linkOn the application of a two-frame 3D Particle Tracking algorithm with multi-frame approach.
No full textUsing optical flow equation for particle identification and velocity prediction in particle tracking
No full textA new algorithm of particle identification suitable for particle tracking technique in fluid mechanics is proposed and tested with synthetic images specifically developed with different particle parameters. The new approach is based on the solution of the optical flow equation via a sum-of-squared-difference method. Particles are detected through the identification of corner features, where image intensity gradients are not null in two orthogonal directions. It is thus possible to identify low intensity and overlapped particles. Furthermore, the feature selection criterion is optimal by construction because it is based on the optical flow solution and therefore a good feature is the one that can be tracked well. This leads to the second advantage of the method, which is the possibility to obtain the local velocity, given by the approximate solution of the optical flow equation, that can be used as a predictor for the subsequent particle pairing step. The proposed algorithm is tested using synthetically generated and experimental images and demonstrates its ability to detect a great number of particles with high reliability in different cases analysed
EXPERIMENTAL STUDY ON THE EVOLUTION OF A THERMALLY FORCED CONVECTIVE BOUNDARY LAYER THROUGH 3D PTV
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Investigation of local winds in a closed valley: An experimental insight using Lagrangian particle tracking
No full textAn experimental study of two-dimensional katabatic and anabatic flows, and their interaction with an urban heat island centred in a closed valley is presented. Down- and up-slope flows are generated via cooling and heating 20 degrees inclined plates. The urban heat island is simulated by an electric heater centred in the valley. In order to understand the main features of the circulation established by thermal effects in an initially stably-stratified environment, an advanced Lagrangian particle tracking technique (Hybrid Lagrangian Particle Tracking) is employed. This allows one to obtain the velocity and acceleration of passive tracer particles as the first and second derivatives of a moving spline function that filters the particle trajectory coordinates. Experiments show the dependence of mean quantities and turbulent statistics on different slope heating. Furthermore, the effects of the slope flows on the circulation in a large city located in a narrow valley appear significant. During day-time simulations the urban heat island circulation is opposed by anabatic winds, creating critical situations for pollutant dispersion. During night-time simulations, the katabatic winds increase the city updraft motion. (C) 2013 Elsevier Ltd. All rights reserved
Spatial-temporal improvements of a two-frame particle-tracking algorithm
No full textA novel algorithm for particle-tracking velocimetry is proposed and tested with both synthetic and real images. It uses nearest-neighbour cluster matching which performs better than fixed area approaches in terms of spatial adaptivity. The algorithm includes several temporal multi-frame improvements, i.e. extrapolation of the expected particle positions in subsequent frames and the frame-gap technique. To further improve the tracking algorithm performances, the particle identification procedure was modified with respect to the traditional background subtraction, local thresholding and grey level weighted averaging by using the optical flow equation. The local maximum of grey levels around each feature extracted is identified and the barycentres of the particle associated with it are calculated by using Gaussian fitting. The novel algorithm works well with several seeding densities, both homogeneously and inhomogeneously distributed. The multi-frame approach substantially improves the average trajectory length and the number of long trajectories in images with and without noise. The number of barycentres correctly identified by employing the feature extraction is significantly larger than when traditional techniques are used, which in turn increases the number of velocity vectors, allowing a better characterization of the flow field under investigation
Fotocamera plenottica per misure 3D di velocità e dimensioni di particelle
No full textSono state proposte differenti tecniche per la misura di campi di velocità tridimensionali all’interno di un fluido: Scanning Particle Image Velocimetry (PIV), Defocusing PIV, Tomographic PIV, Holographic PIV, Synthetic aperture PIV, Stereoscopic Particle Tracking Velocimetry (PTV). Gran parte di queste tecniche prevedono l’uso simultaneo di più telecamere e/o complesse configurazioni ottiche. L’utilizzo di una fotocamera plenottica permette di superare alcuni limiti delle tecniche esistenti consentendo di ottenere da una sola ripresa più immagini focalizzate su piani differenti. Tenendo conto che la minima dimensione dell’immagine di una particella si ha quando la stessa si trova nel piano di messa a fuoco, acquisendo immagini della particella in diverse condizioni di messa a fuoco è possibile determinare la sua posizione anche nella direzione dell’asse ottico del sistema di ripresa. La dimensione della particella potrà essere determinata di conseguenza utilizzando le relazioni dell’ottica Gaussiana o calibrando il sistema di ripresa su oggetti di dimensioni note. Infine, nota la posizione della particella nel tempo è possibile determinare i campi di velocità 3D
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