Experimental and numerical analysis of an unsaturated volcanic ash deposit for the establishment of an early warning system in a quarry in Costa Rica

The paper presents the work carried out for the analysis of the hydro-mechanical behaviour of a volcanic ash in unsaturated conditions for the establishment of a landslide eary warning system (EWS) in a Costa Rican quarry. Rapid mass movements involving ash material occurred in the greater quarry area during heavy rainfall events. Laboratory testing and numerical analyses were combined to provide a better understanding of the triggering mechanism and to provide guidelines for the EWS. Along with conventional laboratory analyses, the experimental program involved tests carried out under suction control to assess the volumetric response in unsaturated conditions of the material and to highlight potential collapse-upon wetting phenomena. The behaviour of the material was analysed in a modern framework for soils in unsaturated conditions developed at the EPFL. Numerical simulations were performed with a coupled hydro-mechanical finite elements code in which the mentioned constitutive model is implemented. The analyses allowed a better understanding of the landslide triggering mechanism and provided pore water pressure values which are associated to the failure mechanism and can be used as threshold values for the EWS.

Numerical simulation of a sailing boat

Matteo Lombardi

In this work, various aspects concerning the numerical simulation of a sailing boat are investigated. The attention is focused on simulation of the free-surface, the fluid-structure interaction (FSI) between wind and sails, and the dynamics of the whole boat. Some preliminary work on shape optimization is also presented. Free-surface simulations are carried out both on classical academic benchmark problems and for the prediction of the wave pattern around racing yachts. The comparison with experimental results and numerical data obtained with other CFD codes has proven the validity of the proposed set-up. In this framework, the Level Set method has been implemented and validated as a possible solution to the problem of interface diffusion arising in planing conditions with the standard Volume of Fluid approach. The FSI problem governing the interaction between wind and sails is solved via a strongly coupled segregated approach based on the standard Dirichlet-Neumann coupling. The structural solution is based on a MITC4 finite-element shell code. The fluid and structural meshes are non-conforming and the exchange of information at the interface is obtained via radial basis functions (RBF) interpolation. The fluid mesh motion is accomplished either through the mapping generated using the radial basis functions or via a method based on the inverse distance weighting (IDW) interpolations. The attention is paid to the methodological aspects of this complex problem and to the analysis of the numerical results. The fluid-structure interaction simulations of one and two sail configurations, with different trimmings, are presented; both steady and transient simulations are performed. The results obtained are very encouraging and show the potential of the proposed model. The dynamic motion of the Series 60 hull and the Alinghi’s AC 32 monohull complete of bulb, keel and sails have also been investigated. For the latter in particular, the free sink, trim and roll case has proven to be particularly interesting, with the boat rolling considerably on the side due to the aerodynamic loads exerted by the wind on the sails. Here, the whole boat has been considered as a rigid body but the integration of the FSI module for the sails into the full boat dynamic system is already under development. Finally, a preliminary analysis of shape optimization techniques applied to sailing boat elements have been investigated: in particular the attention has been focused on the drag minimization of (pseudo) bulb geometries under the constraint of fixed volume/fixed righting moment. The shape parametrization have been achieved either using directly the surface element nodes or via the Free Form Deformation (FFD) technique while the optimization algorithm has been based either on the solution of the adjoint Navier-Stokes equations or via pseudo finite-difference algorithms. The algorithm and developments mentioned have been implemented in a common open-source framework, the library OpenFOAM®.

EPFL2012

Numerical Simulation of the Semi-Rigid Behaviour of Integrally Attached Timber Folded Surface Structures

Anh Chi Nguyen, Aryan Rezaei Rad, Andrea Stitic, Yves Weinand

Timber folded surface structures assembled using semi-rigid multiple tab and slot joints (MTSJ) have been shown to form feasible structural systems with high load bearing potential. However, for their further development and use on large building scales, a pertinent model for prediction of their structural behaviour has yet to be developed. This paper focuses on simplified numerical methods for accurately modelling the semi-rigid structural behaviour of bidirectional timber folded surface structures with multiple tab and slot connections. Within this scope, the structure behaviour is considered to be in the elastic stage. Three practical methods of analysis for such structural systems are presented. The first two approaches use the Finite Element Method (FEM), where the theory of plates and shells are applied. In the first method, the MTSJs are modeled using strip element models, while, in the second strategy, spring models are used. The third modeling strategy elaborates on the new macroscopic mechanical models, referred to as macro models. Sets of one-dimensional (1D) elements are used to represent the mechanical behaviour of the entire system. Both linear and geometric nonlinear analysis are performed for all three modeling strategies. The numerical results are then validated against the large scale experiments. Comparison of the strip and spring element model results have shown that the strips represent more accurately the experimentally obtained values. Concerning the macro modelling approach, very good agreement with both detailed FE modelling approaches, as well as experimental results, were obtained. The results indicate that both linear and nonlinear analysis can be used for modelling the displacements within the elastic range. However, it is essential to include geometric nonlinearities in the analysis for accurate modelling of occurring strains as well as for displacements when considering higher load levels. Finally, it is demonstrated that including semi-rigidity in the numerical models is of high importance for analysing the behaviour of timber folded surface structures with MTSJ.

2019

Numerical simulation of a sailing boat

Matteo Lombardi

EPFL2012

Numerical Simulation of the Semi-Rigid Behaviour of Integrally Attached Timber Folded Surface Structures

Anh Chi Nguyen, Aryan Rezaei Rad, Andrea Stitic, Yves Weinand

2019