A new freeware raycasting tool combined with fluorescent coating to allow for object visibility

Abstract Time resolved PIV encompassing moving and/or deformable objects interfering with the light source requires the employment of dynamic masking (DM). A few DM techniques have been recently developed, mainly in microfluidics and multiphase flows fields. Most of them require ad-hoc design of the experimental setup, and may spoil the accuracy of the resulting PIV analysis. A new DM technique is here presented which envisages, along with a dedicated masking algorithm, the employment of fluorescent coating to allow for accurate tracking of the object. We show results from measurements obtained through a validated PIV setup demonstrating the need to include a DM step even for objects featuring limited displacements. We compare the proposed algorithm with both a no-masking and a static masking solution. In the framework of developing low cost, flexible and accurate PIV setups, the proposed algorithm is made available through a freeware application able to generate masks to be used by an existing, freeware PIV analysis package. Graphic abstrac

A Gas-Kinetic Model for Shallow Water Flows in Presence of Wet/Dry Fronts

Experimental and Computational Hydraulic

Suitability of 2D modelling to evaluate flow properties in 3D porous media

AbstractThe employment of 2D models to investigate the properties of 3D flows in porous media is ubiquitous in the literature. The limitations of such approaches are often overlooked. Here, we assess to which extent 2D flows in porous media are suitable representations of 3D flows. To this purpose, we compare representative elementary volume (REV) scales obtained by 2D and 3D numerical simulations of flow in porous media. The stationarity of several quantities, namely porosity, permeability, mean and variance of velocity, is evaluated in terms of both classical and innovative statistics. The variance of velocity, strictly connected to the hydrodynamic dispersion, is included in the analysis in order to extend conclusions to transport phenomena. Pore scale flow is simulated by means of a Lattice Boltzmann model. The results from pore scale simulations point out that the 2D approach often leads to inconsistent results, due to the profound difference between 2D and 3D flows through porous media. We employ the error in the evaluation of REV as a quantitative measure for the reliability of a 2D approach. Moreover, we show that the acceptance threshold for a 2D representation to be valid strongly depends on which flow/transport quantity is sought

Can physical information aid the generalization ability of Neural Networks for hydraulic modeling?

Application of Neural Networks to river hydraulics is fledgling, despite the field suffering from data scarcity, a challenge for machine learning techniques. Consequently, many purely data-driven Neural Networks proved to lack predictive capabilities. In this work, we propose to mitigate such problem by introducing physical information into the training phase. The idea is borrowed from Physics-Informed Neural Networks which have been recently proposed in other contexts. Physics-Informed Neural Networks embed physical information in the form of the residual of the Partial Differential Equations (PDEs) governing the phenomenon and, as such, are conceived as neural solvers, i.e. an alternative to traditional numerical solvers. Such approach is seldom suitable for environmental hydraulics, where epistemic uncertainties are large, and computing residuals of PDEs exhibits difficulties similar to those faced by classical numerical methods. Instead, we envisaged the employment of Neural Networks as neural operators, featuring physical constraints formulated without resorting to PDEs. The proposed novel methodology shares similarities with data augmentation and regularization. We show that incorporating such soft physical information can improve predictive capabilities

Numerical modelling of fluvial inundations

Lo scopo di questa tesi è quello di indagare diversi possibili approcci alla modellazione numerica delle inondazioni fluviali. L'approccio modellistico matematico adottato in questo lavoro è quello maggiormente accettato per la simulazione di inondazioni su larga scala ed è basato sull'utilizzo del set di equazioni dette "delle acque basse" (SWE, Shallow Water Equations), in forma bidimensionale. La moltitudine e la complessità dei fenomeni che intervengono nella formazione di un evento di piena impone inevitabilmente di includere nella presente indagine solo alcuni di essi. In particolare si è voluto concentrare l'attenzione sulla caratterizzazione delle modalità di propagazione di un'inondazione. In una prima fase si sono analizzate le tipologie di eventi in cui l'onda di espansione fosse di tipo non impulsivo, mossa quindi principalmente dalla forza gravitazionale. Tale classe di fenomeni rappresenta gran parte delle inondazioni che si verificano in natura: esondazioni fluviali, inondazioni di aree costiere dovute a correnti di marea, espansioni controllate dei corsi d'acqua in bacini di laminazione, sono solo alcuni esempi. Questa restrizione operata sulla tipologia di eventi analizzati, se esclude una modesta fetta di fenomeni di interesse, consente d'altra parte di operare semplificazioni notevoli al set delle SWE. Dalla forma originaria delle SWE, rappresentata da una set di equazioni differenziali alle derivate parziali (PDE, Partial Differential Equations) di tipo iperbolico, si può dedurre un sistema semplificato di forma parabolica, (PSWE). Grazie a queste assunzioni il sistema originario, la cui soluzione numerica risulta pesante e spesso non applicabile a problemi di larga scala, diviene più facilmente gestibile e consente di allocare le risorse di calcolo resesi disponibili ad aspetti più importanti, quali ad esempio una dettagliata descrizione topografica o una più accurata modellazione delle condizioni al contorno. La limitazione del campo di applicabilità delle PSWE, se interpretabile dal punto di vista analitico grazie ad alcune semplificazioni, non si riflette in una netta distinzione negli eventi naturali. L'eterogeneità dei fenomeni e la loro rapida evoluzione sfuma i contorni che si tenta di tracciare. E' stato quindi necessario verificare l'accuratezza del modello non inerziale nel riprodurre un'inondazione fortemente impulsiva, simulata grazie ad un prototipo in scala. La quantificazione dell'errore derivante dal confronto con un esperimento controllato è generalizzabile a eventi su scala reale, e consente quindi l'applicazione di tali modelli in modo più consapevole. Una seconda parte della tesi ha riguardato lo sviluppo di un codice per la risoluzione delle SWE in forma completa adottando tecniche numeriche all'avanguardia. In particolare si è concentrata l'attenzione sulla capacità di tali modelli di simulare eventi fortemente impulsivi, nei quali si osserva la formazione e la propagazione di discontinuità nelle grandezze caratteristiche. L'approccio numerico conduce alla frontiera della ricerca in questo campo e pone problematiche stimolanti, alle cui soluzioni già proposte in letteratura si è tentato di apportare contributi innovativi. Si è indagata la capacità di mantenere un alto ordine di accuratezza anche in presenza di termini sorgente di pendenza e attrito al fondo, in concomitanza con transizioni asciutto-bagnato. L'inclusione nel modello concettuale del fenomeno del risalto idraulico, matematicamente interpretato come discontinuità e numericamente colto grazie a schemi shock-capturing, esacerba le questioni di propagazione su fondo asciutto e di gestione di topografie accidentate. La trattazione delle transizioni asciutto-bagnato è sempre risultato un aspetto critico della modellazione numerica delle SWE. Spesso le strategie adottate si sono rivelate farraginose e artificiose. Si è quindi proposto un approccio più fisicamente basato, che minimizza l'utilizzo di artifici numerici che spesso inficiano l'accuratezza propria degli complessità degli schemi numerici avanzati. Le soluzioni proposte sono quindi validate mediante il confronto con dati sperimentali e analitici.The scope of this thesis is to investigate different possible approaches to the numerical modelling of fluvial floods. The mathematical model adopted is most commonly used for large scale inundations, based on the bidimensional SWE (Shallow Water Equations). The number and complexity of phenomena involved in a flood event obliges to focus only on some of them. Therefore major attention has been paid to the characterization of the modalities of propagation. In the first part of the thesis non-impulsive inundation waves have been analyzed, where the gravitational force prevails over the inertial one. These features can be found in most of the natural flood events such as fluvial overflows, tidal inundations of coastal areas, controlled flow over flood expansion fields. Even if this restriction on the type of events excludes some of events of interest, it yields important simplifications to the SWE set. These assumptions allow to pose the original hyperbolic set of PDE (Partial Differential Equations) into a parabolic form. The complete SWE set, whose numerical solution is still computationally demanding so that its application to real cases is quite challenging, becomes more easily manageable and allows to assign the saved computational resources to the accurate modelling of some essential aspects such as the description of topography, boundary conditions and resistance forces. The applicability restrictions of the PSWE, if theoretically deductable with some simplifications, does not provide clear criteria applicable to real world events. The extreme heterogeneity and their rapid time variability makes these criteria even more difficult to use. It then became necessary to verify the accuracy of the diffusive model when simulating a highly inertial inundation wave, reproduced with a physical experiment. The second part of the thesis deals with the development of a numerical code to solve the full dynamic form of the SWE, making use of the latest numerical techniques available. The attention has been focused on the ability of these kind of models to simulate highly impulsive floods, where the formation and the propagation of physical shocks often occur. The shock-capturing numerical approach leads to the frontier of the research in this topic and yield several issues whose solution was given some innovative contributions in this thesis. The ability of maintaining a high order of accuracy was assessed, even when source terms as bed and friction slope are to be modelled. The inclusion of the hydraulic bore modelling, mathematically interpreted as a shock, exacerbates the issues related to the propagation over dry bed, especially when dealing with complex topography. The wet-dry transitions, always a crucial topic in the SWE numerical treatment, have been here managed in a way that minimizes the use of procedures whose lack of numerical foundation often ruins the accuracy of the overall scheme. The proposed results were then validated by means of comparisons with both experimental and analytical solutions

Modelling of Cantilever-Based Flow Energy Harvesters Featuring C-Shaped Vibration Inducers: The Role of the Fluid/Beam Interaction

Flow Energy Harvesters (FEHs), equipped with piezoelectric active layers, are designed to extract energy from non-pulsating flows. FEHs featuring cantilevers with tip-mounted Vibration Inducers (VIs) are designed to develop a galloping motion. In this paper, we present the modelling of a recently introduced VI shape, featuring semitubular-shaped winglets, which do not produce a wake interacting with the cantilever. Such peculiarity allows (i) to exploit the contribution of the wake to the formation of the lift, therefore opening to a more compact design; (ii) its performance to be analyzed by means of simple two-dimensional Computational Fluid Dynamics (CFD) simulations. By comparison with experimental data, we show that the minimal framework for the modelling of such new class of VIs needs to account for both the direct action of the fluid onto the cantilever and the drag on the VI, which are usually negligible for other VI shapes