Search

In this paper we propose a mesoscale model for polar fluids and numerically investigate its validity by considering well known benchmark cases: the steady and unsteady Poiseuille and the low Reynolds lid-driven cavity flow. Analytical and numerical results are compared with our numerical results and a very good agreement is found. Then we consider not yet investigated high Reynolds lid-driven cavity flow cases, obtaining consistent and interesting results, which encourage to apply the proposed model to more complex cases. [ABSTRACT FROM AUTHOR]

This paper is aimed at assessing the ability of the Lattice-Boltzmann Method (LBM) in reproducing the fundamental features of lock-exchange gravity currents. Both two- and three-dimensional numerical simulations are presented at different Reynolds numbers ( 1000 ≤ R e ≤ 30,000 ). Turbulence has been accounted for by implementing an equivalent Large Eddy Simulation (LES) model in the LBM framework. The advancement of the front position and the front velocity obtained by LBM numerical simulations are compared with laboratory experiments appositely performed with similar initial and boundary conditions and with previous results from literature, revealing that the dynamics of the gravity current as a whole is correctly reproduced. Lobes and clefts instabilities arising in three-dimensional simulations and the entrainment parameter are also analysed and comparisons with previous studies are presented. [ABSTRACT FROM AUTHOR]

We present an entropic version of the lattice Boltzmann pseudo-potential approach for the simulation of multiphase flows. The method is shown to correctly simulate the dynamics of impinging droplets on hydrophobic surfaces and head-on and grazing collisions between droplets, at Weber and Reynolds number regimes not accessible to previous pseudo-potential methods at comparable resolution. [ABSTRACT FROM AUTHOR]

Tsunami waves are generated by sea bottom failures, landslides and faults. The concurrent generation of hydro-acoustic waves (HAW), which travel much faster than the tsunami, has received much attention, motivated by their possible exploitation as precursors of tsunamis. This feature makes the detection of HAW particularly well-suited for building an early-warning system. Accuracy and efficiency of the modeling approaches for HAW thus play a pivotal role in the design of such systems. Here, we present a Lattice Boltzmann Method (LBM) for the generation and propagation of HAW resulting from tsunamigenic ground motions and verify it against commonly employed modeling solutions. LBM is well known for providing fast and accurate solutions to both hydrodynamics and acoustics problems, thus it naturally becomes a candidate as a comprehensive computational tool for modeling generation and propagation of HAW. [ABSTRACT FROM AUTHOR]

The ability to accurately and efficiently model the intrusion of salt wedges into river beds is crucial to assay its interaction with human activities and the natural environment. We present a 2D multi-layer Shallow Water Lattice Boltzmann (SWLB) model able to predict the salt wedge intrusion in river estuaries. The formulation usually employed for the simulation of gravity currents is here equipped with proper boundary conditions to handle both the downstream seaside outlet and the upstream river inlet. Firstly, the model is validated against highly accurate semi-analytical solutions of the steady state 1D two-layer Shallow Water model. Secondly, the model is applied to a more complex, fully 3D geometry, to assess its capability to handle realistic cases. The simple formulation proposed for the shear interlayer stress is proven to be consistent with the general 3D viscous solution. In addition to the accuracy, the model inherits the efficiency of the Lattice Boltzmann approach to fluid dynamics problems. [ABSTRACT FROM AUTHOR]

It is shown that the single relaxation time (SRT) version of the Lattice Boltzmann (LB) equation permits to compute the permeability of Darcy's flows in porous media within a few percent accuracy. This stands in contrast with previous claims of inaccuracy, which we relate to the lack of recognition of the physical dependence of the permeability on the Knudsen number. [ABSTRACT FROM AUTHOR]

We present a lattice Boltzmann realization of Grad's extended hydrodynamic approach to nonequilibrium flows. This is achieved by using higher-order isotropic lattices coupled with a higher-order regularization procedure. The method is assessed for (low across parallel plates and three-dimensional flows in porous media, showing excellent agreement of the mass flow with analytical and numerical solutions of the Boltzmann equation across the full range of Knudsen numbers, from the hydrodynamic regime to ballistic motion. [ABSTRACT FROM AUTHOR]

The solution of fluid dynamics problems has recently witnessed a remarkable increase in the formulation of Boltzmann-based approaches, mainly triggered by the numerous advantages stemming from the concrete possibility of employing a low number of allowed velocities, which is the basis of the Lattice-Boltzmann (LB) methods. The field of Shallow Water (SW) modeling also took advantage of these techniques, but there are still open problems related to the practical impossibility to simulate transcritical flows while retaining the intrinsic simplicity of the LB approaches. This problem is even more crucial if one considers that transcritical flows always develop whenever a transition over dry bed occurs. Since such vertically integrated models are currently the mostly employed ones for simulating technically interesting flows, and since these flows often require the flooding of an initially dry bed, the application of LB methods seems to be facing its limits. The Gas Kinetic Method (GKM) overcomes this issue, integrating the Boltzmann equations in continuous velocity space. In this work, we formulate and extensively test a GKM-based model for solving the SW equations, which is able to manage the propagation over uneven dry bed. The benchmarking, carried out against analytical, experimental and previously proposed numerical reference solutions, shows promising results of the proposed approach. [ABSTRACT FROM AUTHOR]

• Depth averaged equations for submerged polydisperse shallow granular flows in water are obtained. • The Discrete Boltzmann Equation model is presented and proposed as a viable solution method. • The gravitational collapse of a loose rectangular granular pile in water is numerically simulated and compared to benchmark data. The collapse of a submerged granular pile under the action of the gravity force is investigated by means of a depth averaged model (hereinafter DAM). The granular pile is a mixture of solid particles and ambient liquid, totally covered by a layer of ambient liquid. The DAM is obtained by depth-averaging the mass and momentum equations of the mixture and the ambient fluid and is aimed at providing estimates of technical quantities regarding the considered phenomenon. The DAM is numerically solved by adopting an equivalent Discrete Boltzmann Equation model (hereinafter DBE) as solution method. The main advantage yielded by the DBE is the possibility to turn the original system of strongly non linear partial differential equations, which would require the resort to complex numerical schemes, into a purely advective, linear, first order one. Previously published experimental and numerical data, relative to the collapse of loose rectangular granular piles in water, are reproduced by the proposed model and a satisfactory agreement is found, revealing that the DAM is able to describe reliably the overall dynamics, in terms of quantities of technical interest as the height of the submerged granular flow and the instantaneous position of its front. [ABSTRACT FROM AUTHOR]

We provide numerical evidence of the transition from rank-ordered trains of internal solitary waves (ISWs) to multi-humped packets. We ascribe such peculiar and unexplored dynamics to the emergence of a long-range influence among waves, mediated by the hydrodynamic conditions developing in the wake of each ISW and promoting acceleration of the trailing waves. Both the train during the transition and its final multi-humped configuration share hydrodynamic features which are consistent with the recently proposed theory of ISWs propagating through moving stratified environments. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, we numerically investigate the breakup dynamics of droplets in an emulsion flowing in a tapered microchannel with a narrow constriction. The mesoscale approach for multicomponent fluids with near contact interactions is shown to capture the deformation and breakup dynamics of droplets interacting within the constriction, in agreement with experimental evidence. In addition, it permits us to investigate in detail the hydrodynamic phenomena occurring during breakup stages. Finally, a suitable deformation parameter is introduced and analyzed to characterize the state of deformation of the system by inspecting pairs of interacting droplets flowing in the narrow channel. [ABSTRACT FROM AUTHOR]

The Lattice Boltzmann Method (LBM) is a mesoscopic method founded on the kinetic theoryof Boltzmann and has been proven to describe successfully flows of hydraulic interest such asturbulent, multicomponent and multiphase flows. The main advantage of the LBM, comparedto more traditional methods of the Computational Fluid Dynamics, are: the simplicity of theset of explicit algebraic equations forming the computational algorithm, the ease ofdescription of complex boundaries and the proneness of the LBM to provide unprecedentedparallel scalability on a large number of architectures. Thus, the computational resourcesavailable nowadays make the employment of 3D models feasible to study large-scaleproblems as well. In this work, a fully 3D LBM model is developed and applied to the simulation of freesurface impulsive flows. The evolution of the free surface is here tracked by means of a 3Dinterface tracking model and coupled to the traditional LBM algorithm. Dam-breakevents are simulated and LBM numerical results are compared to both analytical andexperimental reference results. The latter are relative to symmetrical dam-break,asymmetrical dam-break, dam-break over a triangular obstacle, dam-break with anisolated building and the laboratory model of the Toce river test case. The accuracy ofthe model is assessed by comparing depth and velocity values, when available,at some gauge locations. The study shows that the LBM provides a remarkablyaccurate description of the phenomena. The results are very promising for futureapplications of this kind of models for the study of realistic hydraulic problems. [ABSTRACT FROM AUTHOR]

• Polydisperse shallow granular flow equations are obtained. • The Discrete Boltzmann Equation model equivalent to the polydisperse shallow granular flow equations is presented and proposed as solution method. • Several 1 D and 2 D flows, concerned with the propagation of discontinuities in three-phase shallow granular flows are investigated. A polydisperse shallow mixture consisting of N p solid phases (N p ≥ 1) and one fluid phase (the ambient fluid) is a reliable model for several industrial and environmental flows, as e.g. landslides, avalanches, debris flows and fluidized beds. The description and prediction of these flows is of primary importance, mainly with respect to the mitigation and protection from natural hazards. This paper is aimed at deriving the polydisperse shallow granular flow equations by depth-averaging mass and momentum equations of the mixture and at formulating an equivalent Discrete Boltzmann Equation (hereinafter DBE) model as solution method. The reason is the simplicity and the versatility of the DBE, which consists of a set of purely advective, linear, first order partial differential equations, whose numerical integration does not need sophisticated methods. Both 1 D and 2 D benchmarks, concerning with the propagation of discontinuities in three-phase shallow granular flows, are obtained by applying the finite differences Lax-Friedrichs (hereinafter LF) method to the polydisperse shallow granular flow equations. The overall agreement is good, showing that the DBE and the LF numerical results are equivalent. [ABSTRACT FROM AUTHOR]

In this paper a Discrete Boltzmann Equation model (hereinafter DBE) is proposed as solution method of the two-phase shallow granular flow equations, a complex nonlinear partial differential system, resulting from the depth-averaging procedure of mass and momentum equations of granular flows. The latter, as e.g. a debris flow, are flows of mixtures of solid particles dispersed in an ambient fluid. The reason to use a DBE, instead of a more conventional numerical model (e.g. based on Riemann solvers), is that the DBE is a set of linear advection equations, which replaces the original complex nonlinear partial differential system, while preserving the features of its solutions. The interphase drag function, an essential characteristic of any two-phase model, is accounted for easily in the DBE by adding a physically based term. In order to show the validity of the proposed approach, the following relevant benchmark tests have been considered: the 1D simple Riemann problem, the dam break problem with the wet–dry transition of the liquid phase, the dry bed generation and the perturbation of a state at rest in 2D. Results are satisfactory and show how the DBE is able to reproduce the dynamics of the two-phase shallow granular flow. [ABSTRACT FROM AUTHOR]

In this work a Discrete Boltzmann Model for the solution of transcritical 2D shallow water flows is presented and validated. In order to provide the model with transcritical capabilities, a particular multispeed velocity set has been employed for the discretization of the Boltzmann equation. It is shown that this particular set naturally yields a simple and closed procedure to determine higher order equilibrium distribution functions needed to simulate transcritical flow. The model is validated through several classical benchmarks and is proven to correctly and accurately simulate both 1D and 2D transitions between the two flow regimes. [ABSTRACT FROM AUTHOR]

Free-surface flows play an important role in many environmental hydraulic problems. Owing to their characteristic large spatial and temporal scales, reduced complexity models, such as depth-integrated shallow water (DI-SW) models, have so far been largely preferred to three-dimensional (3D) approaches. Even if they can reproduce water levels satisfactorily, DI-SW models do not natively provide flow velocity at the bed, which is pivotal for estimating scour effects induced by impulsive flows, such as those resulting from dam breaks. However, resort to fully 3D modeling is often impractical due to the high computational demand required; nonetheless, recent works have shown impressive performance of the lattice Boltzmann method (LBM) compared to classical numerical methods while proving to be as accurate in many fields, suggesting LBM makes such computations affordable on desktop computers. So far, a comprehensive assessment of a LBM-based, fully 3D, free-surface model is still lacking. In this work, a LBM model is compared to data from a large series of dam-break-flow experiments, in terms of water levels and, where available, flow velocities. Comparisons with available DI-SW models are also carried out. Results show that the LBM model can correctly simulate both water levels and velocity distributions, the latter often being inaccurately reproduced by DI-SW models. To provide guidelines for the correct modeling of such flows, considerations are also given to opportunities to use either no-slip or free-slip boundary conditions, the latter being preferable if underresolved boundary layers are present. [ABSTRACT FROM AUTHOR]

COVID-19 pandemic in Italy displayed a spatial distribution that made the tracking of its time course quite difficult. The most relevant anomaly was the marked spatial heterogeneity of COVID-19 diffusion. Lombardia region accounted for around 60% of fatal cases (while hosting 15% of Italian population). Moreover, 86% of fatalities concentrated in four Northern Italy regions. The 'explosive' outbreak of COVID-19 in Lombardia at the very beginning of pandemic fatally biased the R-like statistics routinely used to control the disease dynamics. To (at least partially) overcome this bias, we propose a new index RI = dH/dI (daily derivative ratio of H and I, given H = Healed and I = Infected), corresponding to the ratio between healed and infected patients relative daily changes. The proposed index is less flawed than R by the uncertainty related to the estimated number of infected persons and allows to follow (and possibly forecast) epidemic dynamics in a largely model-independent way. To analyze the dynamics of the epidemic, starting from the beginning of the virus spreading—when data are insufficient to make an estimate by adopting SIR model—a "sigmoidal family with delay" logistic model was introduced. That approach allowed in estimating the epidemic peak using the few data gathered even before mid-March. Based on this analysis, the peak was correctly predicted to occur by end of April. Analytical methodology of the dynamics of the epidemic we are proposing herein aims to forecast the time and intensity of the epidemic peak (forward prediction), while allowing identifying the (more likely) beginning of the epidemic (backward prediction). In addition, we established a relationship between hospitalization in intensive care units (ICU) versus deaths daily rates by avoiding the necessity to rely on precise estimates of the infected fraction of the population The joint evolution of the above parameters over time allows for a trustworthy and unbiased estimation of the dynamics of the epidemic, allowing us to clearly detect the qualitatively different character of the 'so-called' second wave with respect to the previous epidemic peak. [ABSTRACT FROM AUTHOR]

Nowadays the Lattice Boltzmann Method (LBM), thanks to its mesoscale approach, is a powerful tool when dealing with upscaling problems in Fluid Dynamics. In this study a numerical investigation on the flow through fractured porous media has been performed, based on a three dimensional LBM model developed at the Engineering Department of the Roma Tre University. Two different kinds of porous matrices, consisting of deterministic and random arrays of spheres, have been considered. The formers have been arranged both as a primitive and body centred cubic system. The latters are characterized both by monodisperse and by polydisperse diameters. In order to calculate the permeability of every considered porous medium, the Darcy's experiment has been simulated. Then a channel has been considered in every porous matrix, giving raise to different fractured porous media. The Darcy's experiment has been repeated with the fractured porous media and the LBM has been used as a numerical lab to verify the ranges of validity of upscaled models based on the analytical solution of flow in equivalent porous media. It has been found that numerical and analytical results are in good agreement when a fractured deterministic porous medium is considered. In this case the order of magnitude of the relative error, defined as the absolute value of the percentage difference between the numerical and analytical velocity on the axis of the channel, is equal to few percents. Moreover, the error decreases with the permeability: the greater the permeability, the smaller the error. LBM numerical results obtained considering random fractured porous media are characterised by a relative error larger than that corresponding to numerical results obtained considering deterministic fractured porous media, being equal permeability and hydrodynamic conditions. Future work is addressed to the understanding of how the parameters characterizing the random fractured porous media affect the results obtained by applying the LBM model and the difference with the reference results. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate that vertically vibrating a plate in a cornstarch suspension causes the suspension to vigorously ratchet up the plate. We show that this is a necessary consequence of the fact that cornstarch inwater is shear thickening: when the plate moves up it opposes gravity and so the fluid stiffens; when it moves down itworks with gravity and so the fluid flows. This produces asymmetric ratcheting that opposes gravity. We find several unusual states that result in this simple experimental system, and we reproduce the essential effect in two different numerical simulations. [ABSTRACT FROM AUTHOR]

Regional and teleseismic earthquakes can induce displacements along joints in a landslideinvolved rocky slope in Central Italy. The rarity of these effects is due to specific physical properties of the seismic signals associated with: (i) the energy content, (ii) the distribution of relative energy and peak of ground acceleration related to the ground motion components and (iii) the spectral amplitude distribution in the frequency domain; these properties allow the triggering earthquakes to be distinguished from the others. The observed effects are relevant when compared to the direction of the landslide movement and the dimensions of the involved rock mass volume. The landslide movement is less constrained in the direction parallel to the dip of the slope and the landslide dimensions are associated with characteristic periods that control the landslide deformational response in relation to the spectral content of the ground motion. The earthquake-induced displacements are significant because they have the same order of magnitude as the average annual cumulative displacement based on a decade of strain measurements within the slope. [ABSTRACT FROM AUTHOR]

Ten small rock slides (with a volume ranging from 10 to 10 m) on a slope affected by working activities were detected, located, and timed using pictures collected by an automatic camera during 40 months of continuous monitoring with terrestrial SAR interferometry (TInSAR). These landslides were analyzed in detail by examining their pre-failure time series of displacement inferred from high-sampling frequency (approximately 5 min) TInSAR monitoring. In most of these cases, a typical creep behavior was observed with the displacement starting 370 to 12 h before the collapse. Additionally, an evident acceleration decrease of the displacement a few hours before the failure was observed in some rock/debris slides, thus suggesting the possibility of a mechanical feature of the slope that differs from the classical creep theory. The efficacy of the linear Fukuzono approach for the prediction of time of failure was tested by back-analyzing the ten landslides. Furthermore, a modified Fukuzono approach named average data Fukuzono (ADF) was implemented and applied to our dataset. Such an approach is able to improve forecasting effectiveness by reducing the error due to anomalies in the time series of displacement, like the acceleration decrease before failure. A prediction with a temporal accuracy of at least 2 h was obtained for all the analyzed rock/debris slides. [ABSTRACT FROM AUTHOR]