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Experimental study on the effects of fixed boundaries in channelized free surface dry granular flows
No full textINDAGINE SPERIMENTALE SULLA DINAMICA DI CORRENTI GRANULARI A SUPERFICIE LIBERA IN REGIME DI MOTO PERMANENTE
No full textExperimental investigations on free surface steady dry granular flows
No full textGeophysical flows, like rock avalanches, snow
avalanches and debris flows, involve the rapid motion of
granular media. The dynamics of these flows is still an
open problem due to the complexity of the flow
resistance mechanisms involved. In case of dry granular
flows two main resistance mechanisms have been
individuated: friction and collisions. In order to better
understand the constitutive behaviour, it is necessary to
provide local measurements of the main flow variables:
velocity and volume fraction.
To do so, we set up an experimental apparatus in which
a dry granular material was let flow in a flume with
transparent sidewalls and recorded by two high-speed
cameras to get both side-wall and free surface views.
Velocity profiles were, then, derived by PIV analysis
and a novel optical method was developed to estimate
the near-wall volume fractio
An optical method for measuring the near-wall volume fraction in granular dispersions
No full textThe volume fraction plays a crucial role in the dynamics of granular flows. This work is devoted to developing a novel cost-effective optical method for determining the near-wall volume fraction. By means of a numerical investigation, performed by Monte Carlo generations of mono-disperse sphere distributions under controlled illumination conditions, the stochastic relationship between the near-wall volume fraction and a measurable quantity, referred to as two-dimensional volume fraction, is figured out. A binarization algorithm is proposed for estimating the two-dimensional volume fraction from gray-scale digital images. The relation is found to be of exponential type, with parameters only depending on the angle of incidence of light. An experimental investigation is designed for implementing the proposed method to a real laboratory context. The laboratory campaign, performed on dispersions of white plastic grains immersed in different ambient fluids, enables us to validate the proposed approach. It is found that the exponential law provides results in sound agreement with experimental data. Sensitivity analyses are also performed to confirm and evaluate the robustness and the accuracy of the proposed method
Experimental Investigation on the Effects of the Fixed Boundaries in Channelized Dry Granular Flows
No full textThe dynamics of granular mixtures, involved in several geophysical phenomena like rock avalanches and debris flows, is far from being completely understood. Several features of their motion, such as non-local and boundary effects, still represent open problems. An extensive experimental study on free-surface channelized granular flows is here presented, where the effects of the fixed boundaries are systematically investigated. The entire experimental data set is obtained by using a homogenous acetal-polymeric granular material and three different basal surfaces, allowing different kinematic boundary conditions. Velocity profiles at both the sidewall and the free surface are obtained by using high-speed cameras and the open-source particle image velocimetry code, PIVlab. Significantly, different sidewall velocity profiles are observed by varying the basal roughness and the flow depth. Owing to sidewall friction and non-local effects, such profiles exhibit a clear rheological stratification for high enough flow depths and they can be well described by recurring to composite functions, variously formed of linear, Bagnold and exponential scalings. Moreover, it has been discovered that transitions from one velocity profile to another are also possible on the same basal surface by merely varying the flow depth. This shape transition is due partly to the sidewall resistances, the basal boundary condition and, in particular, the occurrence/inhibition of basal grain rolling. In most of the experiments, the normal-to-bed velocity profiles and the velocity measurements at the free surface strongly suggest the occurrence of a secondary circulating flow, made possible by a chiefly collisional regime beneath the free surface
MONITORAGGIO E MODELLAZIONE DI FENOMENI DI COLATE DETRITICHE DI POMICI CON TECNICHE DI REMOTE SENSING. UN CASO STUDIO IN COSTIERA AMALFITANA
No full textL’elevato rischio associato ai fenomeni di colata detritica che interessano
gli ammassi di pomici della Costiera Amalfitana richiede la messa a punto
di specifici modelli previsionali ai fini di un’efficace salvaguardia. La
maggiore difficoltà è legata all’assenza dei dati osservati di volume, tirante
e portata, che sono necessari per calibrare e testare i modelli di
simulazione. In questo lavoro è presentato un approccio che integra
l’utilizzo di dati pre e post evento, rilevati anche da drone, con la relativa
modellazione matematico-numerica. Il caso studio è un impluvio naturale a
elevata pendenza nel quale si verificano, con forte ricorrenza, fenomeni di
correnti detritiche di pomici che mettono a rischio un tratto della Strada
Provinciale SP1 ubicato in Costiera Amalfitana (nel Comune di Tramonti).
L’utilizzo integrato di laser scanner terrestre (TLS) e di tecniche
fotogrammetriche ha consentito di acquisire la topografia dell’impluvio e
del conoide sia pre che post evento, ottenendo così il modello digitale del
terreno (DTM) e la stima del volume di materiale mobilitato. L’osservazione
delle tracce lasciate dalla corrente ha consentito di stimarne il tirante in
alcuni punti; l’angolo di attrito del materiale è stato misurato con prove di
laboratorio eseguite su un campione prelevato in situ. I dati così ottenuti
sono stati, quindi, utilizzati per calibrare e validare un modello di
propagazione bidimensionale FLATmodel, basato sulle equazioni delle
shallow-water. Le resistenze al fondo sono state stimate utilizzando il
modello di Voellmy, che si compone di un termine attritivo e di un termine
collisionale dipendente dalla velocità della colata. Tramite back-analysis
sulla forma del deposito misurata è stato valutato il valore ottimale del coefficiente del termine collisionale nella legge di resistenza. La buona
corrispondenza tra simulazione e dati sperimentali conferma
l’appropriatezza del modello proposto come strumento per descrivere la
propagazione e le distanze di run-out di fenomeni con analoghe
caratteristiche
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