Research

Nonsmooth simulations of 3D Drucker-Prager granular flows and validation against experimental column collapses

In collaboration with Gauthier Rousseau (TU Wien, formerly post-doc in the team), Hugo Rousseau (INRAE) and Gilles Daviet (NVIDIA, formerly Ph.D. student in the team), we have performed thorough comparisons between the predictions of our numerical solver Sand6 for granular flows 7.1.2, and collapse experiments conducted in a narrow channel (in collaboration with EPFL). We have shown that our nonsmooth simulator, which relies on a constant friction coefficient corresponding to the yield angle of a granular heap, is able to reproduce with high fidelity various experimental granular collapses over inclined erodible beds. Our results, obtained for two different granular materials and for various bed inclinations, suggest that a simple constant friction rheology choice remains reasonable for capturing a large variety of unsteady granular flows at low inertial number. Using the versatility of our numerical approach, we have further analysed the possible biases pertaining to laboratory-scale experiments, and shown that, in the case of granular collapses, accurate predictions could be performed as long as care was taken in measuring yield angle of the granular material appropriately. This study has been published in the Journal of Fluid Mechanics 8, and all the research data provided for reproducibility at doi.org/10.5281/zenodo.7288829.

Randomly stacked open-cylindrical shells as a functional mechanical device

In collaboration with Tomohiko Sano (Keio University), we have studied the mechanical behaviour of open cylindrical shells, randomly stacked in 2D configurations. Using both numerical simulations (relying on our validated curvature-based fibre model with non-smooth frictional contact 7) and experiments, we have shown that despite the randomness of the configurations, the stacked shells exhibit robust macroscopic dissipative properties, involving complex interplay between elasticity and friction, which control the occurrence of snap-fit events at the micro-scale. Our results demonstrate that the rearrangement of flexible components could yield versatile but predictive mechanical responses, paving the way to new kinds of metamaterials. These results have been presented at the HFSS 2023 international conference 12 and published in Nature Communications Materials 9.

Hydrodynamic model for fish locomotion

In collaboration with Bruno Ventéjou (co-supervised post-doc) and Philippe Peyla (LIPhy, UGA), we study the respective roles of hydrodynamic and social interactions within schools of fish, in the context of the FISHSIF ANR project. As a first step toward the simulation of large assemblies of swimming fish, we have developed a simplified hydrodynamic model, able to account for individual fish swimming and stigmergy, in particular regarding the generation of vortices wakes, without the need to introduce deformation of the body of the fish. The scaling analysis of our model falls accurately within universal swimming laws observed among many aquatic species  38, and further extends the laws of swimming efficiency to the low-inertial regime. Our model has been presented at the 2023 APS Division of Fluid Dynamics international conference 14, and a publication is in preparation.

Mixed elements for a curvature-based ribbon model

In collaboration with Sébastien Neukirch (Institut D’Alembert, Sorbonne Université) and our former Ph.D. student Raphaël Charrondière, we have improved our initial curvature-based numerical model for thin elastic ribbons  2 by developing a mixed strategy, where each ribbon element is treated independently, and glued to each other only at the final solving stage through well-chosen bilateral constraints.

Thanks to this mixed variational strategy, which yields a banded Hessian, our algorithm recovers the linear complexity of low-order models while preserving the quadratic convergence of curvature-based models. As a result, our approach is scalable to a large number of elements, and suitable for various boundary conditions and unilateral contact constraints, making it possible to handle challenging scenarios such as confined buckling experiments or Möbius bands with contact. Additionally, our numerical model can incorporate various ribbon energies, including the recent 16 model for quasi-developable ribbons recently introduced in Physics, which allows to transition smoothly between a rectangular Kirchhoff rod and a (developable) Sadowsky ribbon. Our numerical scheme is carefully validated against demanding experiments of the Physics literature, which demonstrates its accuracy, efficiency, robustness, and versatility.

This work is currently under review for a journal publication.

High-order contact detection between fibres

In this work we analyse the contact forces yielded by fibre models coupled to a segment-based contact detection algorithm. We note the occurrence of spurious jumps which, to the best of our knowledge, were never reported before. We demonstrate that these artifacts are actually directly linked to the low-order contact detection being used between the fibre and the obstacles, and that they worsen as curvature at contact increases. Low-order detection, based on segment proxys, is classically used due to its simplicity of treatment, even for fibre models possessing a higher-order geometry like super-helices. We show that spurious artifacts occur whatever the fibre model and contact response solver used, as soon as a segment-segment detection scheme is employed. To remove such numerical artifacts in the force profile, which can accumulate to yield large force errors, we introduce an efficient high-order detection algorithm, relying on an efficient adaptive pruning strategy.

This work, available as a preprint 15, is currently under review for a journal publication.

Numerical modeling of a feather’s vane, a highly anisotropic membrane

In the context of Jean Jouve’s doctoral thesis, in collaboration with Rahul Narain (Indian Institute of Technology Delhi, India) and Theodore Kim (Yale University, United States), we have developed a macroscopic shell simulator capable of dealing with highly anisotropic materials, such as feathers.

Feathers have a multi-scale structure with very interesting properties. From a central stiff rod (the rachis) hundreds of small fibers (barbs) come out to form a membrane-like structure, which is held together by yet another set of smaller fibers (barbules). This intricate structure drives the elastic response of the membrane and gives feathers the capability of reversible rupture. Our interest is to produce a simulator capable of realistically portray the deformation of a feather. This simulation is challenging because the membrane has two main directions with elastic coefficients differing by 4 orders of magnitude. We have compared and validated our results with experimental data, and an article has been submitted for publication.

Untangling the physics of self-locking in tight knots

In the context of the Ph.D. of Alexandre Teixeira Da Silva, co-supervised by Florence Bertails-Descoubes, Thibaut Métivet, Victor Romero, and Mélina Skouras (Anima, Inria GRA), we study the mechanical behaviour of tight knots, with a particular focus on the respective roles of elasticity and friction in the self-locking events – knot configurations where the knot remains tight even in the absence of external charge. We investigate such behaviour using both numerical simulations, performed with the PolyFEM software, and experiments, conducted within our 7.1.4 platform. Preliminary results have been presented for the overhand knot configuration at the HFSS international conference 13, along with an important validation study of the PolyFEM numerical methods in the context of large-strains mechanics.

Frictional three-point bending test: disentangling the role of friction through real and numerical experiments

In collaboration with Joël Marthelot, Ignacio Andrade-Silva and Olivier Pouliquen (IUSTI, Aix Marseille Université), we have investigated the role of friction in the well-known three-point bending test, traditionally used to measure the bending modulus of slender structures. By performing experiments and numerical simulations, both compared to our new theoretical model, we have devised an efficient protocol to disentangle the respective roles of elasticity and friction in the force response of an indented rod lying on frictional supports, thereby allowing accurate and independent measurements of both the bending modulus and the friction coefficient of the considered material. This work has been presented at the ESMC 2022 international conference  41, and an article is in preparation.