Recent researches on the dynamics of sediment transport have been focused on the particle motion characteristics at small spatial scale, down to the particle scale or even smaller. Quantitative imaging techniques have been successfully introduced into the area of sediment transport to ensure the level of detail requested by the above-mentioned research. The research documented in this Thesis has considered the characteristics of motion of bed-load particles on a smooth bed, identifying and tracking each moving grain. The data used in this work were obtained from laboratory experiment which had been previously carried out under an extensive project at the Politecnico di Milano. The software package Streams was used for image processing to obtain the particle trajectories, from which characteristics of individual particle motion were extracted for quantitative analysis. In total, 55 movies were processed for three experimental discharges of 8 l/s, 11.1 l/s and 13 l/s. All the results from particle tracking have been manually validated superimposing the measured trajectories to ongoing movies of particle motion, for full reliability of the results. The measurement finally produced a data sample of 2219 long trajectories that were then used for a statistical analysis. The statistical parameters mean (μ), standard deviation (δ), coefficient of variation (Cv), skewness (Sk) and kurtosis (Ku) have been analysed for the variables angle of deviation (α), velocity along the flowing direction (Vx), velocity along the transverse direction of flow (Vy) and tortuosity (Ttor). All the mentioned properties were trajectory-averaged, leaving the analysis of instantaneous values for follow-up studies. During the analysis, the longitudinal velocity (Vx) was recognized as being the most significant property for the smooth bed configuration, since the particle motion was substantially straight and thus the angle of deviation, transverse velocity and tortuosity yielded somehow trivial results. The trajectory-averaged particle velocity over a smooth bed resulted to be an increasing function of the hydrodynamic properties of the flow (e.g., the shear velocity). The pdfs and cdfs of longitudinal velocity were also studied, finding that they became progressively more right skewed with increasing discharges. The characteristics of particle motions over a smooth bed were finally compared with those over a rough bed configuration, finding significant differences in the two kinds of motion. These data can thus support analysis of the contribution of bed roughness on bed-load sediment motion.
La ricerca sulla dinamica del trasporto solido si è recentemente concentrate sulle caratteristiche del moto delle particelle a scale di dettaglio, uguali o anche inferiori alla dimensione dei singoli grani, con una raffinatezza resa possibile anche dall’introduzione di sofisticate tecniche di analisi di immagine. La ricerca documentata in questa Tesi si inquadra in questo filone, avendo riguardato le caratteristiche del moto di granelli su un fondo liscio e avendo analizzato il moto di ogni singola particella. Sono stati usati dati sperimentali prodotti durante una campagna di laboratorio precedentemente condotta presso il Politecnico di Milano. Le traiettorie delle particelle sono state ottenute da filmati degli esperimenti tramite il software Streams; successivamente, le quantità di interesse sono state dedotte dalle traiettorie per l’analisi successiva. In totale sono stati processati 55 filmati per le tre portate di 8 l/s, 11.1 l/s e 13 l/s. Tutti i risultati relativi alla fase di tracciamento delle particelle sono stati manualmente validati sovrapponendo le immagini degli esperimenti alle traiettorie misurate, in modo da garantire l’affidabilità delle misure. Il campione ottenuto contiene i dati di 2219 traiettorie, che sono stati usati per un’analisi statistica. Sono state considerate le seguenti quantità: angolo di deviazione (α), velocità parallela al flusso idrico (Vx), velocità trasversale (Vy) e tortuosità (Ttor). Tutte queste proprietà sono state considerate con riferimento al valore mediato sulla traiettoria, mentre l’analisi dei valori istantanei dovrà essere oggetto degli studi successivi. Per tutte le quantità sono state considerate le statistiche di sintesi: media (μ), deviazione standard (δ), coefficiente di variazione (Cv), skewness (Sk) e kurtosi (Ku). l’analisi ha mostrato come la proprietà di maggiore interesse fosse la velocità parallela al flusso, in quanto il moto delle particelle era sostanzialmente allineato con la corrente quindi le statistiche delle altre proprietà non fornivano risultati particolarmente significativi. La velocità media della particelle è risultata essere una funzione crescente della sollecitazione idrodinamica (espressa per esempio tramite le velocità di attrito). Le distribuzioni (pdf, cdf) della Vx sono risultate progressivamente più asimmetriche al crescere della portata. Le caratteristiche del moto dei grani su fondo liscio sono state confrontate con le omologhe su fondo scabro, ottenendo significative differenze la cui analisi potrà essere di supporto all’analisi del contributo della scabrezza del fondo alle proprietà del trasporto solido.
Analysis of sediment particle motion on a smooth bed
HASAN, MD. SAZADUL
2012/2013
Abstract
Recent researches on the dynamics of sediment transport have been focused on the particle motion characteristics at small spatial scale, down to the particle scale or even smaller. Quantitative imaging techniques have been successfully introduced into the area of sediment transport to ensure the level of detail requested by the above-mentioned research. The research documented in this Thesis has considered the characteristics of motion of bed-load particles on a smooth bed, identifying and tracking each moving grain. The data used in this work were obtained from laboratory experiment which had been previously carried out under an extensive project at the Politecnico di Milano. The software package Streams was used for image processing to obtain the particle trajectories, from which characteristics of individual particle motion were extracted for quantitative analysis. In total, 55 movies were processed for three experimental discharges of 8 l/s, 11.1 l/s and 13 l/s. All the results from particle tracking have been manually validated superimposing the measured trajectories to ongoing movies of particle motion, for full reliability of the results. The measurement finally produced a data sample of 2219 long trajectories that were then used for a statistical analysis. The statistical parameters mean (μ), standard deviation (δ), coefficient of variation (Cv), skewness (Sk) and kurtosis (Ku) have been analysed for the variables angle of deviation (α), velocity along the flowing direction (Vx), velocity along the transverse direction of flow (Vy) and tortuosity (Ttor). All the mentioned properties were trajectory-averaged, leaving the analysis of instantaneous values for follow-up studies. During the analysis, the longitudinal velocity (Vx) was recognized as being the most significant property for the smooth bed configuration, since the particle motion was substantially straight and thus the angle of deviation, transverse velocity and tortuosity yielded somehow trivial results. The trajectory-averaged particle velocity over a smooth bed resulted to be an increasing function of the hydrodynamic properties of the flow (e.g., the shear velocity). The pdfs and cdfs of longitudinal velocity were also studied, finding that they became progressively more right skewed with increasing discharges. The characteristics of particle motions over a smooth bed were finally compared with those over a rough bed configuration, finding significant differences in the two kinds of motion. These data can thus support analysis of the contribution of bed roughness on bed-load sediment motion.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/82785