Fluid dynamic analysis of trees influence in dispersion of pollutant in urban street canyon

degli elementi vegetali nella dinamica e nella dispersione degli inquinanti nello street canyon urbano. In particolare, è stato analizzata la risposta fluidodinamica di cespugli con altezze diverse e di alberi con porosità e altezza del tronco varianti. Il modello analizzato consiste in due edifici di altezza e larghezza pari ad H e lunghezza di 10H, tra i quali corre una strada in cui sono stati modellizati una sorgente rappresentativa del traffico veicolare e, ai lati, due linee di componenti vegetali. Le simulazioni sono state fatte con ANSYS Fluent, un software di "Computational Fluid Dynamics"(CFD) che ha permesso di modellizare la dinamica dei flussi e di simulare le concentrazioni emesse dalla sorgente di CO posta lungo la strada. Per la simulazione è stato impiegato un modello RANS a chiusura k-epsilon, che permette di parametrizzare i momenti secondi nell'equazione di Navier Stokes per permettere una loro più facile risoluzione. I risultati sono stati espressi in termini di profili di velocità e concentrazione molare di CO, unitamente al calcolo della exchange velocity per quantificare gli scambi tra lo street canyon e l'esterno. Per quanto riguarda l'influenza dell'altezza dei tronchi è stata riscontrata una tendenza non lineare tra di essi e la exchange velocity. Analizzando invece la altezza dei cespugli è stato visto che all'aumentare della loro altezza esiste una relazione univoca con l'abbassamento della exchange velocity. Infine, andando a variare la permeabilità delle chiome degli alberi è stata trovatta una variazione non monotonica che correla la exchange velocity con il parametro C_2, che è stata interpretata attraverso i diversi andamenti dei profili sopravento e sottovento. In conclusione, allo stadio attuale della ricerca presentata in questa tesi, non è ancora possibile correlare direttamente la exchange velocity con alcun parametro analizzato.

Dispersion and energy relations in periodic chains and grids

In this study wave propagation, dispersion relations, and energy relations for linear elastic periodic systems are analyzed. In particular, the dispersion relations for monoatomic chain of infinite dimension are obtained analytically by writing the Block-type wave equation for a unit cell in order to capture the dynamic behavior for chains under prescribed vibration. By comparing the discretized model (mass-spring chain) with the solid bar system, the nonlinearity of the dispersion relation for chain indicates that the periodic lattice is dispersive in contrast to the continuous rod, which is non dispersive. Further investigations have been performed considering one-dimensional diatomic linear elastic mass-spring chain. The dispersion relations, energy velocity, and group velocity have been derived. At certain range of frequencies harmonic plane waves do not propagate in contrast with monoatomic chain. Also, since the diatomic chain considered is a linear elastic chain, both of the energy velocity and the group velocity are identical. As long as the linear elastic condition is considered the results show zero flux condition without residual energy. In addition, this paper shows that the diatomic chain dispersion relations are independent on the unit cell scheme. Finally, an extension for the study covers the dispersion and energy relations for 2D- grid system. The 2x2 grid system show a periodicity of the dispersion surface in the wavenumber domain. In addition, the symmetry of the surface can be exploited to identify an Irreducible Brillouin Zone (IBZ). Compact representations of the dispersion properties of multidimensional periodic systems are obtained by plotting frequency as the wave vector’s components vary along the boundary of the IBZ, which leads to a widely accepted and effective visualization of bandgaps and overall dispersion properties.

Digital predistortion for compensation of nonlinearities in Radio over Fiber Links

In order to cope up with the ever increasing demand for larger transmission bandwidth, Radio over Fiber technology is a very beneficial solution. These systems are expected to play a major role within future fifth generation wireless networks due to their inherent capillary distribution properties. Nonlinear compensation techniques are becoming increasingly important to improve the performance of telecommunication channels by compensating for channel nonlinearities. Indeed, significant bounds on the technology usability and performance degradation occur due to nonlinear characteristics of optical transmitter, nonlinear generation of spurious frequencies, which, in the case of RoF links exploiting Directly Modulated Lasers , has the combined effect of laser chirp and optical fiber dispersion among its prevailing causes. The purpose of the research is to analyze some of the main causes of harmonic and intermodulation distortion present in Radio over Fiber (RoF) links, and to suggest a solution to reduce their effects, through a digital predistortion technique. Predistortion is an effective and interesting solution to linearize and this allows to demonstrate that the laser’s chirp and the optical fiber’s dispersion are the main causes which generate harmonic distortion. The improvements illustrated are only theoretical, based on a feasibility point of view. The simulations performed lead to significant improvements for short and long distances of radio over fiber link lengths. The algorithm utilized for simulation has been implemented on MATLAB. The effects of chirp and fiber nonlinearity in a directly modulated fiber transmission system are investigated by simulation, and a cost effective and rather simple technique for compensating these effects is discussed. A detailed description of its functional model is given, and its attractive features both in terms of quality improvement of the received signal, and cost effectiveness of the system are illustrated.