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A parallel scheme for a multi-domain truly incompressible smoothed particle hydrodynamics (SPH) approach is presented. The proposed method is developed for distributed-memory architectures through the Message Passing Interface (MPI) paradigm as communication between partitions. The proposal aims to overcome one of the main drawbacks of the SPH method, which is the high computational cost with respect to mesh-based methods, by coupling a multi-resolution approach with parallel computing techniques. The multi-domain approach aims to employ different resolutions by subdividing the computational domain into non-overlapping blocks separated by block interfaces. The particles belonging to different blocks are efficiently distributed among processors ensuring well balanced loads. The parallelization procedure handles particle exchanges both throughout the blocks and the competence domains of the processors. The matching of the velocity values between neighbouring blocks is obtained solving a system of interpolation equations at each block interfaces through a parallelized BiCGSTAB algorithm. Otherwise, a whole pseudo-pressure system is solved in parallel considering the Pressure Poisson equations of the fluid particles of all the blocks and the interpolation equations of all the block interfaces. The employed test cases show the strong reduction of the computational efforts of the SPH method thanks to the interaction of the employed multi-resolution approach and the proposed parallel algorithms.

Over the last few decades, the increase in extreme weather events has caused urban floods to become more and more frequent, with increased risks for road users. Among the different types of pavement structures, permeable pavements (PPs) can effectively increase the hydraulic resilience of the cities since they are specifically designed for reducing runoff by allowing water to infiltrate and be discharged into the soil and/or by drains. This study aims at evaluating the hydrological performance of a Porous Asphalt (PA) using numerical simulations for a case study carried out using EPA SWMM (Storm Water Management Model) software. Several simulations have been carried out, considering different rainfall events, covered areas, and spatial layouts of the permeable pavement itself. The results provide useful information on the use of PA.

Il lavoro riguarda l’implementazione di una catena modellistica per l’analisi delle colate detritiche in grado di simulare sia l’evento critico di innesco di precipitazione, attraverso modelli idrologici di piena, che la successiva propagazione a valle del materiale già distaccato posto in alveo, attraverso la modellazione idrodinamica. Gli aspetti di tale modellazione affrontati nella tesi riguardano: (i) la calibrazione e validazione di un modello idrologico in bacini montani, spesso caratterizzati da mancanza di misure o dati poco attendibili; (ii) la stima della portata solida a partire dalla portata liquida; (iii) la scelta di un modello reologico per la corretta caratterizzazione della miscela solido-liquida; (iv) l’implementazione di un modello di propagazione che tenga conto delle pendenze ripide tipiche degli alvei torrentizi e di un criterio di arresto. La framework modellistica è costituita dal modello idrologico semi-distribuito GEOframe-NewAge, qui adottato per la prima volta ad una scala temporale di 5 minuti, adeguata a catturare gli eventi meteorici intensi che causano l’innesco; dal modello idrodinamico in-house DAM_DEBRIS, che integra le classiche equazioni alle basse, qui appositamente modificate per tener conto delle alte pendenze di cui sono caratterizzati i bacini montani. Inoltre, è stato implementato un opportuno criterio di arresto e sono state confrontate più leggi reologiche per la caratterizzazione della miscela. L’area di studio analizzata è la parte montana del bacino del fiume Fella (Alpi Giulie), situato nella parte nord-est della regione Friuli Venezia Giulia. Quest’area, infatti, è altamente suscettibile a fenomeni di colate detritiche in virtù sia delle caratteristiche geomorfologiche che di quelle pluviometriche, con precipitazioni particolarmente abbondanti ed intense. I risultati della modellazione idrologica hanno dimostrato l’attendibilità degli idrogrammi simulati anche in sezioni non strumentate. Relativamente alla parte idrodinamica, primi test effettuati su prove di laboratorio hanno mostrato risultati incoraggianti nell’ottica di una sua applicazione al caso reale. La disponibilità di un simile strumento modellistico, potrà permettere lo studio di possibili scenari futuri indotti, per esempio, dall’intensificarsi di eventi meteorici particolarmente estremi, fornendo un ulteriore strumento per una efficiente pianificazione territoriale. The work of this thesis aims at implementing a hydro-morphodynamic modelling framework for debris flow analysis capable of simulating both the precipitation triggering event, through the hydrological modelling, and the subsequent downstream propagation of debris flow, through the hydrodynamic modelling. The aspects addressed in the thesis concern: (i) the calibration and validation of an hydrological model in mountain basins, often characterized by scarcity of measurements or unreliable data; (ii) the estimation of solid hydrograph from the liquid one; (iii) the choice of a rheological model to characterize the solid-liquid mixture; and (iv) the implementation of a propagation model, which takes into account the steep slopes typical of river beds, and a runout criterion. The framework consists of the semi-distributed hydrologic model GEOframe-NewAge, here adopted for the first time at a 5-minute time-step, which is adequate to capture the intense precipitation events that trigger the debris flowing; the in-house hydrodynamic model DAM_DEBRIS, which integrates the classical shallow water equations, here modified to account for the steep slopes of mountains basins. In addition, an appropriate runout criterion was implemented, and multiple rheological laws were compared for the mixture characterization. The study area is the mountainous side of the Fella River basin (across the Julian Alps), located in the northeastern part of the Friuli Venezia Giulia region. This area is highly susceptible to debris flow phenomena due to both geomorphologic and rainfall characteristics, characterized by abundant and intense precipitations. The results of the hydrological modelling demonstrated the reliability of the simulated hydrographs in ungauged areas. With regard to the hydrodynamic part, first numerical tests carried out on laboratory tests showed encouraging results with a view to its application to the real case. The availability of such a modelling tool, may allow the study of possible future scenarios induced, for example, by the intensification of extreme weather events, providing an additional tool for an efficient territorial planning.

The need to develop more accurate numerical tools for the propagation of noise in underwater environments is driven by the continuous increase of human activity in the sea and coastal areas. Noise has been shown to be dangerous to marine wildlife, and steps should be taken soon to mitigate it. Knowing that the primary sources of noise pollution at sea are marine propellers, one of the problems is assessing how the noise generated interacts with the environment, since up to now, the main focus was the characterization of the acoustic signature in the near field or, alternatively, the propagation of simplified acoustic sources in sea-like domains. The work conducted in this thesis assesses the modelling of complex acoustic sources and the propagation of acoustic pressure in realistic domains. A propagation model based on the solution of the acoustic wave equation in the time and space domain is implemented and used in conjunction with the Ffowcs Williams and Hawkings (FW-H) to analyze the possible patterns occurring in the underwater environment. Specifically, we analyzed the noise radiated by a marine propeller in a canal, focusing on the effects of the boundaries on the acoustic field and, secondly, the consequence of a rotating body placed underneath a free surface. We defined a new methodology called Full Acoustic Analogy (FAA) to achieve these results. This methodology aims to overcome some intrinsic limitations of the known Acoustic Analogies. The study presented here attempts to bridge the gap between noise characterization and its propagation by introducing a new methodology for evaluating flow-induced noise in a realistic environment. The propagation model developed, which used the finite-difference-time-domain method, has been compared against benchmark cases (monopole source propagating in classical waveguides) for which an analytical solution is available, and it provides accurate results of the acoustic field. Furthermore, a second analysis is conducted on two classical waveguides: the Ideal one and the Pekeris one. The solution of the wave equation in time and physical space enables the implementation of different sources, such as dipole and quadrupole; therefore, we analyzed the acoustic response of the Pekeris waveguide. The results show that the propagation of the acoustic pressure is strongly affected by the directivity pattern of the source. This was the first step in evaluating the capabilities of the solution of the acoustic equation in the presence of sources characterized by complex directivity since our ultimate goal is to evaluate the noise emitted by a propeller. In the second part, the FAA analogy is introduced, and we describe how the acoustic pressure obtained with the FW-H equation is used as a source term in the propagation model. After the validation of the new proposed methodology in an unbounded homogeneous domain, we investigate the propagation of the linear part of the noise generated by a naval propeller within a canal. Local maxima and minima of the acoustic fields arise from the interaction between the noise source and the environment; in particular, they derive from the superposition of direct and reflected waves. Moreover, a rotating body placed underneath a free surface generates a peculiar asymmetry of the acoustic field associated with the interaction between the acoustic waves and the free surface. The need to develop more accurate numerical tools for the propagation of noise in underwater environments is driven by the continuous increase of human activity in the sea and coastal areas. Noise has been shown to be dangerous to marine wildlife, and steps should be taken soon to mitigate it. Knowing that the primary sources of noise pollution at sea are marine propellers, one of the problems is assessing how the noise generated interacts with the environment, since up to now, the main focus was the characterization of the acoustic signature in the near field or, alternatively, the propagation of simplified acoustic sources in sea-like domains. The work conducted in this thesis assesses the modelling of complex acoustic sources and the propagation of acoustic pressure in realistic domains. A propagation model based on the solution of the acoustic wave equation in the time and space domain is implemented and used in conjunction with the Ffowcs Williams and Hawkings (FW-H) to analyze the possible patterns occurring in the underwater environment. Specifically, we analyzed the noise radiated by a marine propeller in a canal, focusing on the effects of the boundaries on the acoustic field and, secondly, the consequence of a rotating body placed underneath a free surface. We defined a new methodology called Full Acoustic Analogy (FAA) to achieve these results. This methodology aims to overcome some intrinsic limitations of the known Acoustic Analogies. The study presented here attempts to bridge the gap between noise characterization and its propagation by introducing a new methodology for evaluating flow-induced noise in a realistic environment. The propagation model developed, which used the finite-difference-time-domain method, has been compared against benchmark cases (monopole source propagating in classical waveguides) for which an analytical solution is available, and it provides accurate results of the acoustic field. Furthermore, a second analysis is conducted on two classical waveguides: the Ideal one and the Pekeris one. The solution of the wave equation in time and physical space enables the implementation of different sources, such as dipole and quadrupole; therefore, we analyzed the acoustic response of the Pekeris waveguide. The results show that the propagation of the acoustic pressure is strongly affected by the directivity pattern of the source. This was the first step in evaluating the capabilities of the solution of the acoustic equation in the presence of sources characterized by complex directivity since our ultimate goal is to evaluate the noise emitted by a propeller. In the second part, the FAA analogy is introduced, and we describe how the acoustic pressure obtained with the FW-H equation is used as a source term in the propagation model. After the validation of the new proposed methodology in an unbounded homogeneous domain, we investigate the propagation of the linear part of the noise generated by a naval propeller within a canal. Local maxima and minima of the acoustic fields arise from the interaction between the noise source and the environment; in particular, they derive from the superposition of direct and reflected waves. Moreover, a rotating body placed underneath a free surface generates a peculiar asymmetry of the acoustic field associated with the interaction between the acoustic waves and the free surface.

In the last few decades, hydropower production has been moving toward a new paradigm of low and diffused power density production of energy with small and mini-hydro plants, which usually do not require significant water storage. In the case of nominal power lower than 20 kW and ultra-low head H (H < 5 m), Archimedes screw or Kaplan type turbines are usually chosen due to their efficiency, which is higher than 0.85. A new cross-flow type turbine called Ultra-low Power Recovery System (UL-PRS) is proposed and its geometry and design criteria are validated in a wide range of operating conditions through 2D numerical analysis computed using the ANSYS Fluent solver. The new proposed solution is much simpler than the previously mentioned competitors; its outlet flow has a horizontal direction and attains similar efficiency. The costs of the UL-PRS turbine are compared with the costs of one Kaplan and one cross-flow turbine (CFT) in the case study of the main water treatment plant of the city of Palermo in Italy. In this case, the UL-PRS efficiency is estimated using a URANS 3D numerical analysis computed with the CFX solver.

The present study uses eddy-resolving numerical simulations to investigate how bed roughness affects flow and turbulence structure around an isolated, partially-buried mussel (Unio elongatulus) aligned with the incoming flow. The rough-bed simulations resolve the flow past the exposed part of a gravel bed, whose surface is obtained from a laboratory experiment that also provides some additional data for validation of the numerical model. Results are also discussed for the limiting case of a horizontal smooth bed. Additionally, the effects of varying the level of burial of the mussel inside the substrate and the discharge through the two mussel siphons are investigated via a set of simulations in which the ratio between the median diameter of the (gravel) particles forming the rough bed, d50, and the height of the exposed part of the mussel, h, varies between 0.10 and 0.22. The increase of the bed roughness is associated with a strong amplification of the turbulence kinetic energy in the near-wake region. Increasing the bed roughness and/or reducing h intensifies the interactions of the eddies generated by the bed particles with the base and tip vortices induced by the active filtering and by the mussel shell, respectively, which, in turn, induces a more rapid dissipation of these vortices. Increasing the bed roughness also reduces the strength of the main downwelling flow region forming in the wake. The strong downwelling near the symmetry plane is the main reason why the symmetric wake shedding mode dominates in the smooth bed simulations with negligible active filtering. By contrast, the anti-symmetric wake shedding mode dominates in the simulations conduced with a high value of the bed roughness. The mean streamwise drag force coefficient for the emerged part of the shell and the dilution of the excurrent siphon jet increase with increasing bed roughness.

The valorisation, protection and preservation policies for the underground building heritage are often difficult to implement due to an inadequate knowledge of hypogeal constructions. The complex and widespread underground structures of the vast “Cavo” Heritage (“horizontal wells”, “shelf wells” or “well tunnels”), so called qanāts, hidden underground and built over the centuries in Palermo, representing an evocative testimony to the history of water culture in the ancient city. Through the historical and constructive analyses and the implementation and development of measurement and 3D representation and visualization, first actions have been carried out. The paper will present the first results of the restoration project and the path of re-introduction in the fruition network of the qanāt “Gesuitico alto”, developed also in the field of “iHeritage. Mediterranean Platform for UNESCO Cultural Heritage” project, financed by ENI CBC MED Programme 2014–2020. The paper presents an experimentation of a procedural workflow of data acquisition, analysis and subsequent 3D virtual navigation of hypogeal environments. The methodology used is the SLAM with a last generation WMLS. The platform of virtual reality visualization, within UnReal Engine, allows the user to immerse and navigate in the anthropic environment by engaging it with a set of infographics that highlight the virtual visit.

Vegetation uprooting is a complex process which depends on many interrelated factors. In this paper, attention is focused on the flow-induced uprooting in river bends, where the flow characteristics vary as effected by the channel's curvature and its continuous changing in the downstream direction. Results are presented by an experimental work done in a high-amplitude meandering flume with mature herbaceous vegetation on the bed. In the first part of the paper, the variation along the bend of the geometrical (length, thickness, and radical architecture) and the mechanical (resistance force) characteristics of the roots are analyzed. In the second part of the paper, the transition from the condition of vegetation distributed on the bed to the condition of vegetation uprooted by the flow is experimentally examined. The results show that the flow-induced uprooting varies along the bend depending on the roots' geometrical and mechanical characteristics which are, in turn, influenced by the variation of the water surface elevation along the bend itself. It has been found that lower values of the resistance force occur at the apex section where the roots are characterized by higher values of length and lower values of thickness and have radical apparatuses characterized by limited encumbrance, a high number of roots, and low bushyness. The flow-induced uprooting especially develops close to the outer bank along the channel reach between the bend entrance and the apex section, where the root resistance force and the water surface slope tend to decrease in value.

Hydraulic turbines for energy recovery in wastewater treatment plants, with relatively large discharges values and small head jumps, are usually screw Archimedes or Kaplan types. In the specific case of a small head jump (about 3 m) underlying a rectangular weir in the major Palermo (Italy) water treatment plant, a traditional Kaplan solution is compared with two other new proposals: a Hydrostatic Pressure Machine (HPM) located at the upstream channel and a cross-flow turbine (CFT) located in a specific underground room downstream of the same channel. The fluid mechanical formulations of the flow through these turbines are analyzed and the characteristic parameters are stated. Numerical analysis was carried out for the validation of the HPM design criteria. The efficiency at the design point of the CFT and HPM are estimated using the ANSYS CFX solver for resolution of 3D URANS analysis. The strong and weak points of the three devices are compared. Finally, a viability analysis is developed based on several economic indicators. This innovative study with a theoretical formulation of the most suitable turbomachine characterization, the potential energy estimation based on hydraulic energy recovery in a real case study of a wastewater treatment plant and the comparison of the three different low-head turbines, enhancing the main advantages, is of utmost importance towards the net-zero water sector decarbonization.

Floods are becoming more frequent, especially in urban environments where most of the surface is waterproofed. Permeable pavement (PP) can be applied as low impact development (LID) systems for runoff mitigation in urban areas. Their effectiveness can be assessed, case by case, by numerical simulations. In this study, the effectiveness of mitigating runoff of different permeable pavements has been evaluated. In particular, porous asphalt (PA), pervious concrete (PC), permeable interlocking concrete pavement (PICP) and grid pavement (GP) have been investigated using EPA Storm Water Management Model (SWMM) software. To this aim, a car parking area located in the University Campus of Palermo (Italy) has been taken as a case study, considering several scenarios, each having a different percentage and planimetric layout of a PP type combined with an impermeable pavement. All the scenarios were tested assuming four synthetic rainfall events, referring to return periods of 5, 10, 50 and 100 years, and a real high return period event that occurred in Palermo in 2020. The results showed that amongst the different PPs considered, only the PA, bounded at the bottom by an impermeable layer, was practically ineffective. The other three PPs, proved to be effective in a noticeable way and furthermore for each scenario studied, they proved to bear almost the same mitigated runoff. The results proved appreciable differences in runoff as a function of the location of the PP over the study area.

The accurate delineation of flood hazard maps is a key element of flood risk management policy. Flood inundation models are fundamental for reproducing the boundaries of flood-prone areas, but their calibration is limited to the information available on the areas affected by inundation during observed flood events (typically fragmentary photo, video or partial surveys). In recent years, Earth Observation data have supported flood monitoring and emergency response (e.g., the Copernicus Emergency Service) thanks to the proliferation of available satellite sensors, also at high spatial resolution. Under this umbrella, the study investigates a levee breach that occurred in December 2017 along the Enza River, a right tributary of the Po River, that caused the inundation of a large area including Lentigione village. The flood event is simulated with a 2D hydraulic model using satellite images to calibrate the roughness coefficients. The results show that the processing and the timing of the high-resolution satellite imagery is fundamental for a reliable representation of the flooded area.

In this paper a novel model, based on the smoothed particle hydrodynamics (SPH) method, is proposed to simulate thrombus formation. This describes the main phases of the coagulative cascade through the balance of four biochemical species and three type of platelets. SPH particles can switch from fluid to solid phase when specific biochemical and physical conditions are satisfied. The interaction between blood and the forming blood clot is easily handled by an innovative monolithic FSI approach. Fluid-solid coupling is modelled by introducing elastic binds between solid particles, without requiring detention and management of the interface between the two media. The proposed model is able to realistically reproduce the thromboembolic process, as confirmed by the comparison of numerical results with experimental data available in the literature.

Cross-flow turbines have recently been proposed for energy recovery in aqueducts when the outlet pressure is greater than zero, owing to their constructive simplicity and good efficiency within a large range of flow rates and head drops. In the case of high head drop (higher than 150 m) and relatively small discharge (lower than 0.2 m3/s), the traditional design of these turbines leads to very small widths of the nozzle and the runner; as a consequence, friction losses grow dramatically and efficiency drops down to very low values. Standard Pelton turbines require zero outlet pressure and cannot be used as alternatives. A new counter-pressure hydraulic turbine for high head and low flow rate, called the High Power Recovery System (H-PRS) is proposed. H-PRS presents a different geometry to reduce friction losses inside the nozzle and the runner by widening the two external walls. Several curved baffles are proposed to guide the fluid particles inside the nozzle and to guarantee the right velocity direction at the inlet surface of the runner. Computational Fluid Dynamics (CFD) 3D transient analyses are carried out to measure H-PRS efficiency for different operating conditions and to compute its characteristic curve for different positions of the regulating flap.

A new numerical methodology to solve the 3D Navier-Stokes equations for incompressible fluids within complex boundaries and unstructured body-fitted tetrahedral mesh is presented and validated with three literature and one real-case tests. We apply a fractional time step procedure where a predictor and a corrector problem are sequentially solved. The predictor step is solved applying the MAST (Marching in Space and Time) procedure, which explicitly handles the non-linear terms in the momentum equations, allowing numerical stability for Courant number greater than one. Correction steps are solved by a Mixed Hybrid Finite Elements discretization that assumes positive distances among tetrahedrons circumcentres. In 3D problems, non-Delaunay meshes are provided by most of the mesh generators. To maintain good matrix properties for non-Delaunay meshes, a continuity equation is integrated over each tetrahedron, but the momentum equations are integrated over clusters of tetrahedrons, such that each external face shared by two clusters belongs to two tetrahedrons whose circumcentres have positive distance. A numerical procedure is proposed to compute the velocities inside clusters with more than one tetrahedron. Model preserves mass balance at the machine error and there is no need to compute pressure at each time iteration, but only at target simulation times.

This paper presents a novel three-dimensional fluid-structure interaction (FSI) approach, where the meshless smoothed particle hydrodynamics (SPH) method is used to simulate the motion of incompressible fluid flows, whilst structures are represented by a simplified approach based on particle-spring systems. The proposed FSI technique allows to use independent spatial-temporal resolutions for the fluid and structural computational domains. The particle-spring elastic constants are calibrated and relationships with the mechanical material properties, Young's modulus and Poisson's ratio, are determined. Fluid and structure computational domains are separated by interfaces made of triangular elements whose position is updated during the simulation following the structural deformation. The coupling of the two media at the fluid-structure interfaces is handled by the introduction of solid and fluid boundary particles. This approach, automatically and without introducing further complexity, avoids the penetration of fluid particles into the solid domain. The efficiency and accuracy and the present method are validated with analytical/benchmark solutions from the literature. (c) 2022 Elsevier B.V. All rights reserved.

In the present paper we propose a new numerical methodology for the solution of 2D non-isothermal incompressible flows for natural and mixed convection in irregular geometries. The governing equations are the Incompressible Navier-Stokes Equations and the Energy Conservation Equation. Fluid velocity and temperature are coupled in the buoyancy term of the momentum equations according to the Oberbeck–Boussinesq approximation. The governing equations are discretized over unstructured triangular meshes satisfying the Delaunay property. Thanks to the Oberbeck–Boussinesq hypothesis, the flow and energy problems are solved in an uncoupled way, and two fractional time step procedures are sequentially applied to solve each problem. The prediction steps of both procedures are solved applying a Marching in Space and Time (MAST) numerical Eulerian scheme, which explicitly handles the non-linear terms in the momentum equations of the fluid problem, and allows numerical stability for Courant numbers greater than one. An analytical solution is applied for the prediction thermal problem. The correction steps of the two fractional time step procedures involve the solution of large linear systems, whose matrices are sparse and symmetric and have the “M” property if the mesh satisfies the Delaunay condition. This allows a well performing condition number. The matrix coefficients are constant in time, so that they are calculated and factorized only once, before the simulations loop starts, saving a lot of computational time. We present four numerical applications. For the first test, an analytical solution is available, and this makes it possible to analyze the spatial convergence order of the numerical solver. In the other applications, we investigate the capability of the proposed algorithm to handle irregular geometries, and we compare the computed results with experimental data and the outputs of literature models. A study of the required computational costs is also presented.

The purpose of this paper is to understand the capability and consistency of large eddy simulation (LES) in Eulerian–Lagrangian studies aimed at predicting inertial particle dispersion in turbulent wall-bounded flows, in the absence of ad hoc closure models in the Lagrangian equations of particle motion. The degree of improvement granted by LES models is object of debate, in terms of both accurate prediction of particle accumulation and local particle segregation; therefore, we assessed the accuracy in the prediction of the particle velocity statistics by comparison against direct numerical simulation (DNS) of a finer computational mesh, under both one-way and two-way coupling regimes. We performed DNS and LES at friction Reynolds number Re τ= 180 in smooth and rough channels, tracking particles with different inertia, with the aim to conduct a parametric study that examines the accuracy of particle statistics obtained from LES computations. The issue has been widely analysed in turbulent flow bounded by smooth walls, whereas the effect of rough boundaries on momentum coupled two-phase flows has been much less investigated until now. The action of the roughness of the wall is studied in terms of both turbulence modification and particle interaction with the wall surface due to particle rebounding off the solid boundary, without the introduction of a virtual rebound model. Results show that resolved LES adequately predicts particle-induced changes in both fluid and particle statistics in rough channels, at least for the range of parameters considered here. © 2020, Springer-Verlag GmbH Austria, part of Springer Nature.

The downward shift of the mean velocity profile in the logarithmic region, known as roughness function, $$\Delta U^+$$ , is the major macroscopic effect of roughness in wall bounded flows. This speed decrease, which is strictly linked to the friction Reynolds number and the geometrical properties which define the roughness pattern such as roughness height, density, shape parameters, has been deeply investigated in the past decades. Among the geometrical parameters, the effective slope (ES) seems to be suitable to estimate the roughness function at fixed friction Reynolds number, Re $$_{\tau }$$ . In the present work, the effects of several geometrical parameters on the roughness function, in both transitional and fully rough regimes, are investigated by means of large eddy simulation of channel flows characterized by different wall-roughness textures at different values of Re $$_{\tau }$$ up to 1000. The roughness geometry is generated by the superimposition of sinusoidal functions with random amplitudes and it is exactly resolved in the simulations. A total number of 10 cases are solved. With the aim to find a universal correlation between the roughness geometry and the induced roughness function, we analyzed the effect of more than a single geometrical parameter, including the effective slope, which takes into account both the roughness height and its texture. Based on data obtained from our simulations and a number of data points from the literature, a correlation between the ES and the root mean square of the roughness oscillation, as well as between ES and the mean absolute deviation of the roughness, satisfactorily predicts the roughness function.

The present work presents a numerical study of the evaporation of a sessile liquid droplet deposited on a substrate and subjected to different heating configurations. The physical formulation accounts for evaporation, the Marangoni effect, conductive transfer in the support, radiative heating, and diffusion–convection in the droplet itself. The moving interface is solved using the Arbitrary Lagrangian–Eulerian (ALE) method. Simulations were performed using COMSOL Multiphysics. Different configurations were performed to investigate the effect of the heating conditions on the shape and intensity of the Marangoni circulations. A droplet can be heated by the substrate (different natures and thicknesses were tested) and/or by a heat flux supplied at the top of the droplet. The results show that the Marangoni flow can be controlled by the heating configuration. An upward Marangoni flow was obtained for a heated substrate and a downward Marangoni flow for a flux imposed at the top of the droplet. Using both heat sources generated two vortices with an upward flow from the bottom and a downward flow from the top. The position of the stagnation zone depended on the respective intensities of the heating fluxes. Controlling the circulation in the droplet might have interesting applications, such as the control of the deposition of microparticles in suspension in the liquid, the deposition of the solved constituent, and the enhancement of the evaporation rate.

Cavitation is a relevant phenomenon for structural safety and noise level in hydraulic turbines, occurring when water pressure falls below the vapor value at a given temperature. In this case bubbles of vapor grow inside the liquid and move along with it. When the pressure returns above the vapor value the bubble collapses, and the pressure can locally achieve very high values, up to 7000 bars (Kumar & Saini, 2010). Moreover, if the bubble was confined also by the solid wall of a blade, the solid particles suspended in the fluid can be transported by the fluid ones and hit the solid wall at very high velocity, generating erosion. Cavitation is also the source of high frequency noise, very disturbing for humans. Cavitation has been extensively studied mainly in reaction turbines, especially Francis and Kaplan types (Luo &Tsujimoto, 2016, Turi et al., 2019; Alligne et al., 2014). Adhikari et al. (2016) carried on a numerical analysis occurring at the tip of the blades of a Crossflow turbine. The conclusion was that, at least in the case study, cavitation occurred only at rotational velocity greater than the design one. Power Recovery System (PRS) turbine is a reaction turbine with a rotor like the Crossflow one, that can be used instead of the Crossflow in the case of outlet pressure greater than zero. PRS has a very simple design and can provide hydraulic regulation with the use of a mobile flap that can reduce the rotor inlet area. Due to its simplicity and small size, PRS can be easily installed by replacing a short part of the pipeline where the hydraulic jump is available. In the following paper it is first shown, by means of CFD simulations, that low pressure and cavitation can easily show up in PRS turbines, especially when the downstream outlet pressure is very low, and that pressure attains the minimum value immediately after the tip of the nozzle, according to the rotational direction. The physical explanation is that the momentum Пout of the water volume trapped in the blade channel, immediately after the tip of the nozzle, cannot be balanced by the Пin, inlet one and this leads to strong negative pressure gradients. A possible countermeasure is the use of external recirculation by means of a small pipe connecting the outlet pipe with a rectangular opening of the case immediately after the tip of the nozzle. The connection leads to a small recirculation flow and to a pressure rise in the target area. The length of the arc between the end of the nozzle and the rectangular opening is a fraction of the arc of the blade channel, so that an inlet momentum is always provided to the blade channel up to the end of the rectangular opening. The proposed change has been numerically and experimentally tested on a 5kW PRS prototype, installed in the experimental loop of the hydraulic lab of the University of Palermo.

We present a new numerical solver for free-fluid flowing over and inside a porous medium. It is based over a macroscopic approach and one fictitious medium is assumed inside the domain, according to the One Domain Approach. Preliminary results are shown and compared with the ones provided by the well-known DuMux solver which applies a two Domain Approach.

Hydraulic turbines for energy recovery in wastewater treatment plants, with relatively large discharges and small head jumps, are usually screw or Kaplan types. In the specific case of a small head jump (about 3 m) underlying a rectangular weir in the major Palermo (Italy) treat-ment plant, a traditional Kaplan solution is compared with two other ones: a Hydrostatic Pres-sure Machine (HPM) located in the upstream channel and a cross-flow turbine located in a specif-ic underground room downstream the same channel.

The present study has the main purpose to experimentally investigate a turbulent momentum jet issued in a basin affected by rotation and in presence of porous obstructions. The experiments were carried out at the Coriolis Platform at LEGI Grenoble (FR). A large and unique set of velocity data was obtained by means of a Particle Image Velocimetry measurement technique while varying the rotation rate of the tank and the density of the canopy. The main differences in jet behavior in various flow configurations were assessed in terms of mean flow, turbulent kinetic energy and jet spreading. The jet trajectory was also detected. The results prove that obstructions with increasing density and increased rotation rates induce a more rapid abatement of both jet velocity and turbulent kinetic energy. The jet trajectories can be scaled by a characteristic length, which is found to be a function of the jet initial momentum, the rotation rate, and the drag exerted by the obstacles. An empirical expression for the latter is also proposed and validated. Graphic abstract

Savonius water rotor is a prominent drag based turbine able to extract energy available in flowing water with low velocity like river streams, tidal currents or other man made water canals. However, in view of its low performance, an enhanced design of the rotor blades is necessary to better its efficiency. Therefore, the present study aims to improve the efficiency of Savonius rotor by changing the blade design. Different blade shapes were investigated numerically using computational fluid dynamics (CFD). Using conventional design, the peak power coefficient was found to be 0.166 at tip-speed ratio of 0.78. However, the peak power coefficient reaches 0.184 using the optimal blade design. This work may be an important towards further improvement of the Savonius rotor's efficiency.

We present results of numerical simulations of a stratified reservoir with a three-layer stratification, subject to an oscillating surface shear stress. The simulations are carried out at a laboratory scale, using Large Eddy Simulation. We solve the three-dimensional Navier-Stokes equations under the Boussinesq approximation using a second-order accurate finite volume solver. The model was validated by reproducing experimental results for the reservoir response to surface shear stress and resonant frequencies of internal waves. In the first part of our research, we investigate the effect of sloped end-walls on mixing and internal wave adjustment to forcing within the basin, for three different periods of forcing. We find interesting combinations of wave modes and mixing under variation of the forcing frequencies and of the inclination of the end-walls. When the frequency of the forcing is close to the fundamental mode wave frequency, a resonant internal seiche occurs and the response is characterized by the first vertical mode. For forcing periods twice and three times the fundamental period the dominant response is in terms of the second vertical mode. Adjustment to forcing via second vertical mode is accompanied by the cancellation of the fundamental wave and energy transfer towards the high-frequency waves. The study shows that the slope of the end-walls dramatically affects the location of mixing, which has feedback on the wave field promoting higher vertical modes. The second part of our research is devoted to the investigation of the influence of Earth's rotation on the wave field. In this case, the ratio of forcing to inertial period plays the important role in discerning whether Kelvin or Poincare type waves will be excited which has direct consequences on the wave modal structure and the turbulent quantities. Superinertial forcing frequency excites Poincare waves that are observed to increase the amount of turbulent dissipation rate in the system. In the two cases, where the forcing period was half and twice the fundamental one, we observed that superinertial forcing frequency pushed the dominant internal wave response from first to second vertical mode. The study shows that the increase of the importance of the rotation rate over forcing increases the turbulent dissipation rate in the interior while decreasing it in the near-surface and near-boundary regions.

Efficiency improvement of water distribution networks needs to be in place to guarantee a long life period under suitable operating conditions. Excessive pressure is among the well-known issues encountered in water distribution networks which can cause strength damages to the piping system. In order to overcome this drawback and to reach a suitable water pressure delivery, the present work suggests a novel pressure regulation system. This regulation system is consisting of a Banki turbine equipped with a mobile flap as a control device. The suggested pressure regulation system was experimentally and numerically investigated under in-range i.e. like actual water distribution networks and out-range flow conditions. A set of computational fluid dynamic simulations was carried out to study the performance and flow characteristics around the turbine with and without a mobile flap. The flow solution was obtained by solving the Reynolds-averaged Navier–Stokes equations. Different rotor–stator interface and turbulence models, and mesh sizes were investigated to select the most accurate configuration of the numerical method. The validation of the numerical method was performed with an assessment of the standard error with respect to experimental data. Based on the experimental results, three correlations estimating the net head, the torque and the efficiency for given flap angle and volumetric flow rate were presented. Results revealed that the suggested regulation system presents a good efficiency up to 76% to recover the wasted hydrodynamics energy in water distribution networks.

Bedload grains in consecutive meandering bends either move longitudinally or across the channel centerline. This study traces and quantifies the grains’ movement in two laboratorial sine-generated channels, i.e., one with deflection angle θ0 = 30° and the other 110°. The grains originally paved along the channels are uniform in size with D = 1 mm and are dyed in various colors, according to their initial location. The experiments recorded the changes in the flow patterns, bed deformation, and the gain-loss distribution of the colored grains in the pool-bar complexes. We observed the formation of two types of erosion zones during the process of the bed deformation, i.e., Zone 1 in the foreside of the point bars and Zone 2 near the concave bank downstream of the bend apexes. Most grains eroded from Zone 1 are observed moving longitudinally as opposed to crossing the channel centerline. Contrastingly, the dominant moving direction of the grains eroded from Zone 2 changes from the longitudinal direction to the transversal one as the bed topography evolves. Besides, most building material of the point bars comes from the upstream bends, although low- and highly curved channels behave differently.

Twisted Darrieus water turbine is receiving growing attention for small-scale hydropower generation. Accordingly, the need for raised water energy conversion incentivizes researchers to focus on the blade shape optimization of twisted Darrieus turbine. In view of this, experimental analysis has been performed to appraise the efficiency of a spiral Darrieus water rotor in the present work. To better the performance parameters of the studied water rotor with twisted blades, three novel blade shapes, namely U-shaped blade, V-shaped blade, and W-shaped blade, have been numerically tested using a computational fluid dynamics three-dimensional numerical model. The maximum power coefficient of the Darrieus rotor reaches 0.17 at a 0.63 tip-speed ratio using twisted blades. Using V-shaped blades, the maximum power coefficient has risen to 0.185. The current study could be practically applied to provide more effective employment of twisted Darrieus turbines and to improve the generated power from flowing water such as river streams, tidal currents, or other man-made water canals.

Vitrectomy is a surgical procedure by which the vitreous humour is removed from the vitreous chamber of the eye and is replaced with a tamponade fluid. Vitrectomy is performed by means of a system consisting of several components, including the vitreous cutter, which aspirates the vitreous body, a viscoelastic fluid that occupies the vitreous chamber of the eye. The vitreous cutter port geometry may have great influence in vitrectomy surgery from the fluidics and safety standpoints. For this reason, there has been an increase interest from ophthalmic companies in optimising the fluidics within the vitreous chamber during surgical maneuvers, in order to avoid retinal tractions and intraocular pressure (IOP) variations. Once removed, the vitreous is often replaced with Silicone Oils (SOs), which, however, inevitably tend to emulsify in the aqueous solution present in the eye, limiting the chances of using SOs as long term vitreous substitutes. Many months are indeed often required for complete healing of retinal damages. Moreover, formation of emulsions triggers local inflammatory processes, which can cause further complications. This thesis is an experimental work related to the physics of ocular surgical procedures and it consists of two main parts. The first part concerns the fluid mechanics of the vitrectomy procedure. In particular, experiments with different approaches have been performed on the fluidics of various vitrectomy systems, with the objective of identifying ideal working conditions that minimise the risks of retinal damages, while keeping the surgical procedure efficient. The second part is related to the characterisation of the interfacial rheology between SOs and aqueous solutions containing endogenous proteins released by the ocular tissue as a response to an inflammatory state. Aim of this part is to investigate to what extent biomolecules present in the aqueous solution can modify the rheological properties of the interface between the two fluids and, in turn, facilitate the formation of an emulsion. This work is carried out in collaboration with Dr. Libero Liggieri at CNR-ICMATE. Each part consists of several chapters, which address various projects within these two topics and are briefly summarised below. PART I. PERFORMANCE ASSESSMENT OF VITRECTOMY DEVICES An important issue associated with vitrectomy is the risk of generation of retinal tractions by the vitreous cutter. It has been demonstrated that an intermittent flow generates fluid accelerations, with consequent pressure variations within the vitreous chamber, which may result in pulsatile tractions on the retina, with significant risk of iatrogenic retinal damage. Rossi et al. (2014b) proposed a conceptual diagram according to which, for a safe and efficient vitrectomy, optimisation of fluidics can be achieved by maximising the flow rate and minimising the acceleration around the cutter port. Another important aspect of the surgical procedure is prevention of large IOP changes within the eye. In particular, ocular hypertension or ocular hypotony can have very important clinical consequences. In order to maintain a normal IOP value, the balance between the aspiration and the irrigation pressures during vitrectomy plays a very important role. In chapter 1 we reviewed the literature concerning fluid dynamics of vitreoretinal surgery. In chapter 2 we evaluated the performance of vitreous cutters in vitro in terms of flow rate, using vitreous phantoms with different rheological properties (artificial vitreous, AV). On each fluid solution we performed rheological tests in order to obtain a good control of the properties of the working fluid and thus guarantee reproducibility of the experiments. This allowed us to compare the efficiency of the instruments in a highly controlled environment and to assess how their behavior is affected by fluid properties. Our results showed that all cutters are higly sensitive to fluid rheology and this is particularly true for double blade cutters, which, however, always produce larger flow rates compared to single blade ones. These findings imply that large flow rate fluctuations should be expected when a vitreous cutter is operated in a medium with heterogeneous mechanical properties (Nepita et al., 2021). In chapters 3 and 4 we show two experimental studies aimed at assessing the performance of two vitrectomy systems, characterised by different technologies (guillotine and ultrasound probes), tested both with balanced salt solution (BSS) and AVs. The following types of experiments have been performed in a cubic testing chamber. - Flow rate measurements: mean flow rate has been measured using a digital image tracking process for all cutters, changing the operational parameters within a wide range (cutting frequency, aspiration pressure, etc.). Results for guillotine cutters show that for viscoelastic fluids the averaged flow rate only slightly decreases as the cutting frequency increases. The ultrasound cutters produce larger flows regardless the stroke (the amplitude of the ultrasound vibrations of the needle). - Flow fields measurements: two dimensional PIV experiments have been performed with the goal of characterising the flow fields generated by all cutters. For each set of parameters, a sequence of images has been acquired on two different planes: ?frontal view? and ?lateral view?. Moreover, for a specific solution, two areas of interest have been used for the acquisition: a ?large field? of view, aimed at describing the entire circulation induced by the cutters at the scale of the experimental box, and a ?small field? of view, i.e. a zoom around the cutter tip, to describe more accurately flow structures around the cutter port. The flow induced by ultrasound cutters is strongly three dimensional and time dependent. Flow structures are generated at different spatial scales, from large scales similar to the experimental domain to smaller scales close to the cutter port (Stocchino et al., 2020). - Temperature measurements: a specific task was dedicated to measure temperature variations induced by ultrasound cutters as a function of the stroke, maintaining the vacuum pressure equal to zero. The two dimensional variation of the temperature field has been monitored with an infrared thermal camera. The acquired thermal images were used to obtain the temperature variations in time at a particular spot, or to generate two dimensional temperature maps. The temperature slightly grows but this effect is likely to be clinically irrelevant, since infusion of a different fluid will mitigate it. Moreover, the cutter head reaches maximum values close to body temperature, meaning that it will not transmit much heat to the needle and eventually to the vitreous during surgery (Stocchino et al., 2020). - Pressure measurements: for this specific type of experiments a model of the human vitreous chamber at real scale and with a realistic geometry has been employed. The eye model has been designed considering both infusion and aspirations lines, thus mimicking the real surgical procedure. The main goal of this part of the research activity is to perform pressure measurements at two different positions of the vitreous chamber (anterior and posterior): for this purpose two high precision pressure transducers (Viatran model 422) have been used. Results show that dynamic variations of the pressure due to the motion of the cutter blade are always small compared to the average pressure in the eye. Pressure fluctuations related to fluid motion are felt more intensely by the pressure sensor placed on the back of the vitreous chamber, opposite with respect to the irrigation line. This is probably due to the generation of a jet that impinges the chamber wall opposite to its position. When the aspiration phase begins the pressure in the eye model drops, which is related to hydraulic head (pressure) losses along the pipelines of the system. This pressure drop is typically reduced (sometimes very efficiently) when the vitrectomy system can actively compensate pressure variations. Results suggest that maximum irrigation pressure that can be used for compensating pressure losses during aspiration should be selected depending on several parameters such as the minimum irrigation pressure and the aspiration pressure, in order to optimise the desired compensation. PART II. STUDY OF SILICONE OIL EMULSIFICATION IN RELATION TO VITRECTOMY The tendency of SOs to emulsify in water is influenced by the chemico-physical properties of the fluids and interface and by external factors, such as temperature and mechanical energy provided to the system. Exposition of the SO to endogenous molecules (proteins, lipids, etc.) in the vitreous chamber, the presence of which is favored by the post-surgical inflammatory state of ocular tissues, is also considered to be important for the generation of an emulsion. The mechanisms at the basis of the effects of these molecules on the emulsification process depend on their capability to adsorb at the water-oil interface, modifying the mechanical interfacial properties, such as the interfacial tension (IT) and the dilational viscoelasticity (DV), i.e interfacial rheological response to surface variations. In chapters 5 and 6 we give an overview of the possible mechanisms for SO emulsion formation and stabilisation, with a particular focus on biomolecules adsorption processes at the SO-aqueous interface. In chapter 7 we propose an experimental study aimed at achieving a chemico-physical characterisation of the interfacial properties between SOs and aqueous solutions and at investigating the effect of surfactant biomolecules, such as albumin and ?-globulins, possibly responsible for emulsion formation. In particular, measurements of IT on pendant drops of the protein aqueous solution in the SO have been performed. These measurements have been carried out by a Drop Shape Tensiometer (PAT1-Sinterface, Berlin). This technique allows one to quantify the DV by measuring the IT response to small sinusoidal perturbations of frequency imposed on the interfacial area. Results show that adsorption of these proteins at the interface reduces the IT to values compatible with the generation of an emulsion and provides values of the DV compatible with a good stability of the resulting emulsions (Nepita et al., 2020). This study allowed us to understand the effect of individual, key blood proteins on the mechanical properties of the interface. In order to evaluate the overall effect of the presence of these proteins, we conducted a second series of experiments adopting whole human serum blood (WHSB) samples obtained from donors. The results obtained with the serum dissolved in the solution showed that, already in concentrations of 1-2 orders of magnitude smaller than that of single proteins, the IT significantly decreases, up to values compatible with the formation of an emulsion, indicating therefore possible synergistic effects with other components of the serum, in addition to the proteins previously studied. Moreover, the DV modulus increases up to rather high values, compatible with the stabilisation of the emulsions against the coalescence. Drugs used as intraoperative tools in vitrectomy can also act as surfactants. An example is the intraocular injection of an anti-inflammatory steroid, composed of micro crystal particles. To assess whether the drug has a surfactant role at the interface, we conducted a first series of experiments to quantify the IT and the DV properties. We performed the measurements on the aqueous phase of the dispersion (supernatant), which separates after the sedimentation of crystals. The experiments with the supernatant neither show a significant decrease of IT that can be associated with the formation of an emulsion, nor lead to significant DV values. However, it is known that dispersions of partially hydrophobic solid particles can give rise to particularly stable emulsions, known as Pickering emulsions (Binks, 2002). In order to verify the capability of biomolecules to form emulsions stable over the timescale of months, comparative emulsification tests have been performed and shown in chapter 8. Emulsions were produced using to the Double Syringe method (Gaillard et al., 2017). Although this emulsification method may be very different from what occurs in practice, it is significant for investigating the stability of the emulsion formed and is, therefore, used as a simple screening test. For all samples containing blood proteins or the steroid drug, the presence of an emulsion layer was detected, the thickness of which decreases with increasing dilution, and which was stable over long times. On the contrary, the sample containing the Dulbecco buffer alone was already separated a few minutes after the emulsion formation. Thus, we can conclude that blood proteins are able to form and stabilise emulsions. Moreover, the steroid drug is able to stabilise Pickering-type emulsions. Eye movements are the main source of mechanical energy applied to the interface between SO and aqueous solution inside the eye. A set of measurements in a realistic domain have been performed. A qualitative and a quantitative analysis of the risults are reported in chapter 9 and 10, respectively. In particular we have used a Plexiglass model of a vitreous chamber with realistic geometry. The experimental setup is equipped with an optical diagnostics for monitoring the emulsion, a mechanical system controlled by a computer to simulate eye saccadic movements and thermalised at 35�C. Once filled with SO and aqueous solution, the model is mounted on a support connected to a computer-controlled motor. In this study, harmonic rotations were considered. The temporal evolution of the emulsion has been studied through acquisition of images with a digital camera. In the case of emulsions characterised by very small droplets, a microscope (Leica Microsystems, Switzerland) has been used to determine their distribution and size. The main result obtained from these experiments consists in the formation and persistence of emulsions even when a low concentration of albumin has been dissolved in the aqueous solution (1% of the physiological concentration in blood). Moreover, the majority of the drops had a very small diameter: in the range of 0 ? 10 ?m. Droplets with these characteristic dimensions are considered more dangerous from the clinical point of view, since they are able to favor the onset of ocular complications.

The present study investigates flow turbulence and dispersion processes in the presence of flexible and dense vegetation on the bed. The turbulent dispersion coefficients and the terms of the turbulent kinetic energy equation are determined by using data collected in a straight laboratory channel with living vegetation on the bed. Results show that the turbulent integral lengths assume an order of magnitude comparable to the stems’ characteristic dimension independently by the direction and the turbulence assumes an isotropic behavior. The coefficients of dispersion have a trend similar to that of the turbulent lengths and assume low values in the longitudinal, transversal and vertical directions. Results also show that, in the mixing layer, the shear and wake turbulence production terms balance the dissipation; the turbulent diffusion term also assumes low values and its sign varies along the vertical indicating a transport of turbulent energy both from the vegetation to the free surface and from the free surface to vegetation.