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Summary: This paper concerns the rheological characterisation of liquefied sands as non‐Newtonian Bingham fluids. For this purpose, dam‐breaking laboratory tests are often executed and interpreted, offering a viable option to identify the properties of fluidised water‐soil mixtures. However, limited attention has been devoted so far to clarify what variables and measurements would allow unambiguous calibration of Bingham parameters, namely, the viscosity η and the yield stress τy. The numerical results of parametric studies based on the particle finite element method (PFEM) are critically inspected to gain deeper insight into the problem. First, it is confirmed that multiple η − τy pairs may reproduce the same experimental evidence when formed by only one measurement—usually, the post–dam‐breaking displacement of the bottom toe (tip) of the liquefied mass. Then, two alternative procedures are proposed for unambiguous identification of both η and τy: one is based on monitoring the evolving aspect ratio of the fluid mass during free, gravity‐driven flow; the other relies on a slightly different dam‐breaking test, also including impact against a rigid obstacle. In particular, the latter approach reduces the relevant duration of the test, reducing the possible influence of reconsolidation effects on the calibration of rheological parameters. [ABSTRACT FROM AUTHOR]

Highlights • Accurate and efficient numerical simulation of granular flow. • Two regularized models of the μ (I) -rheology. • Application of the PFEM to frictional material simulation. • Validation against 3D experimental tests. Abstract This paper proposes two regularized models of the μ (I) -rheology and shows their application to the numerical simulation of 3D dense granular flows. The proposed regularizations are inspired by the Papanastasiou and Bercovier–Engleman methods, typically used to approximate the Bingham law. The key idea is to keep limited the value of the apparent viscosity for low shear rates without introducing a fixed cutoff. The proposed techniques are introduced into the Particle Finite Element Method (PFEM) framework to deal with the large deformations expected in free-surface granular flows. After showing the numerical drawbacks associated to the standard μ (I) -rheology, the two regularization strategies are derived and discussed. The regularized μ (I) -rheology is then applied to the simulation of the collapse of 2D and 3D granular columns. The numerical results show that the regularization techniques improve substantially the conditioning of the linear system without affecting the solution accuracy. A good agreement with the experimental tests and other numerical methods is obtained in all the analyzed problems. [ABSTRACT FROM AUTHOR]

The numerical simulation of airbags is receiving an increasing attention for the remarkable advantages in terms of cost, efficiency, flexibility and amount of data that can be extracted from the analysis. This work proposes an advanced fluid–structure interaction (FSI) numerical technique for the simulation of airbag deployment. The fluid subproblem, described by weakly compressible Navier–Stokes equations, is solved exploiting the advanced features of the Particle Finite Element Method (PFEM) while the solid subproblem is addressed using standard Finite Element method. A domain decomposition approach with a special treatment of the fluid–structure interface conditions has been implemented to couple fluid and structural solvers allowing for non-conforming meshes at the interface and different time step size in the two subdomains. A peculiar feature of the proposed methodology is the explicit time integration, mandatory for the solution of very fast dynamics problems, like the airbag deployment: an explicit fluid solver is coupled explicitly with an explicit structural solver. The proposed technique is first tested on a inflation of a balloon, showing very good agreements and then it has been applied to the real case of airbag deployment. • Advanced fluid–structure interaction technique for the simulation of airbag deployment. • PFEM for the fluid part and standard FEM for the solid part. • Non-conforming meshes at the interface and different time step sizes. • An explicit fluid solver is coupled explicitly with an explicit structural solver. • Validation with a benchmark test and comparisons with experimental data. [ABSTRACT FROM AUTHOR]

Prediction of multi-hazard slope stability events requires an informed and judicious choice of the possible scenarios. An incorrect definition of landslide conditions in terms of expected failure volume, material behavior, or boundary conditions can lead to inaccurate predictions and, in turn, to wrong engineering and risk management decisions. Reduced-scale experiments carried out two years before the Vajont disaster were carried out with a material not representative of the actual rockslide behavior and failed in not considering the simultaneous failure of the whole landslide body. Based on these inappropriate assumptions, the physical models led to wrong estimates of the safety operational level for the Vajont reservoir. This work uses the Particle Finite Element Method (PFEM) to analyze the implications of the wrong hypotheses considered in the pre-event experiments, simulating numerically the Vajont disaster for different sliding volumes and material properties. The use of the PFEM for the accurate assessment of the consequences of landslides impinging in water reservoirs has been already validated in a companion paper. In this work, we demonstrate the capabilities of a robust and reliable numerical modeling approach for the simulation of different scenarios, assessing what could have been a safe operational reservoir level in the case of a landslide generated impulse wave. The three-dimensional analyses were run with a high mesh resolution and demonstrate the suitability and robustness of the PFEM model for large-scale landslide and multi-hazard events simulation. • Application of a Lagrangian numerical method to the 3D simulation of Vajont landslide; • Analysis of the different multi-failure scenarios considered in the physical tests performed before the disaster; • Analysis of Vajont disaster for different initial water levels in the reservoir; • Comparison and discussion of the results given by the different configurations analyzed. [ABSTRACT FROM AUTHOR]

This work presents a numerical method for the simulation of landslides generated impulse waves and its application to the historical Vajont case study. The computational tool is based on the Particle Finite Element Method (PFEM), a Lagrangian strategy that combines the finite element solution of the governing equations with an efficient remeshing strategy to deal with large deformation problems. After presenting the numerical formulation, different landslide impulse wave problems with Froude number ranging from 0.5 to 2.8, are analyzed to validate the proposed methodology. The computational method is shown to be able to reproduce accurately the landslide runout, the momentum transfer between the sliding material and the impounded water, and the consequent wave propagation observed in experimental physical models. Then, the PFEM model is applied to the numerical simulation of the Vajont disaster, which is analyzed with a fully-resolved three-dimensional model. The numerical results are discussed and compared to the post-event observations and the numerical results of other computational methods. The results in terms of landslide velocity and runout, geometry of the deposit, maximum water runup, dam overtopping wave, and water discharge in the downstream valley are in good agreement with observations and reconstructions. The calibration and validation performed for this study form the basis for the PFEM analyses presented in a companion paper finalized to simulate different scenarios of the Vajont rockslide considered in the experimental tests done a year before the disaster. • Description of a Lagrangian method for the numerical simulation of landslides generated impulse waves; • Validation against several 2D and 3D benchmark problems with different Froude number; • Fully 3D simulation of Vajont landslide; • Comparison of the 3D results to post-event observations and other numerical simulations. [ABSTRACT FROM AUTHOR]

We present a Lagrangian nodal integration method for the simulation of Newtonian and non-Newtonian free-surface fluid flows. The proposed nodal Lagrangian method uses a finite element mesh to discretize the computational domain and to define the (linear) shape functions for the unknown nodal variables, as in the standard Particle Finite Element Method (PFEM). In this approach, however, the integrals are performed over nodal patches and not over elements, and strains/stresses are defined at nodes and not at Gauss points. This allows to limit the drawbacks associated with the remeshing and leads to a more accurate stress computation than in the classical elemental PFEM. Several numerical tests, in 2D and in 3D, are presented to validate the proposed nodal PFEM. In all cases, the method has shown a very good agreement with analytical solutions and with experimental and numerical results from the literature. A thorough comparison between nodal and elemental PFEMs is also presented, focusing on crucial issues, such as solution accuracy, convergence, mass conservation and sensitivity to mesh distortion. • Derivation of a new node-based Particle Finite Element Method for free-surface fluid flow. • Comparison of results accuracy and convergence between nodal-PFEM and standard elemental-PFEM. • Application to both Newtonian and non-Newtonian fluid models. • Presentation and analysis of several validation tests, in 2D and 3D. [ABSTRACT FROM AUTHOR]