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The Virtual Element Method (VEM) for the elasticity problem is considered in the framework of the Hu-Washizu variational formulation. In particular, a couple of low-order schemes presented in [1] , are studied for quadrilateral meshes. The methods under consideration avoid the need of the stabilization term typical of the VEM, due to the introduction of a suitable projection on higher-order polynomials. The schemes are proved to be stable and optimally convergent in a compressible regime, including the case where highly distorted (even non-convex) meshes are employed. [ABSTRACT FROM AUTHOR]

An original, variational formulation of the Virtual Element Method (VEM) is proposed, based on a Hu–Washizu mixed variational statement for 2D linear elastostatics. The proposed variational framework appears to be ideal for the formulation of VEs, whereby compatibility is enforced in a weak sense and the strain model can be prescribed a priori, independently of the unknown displacement model. It is shown how the ensuing freedom in the definition of the strain model can be conveniently exploited for the formulation of self-stabilized and possibly locking-free low order VEs. The superior performances of the VEs formulated within this framework has been verified by application to several numerical tests. [ABSTRACT FROM AUTHOR]

A recent mixed formulation of the Virtual Element Method in 2D elastostatics, based on the Hu-Washizu variational principle, is here extended to 2D elastodynamics. The independent modeling of the strain field, allowed by the mixed formulation, is exploited to derive first order quadrilateral Virtual Elements (VEs) not requiring a stabilization (namely, self-stabilized VEs), in contrast to the standard VEs, where an artificial stabilization is always required for first order quads. Lumped mass matrices are derived using a novel approach, based on an integration scheme that makes use of nodal values only, preserving the correct mass in the case of rigid-body modes. In the case of implicit time integration, it is shown how the combination of a self-stabilized stiffness matrix with a self-stabilized lumped mass matrix can produce excellent performances both in the compressible and quasi-incompressible regimes with almost negligible sensitivity to element distortion. Finally, in the case of explicit dynamics, the performances of the different types of derived VEs are analyzed in terms of their critical time-step size. [ABSTRACT FROM AUTHOR]

Submarine buried pipelines interact with shallow soil layers that are often loose and prone to fluidization/liquefaction. Such occurrence is a possible consequence of pore pressure build-up induced by hydrodynamic loading, earthquakes, and/or structural vibrations. When liquefaction is triggered in sand, the soil tends to behave as a viscous solid–fluid mixture of negligible shear strength, possibly unable to constrain pipeline movements. Therefore, pipelines may experience excessive displacement, for instance, in the form of vertical flotation or sinking. To date, there are no well-established methods to predict pipe displacement in the event of liquefaction. To fill such a gap, this work proposes a computational fluid dynamics (CFD) framework enriched with soil mechanics principles. It is shown that the interaction between pipe and liquefied sand can be successfully analyzed via one-phase Bingham fluid modeling of the soil. Postliquefaction enhancement of rheological properties, viscosity, and yield stress can also be accounted for by linking soil–pipe CFD simulations to a separate analysis of the pore pressure dissipation. The proposed approach is thoroughly validated against the results of small-scale pipe flotation and pipe dragging tests from the literature. [ABSTRACT FROM AUTHOR]

3D Concrete Printing (3DCP) is a rapidly evolving technology that allows for the efficient and accurate construction of complex concrete objects. In this paper, a numerical modelling approach is presented for the simulation of the printing process of cementitious materials, based on the homogeneous fluid assumption. To cope with the large deformations of the domain and the nonlinearity resulting from the use of a non-Newtonian rheological law, the Navier–Stokes equations are solved in the framework of the Particle Finite Element Method (PFEM). Furthermore, tailored solutions have been formulated and implemented for the time-dependent moving boundary conditions at the nozzle outlet and for the efficient handling of the inter-layer contact in the same PFEM framework. The overall computational cost is decreased by the implementation of an adaptive de-refinement technique, which drastically reduces the number of degrees of freedom in time. The proposed modelling approach is finally validated by simulating the printing process of six rectilinear layers and one multi-layer "wall". The results show good agreement with the experimental data and provide valuable insights into the printing process, paving the way for the use of numerical modelling tools for the optimization of materials and processes in the field of 3D Concrete Printing. [ABSTRACT FROM AUTHOR]

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]

Structural design with highly flowable Fibre Reinforced Concrete has to duly take into account the preferential alignment of fibers, which can be governed through the rheological properties of the fluid mixture and the casting process and by the geometry of the structure. The possibility of predicting the fiber alignment, by tailoring the casting process, and of non-destructively monitoring it, can foster more efficient structural applications and design approaches. Focusing on UHPFRC slabs with pre-arranged casting defects, the flow-induced alignment of the fibers has been predicted by means of a suitable CFD modelling approach and hence monitored via a non-destructive method based on magnetic inductance properties of the fiber reinforced composite. The comparison between the assessed data on the fiber orientation and the crack patterns as visualized by image analysis supports the effectiveness of casting flow modelling and non-destructive fiber dispersion monitoring in supporting the structural design of elements made with highly flowable fiber reinforced cementitious composites. [ABSTRACT FROM AUTHOR]

SUMMARY A Lagrangian numerical approach for the simulation of rapid landslide runouts is presented and discussed. The simulation approach is based on the so-called Particle Finite Element Method. The moving soil mass is assumed to obey a rigid-viscoplastic, non-dilatant Drucker-Prager constitutive law, which is cast in the form of a regularized, pressure-sensitive Bingham model. Unlike in classical formulations of computational fluid mechanics, where no-slip boundary conditions are assumed, basal slip boundary conditions are introduced to account for the specific nature of the landslide-basal surface interface. The basal slip conditions are formulated in the form of modified Navier boundary conditions, with a pressure-sensitive threshold. A special mixed Eulerian-Lagrangian formulation is used for the elements on the basal interface to accommodate the new slip conditions into the Particle Finite Element Method framework. To avoid inconsistencies in the presence of complex shapes of the basal surface, the no-flux condition through the basal surface is relaxed using a penalty approach. The proposed model is validated by simulating both laboratory tests and a real large-scale problem, and the critical role of the basal slip is elucidated. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We discuss a Lagrangian approach for the simulation of 3D compressible flows on non-structured tetrahedral meshes. The formulation is nodal-based, in the sense that kinematic and thermodynamic variables are all interpolated with continuous P1 polynomials. The equations are solved with an explicit time-marching scheme without stabilizing terms and with the inclusion only of a shock-capturing viscosity. Several examples featuring shock propagations and mixing of fluids are addressed, with particular emphasis on the stability of the approach and on possible strategies for mesh update. [ABSTRACT FROM AUTHOR]

In this paper, we analyze possible sources of dissipation in aluminium nitride (AlN) contour mode resonators for three different resonance frequency devices ( f \!r ) (220 MHz, 370 MHz, and 1.05 GHz). For this purpose, anchors of different widths ( W \!a ) and lengths ( L \!a ) proportional to the acoustic wavelength ( \lambda ) are designed as supports for resonators in which the dimensions of the vibrating body are kept fixed. The {Q} extracted experimentally confirms that anchor losses are the dominant source of damping for most anchor designs when f \!r is equal to 220 and 370 MHz. For specific anchor dimensions ( W \!a / \lambda is in the range of 1/4–1/2) that mitigate energy leakage through the supports, a temperature-dependent dissipation mechanism dominates as seen in higher Q due to anchor losses, we use a finite-element method with absorbing boundary conditions. We also propose a simple analytical formulation for describing the dependence of the temperature-dependent damping mechanism on frequency. In this way, we are able to quantitatively predict Q due to anchor losses and qualitatively describe the trends observed experimentally. [2014-0232] [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]

Abstract: Cementitious composites with customized rheologies are becoming increasingly popular throughout a wide variety of civil engineering applications. Assessing their fundamental rheological properties is crucial for the success of a particular application. Their measurement is not a trivial task and typically requires dedicated and expensive equipment. These equipment may not be compatible with field applications and not even available in every laboratory. Correlations between fundamental rheological properties and field test measurements have been assessed, as for the yield stress versus the slump flow diameter. As for the plastic viscosity, different attempts have been made, with flow time parameters measured from different tests. This work provides further evidence to the aforementioned correlations, with reference to a broad range of cement pastes and mortars formulated from SCCs, as well as employing a tool for Computational Fluid Dynamics (CFD) modeling developed by the authors. [Copyright &y& Elsevier]

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]

First, we define in this paper two benchmark flows readily usable by anyone calibrating a numerical tool for concrete flow prediction. Such benchmark flows shall allow anyone to check the validity of their computational tools no matter the numerical methods and parameters they choose. Second, we compare numerical predictions of the concrete sample final shape for these two benchmark flows obtained by various research teams around the world using various numerical techniques. Our results show that all numerical techniques compared here give very similar results suggesting that numerical simulations of concrete filling ability when neglecting any potential components segregation have reached a technology readiness level bringing them closer to industrial practice. [ABSTRACT FROM AUTHOR]