RESEARCH CENTRE

Inria Centre
at Université Grenoble Alpes

2023
ACTIVITY REPORT

Project-Team

ELAN

modELing the Appearance of Nonlinear
phenomena

IN COLLABORATION WITH: Laboratoire Jean Kuntzmann (LJK)

DOMAIN

Applied Mathematics, Computation and
Simulation

THEME

Numerical schemes and simulations

Contents

Project-Team ELAN

1 Team members, visitors, external collaborators

2 Overall objectives

3 Research program

3.1 Discrete modelling of slender elastic structures . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Discrete and continuous modelling of frictional contact . . . . . . . . . . . . . . . . . . . . .
3.3 Inverse design of slender elastic structures [ERC GEM ] . . . . . . . . . . . . . . . . . . . . . .

4 Application domains

4.1 Mechanical Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Computer Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Soft Matter Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 Social and environmental responsibility

5.1 Footprint of research activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Impact of research results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6 Highlights of the year

6.1 Awards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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7 New software, platforms, open data

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7.1 New software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
7.1.1 Feel++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
7.1.2 Sand6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.3 MERCI
9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.4 ElanFab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1.5 so-bogus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1.6 meche2D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.2 New platforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.2.1 Elanfab 7.1.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

8 New results

11

8.1 Nonsmooth simulations of 3D Drucker-Prager granular flows and validation against experi-

mental column collapses

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.2 Randomly stacked open-cylindrical shells as a functional mechanical device . . . . . . . . . 12
8.3 Hydrodynamic model for fish locomotion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.4 Mixed elements for a curvature-based ribbon model
. . . . . . . . . . . . . . . . . . . . . . . 12
8.5 High-order contact detection between fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.6 Numerical modeling of a feather’s vane, a highly anisotropic membrane . . . . . . . . . . . 13
8.7 Untangling the physics of self-locking in tight knots
. . . . . . . . . . . . . . . . . . . . . . . 13
8.8 Frictional three-point bending test: disentangling the role of friction through real and

numerical experiments

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

9 Partnerships and cooperations

9.1 International research visitors

14
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1.1 Visits of international scientists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1.2 International collaborations
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.3 National initiatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

9.2 European initiatives

9.2.1 H2020 projects

10 Dissemination

16
10.1 Promoting scientific activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1.1 Scientific events: selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1.2 Journal
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1.3 Invited talks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1.4 Research administration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.2 Teaching - Supervision - Juries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.2.1 Teaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.2.2 Supervision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.2.3 Juries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.3 Popularization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

11 Scientific production

18
11.1 Major publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.2 Publications of the year
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.3 Cited publications

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Project-Team ELAN

Creation of the Project-Team: 2019 August 01

Keywords

Computer sciences and digital sciences

A2.5. – Software engineering

A5.5.4. – Animation

A6.1.1. – Continuous Modeling (PDE, ODE)

A6.1.4. – Multiscale modeling

A6.1.5. – Multiphysics modeling

A6.2.1. – Numerical analysis of PDE and ODE

A6.2.5. – Numerical Linear Algebra

A6.2.6. – Optimization

A6.2.7. – High performance computing

A6.2.8. – Computational geometry and meshes

A6.3.1. – Inverse problems

A6.5. – Mathematical modeling for physical sciences

A6.5.1. – Solid mechanics

A6.5.2. – Fluid mechanics

A6.5.3. – Transport

A9.2. – Machine learning

Other research topics and application domains

B1.1.2. – Molecular and cellular biology

B3.3.1. – Earth and subsoil

B5.5. – Materials

B9.2.2. – Cinema, Television

B9.5.3. – Physics

B9.5.5. – Mechanics

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Inria Annual Report 2023

1 Team members, visitors, external collaborators

Research Scientists

• Florence Descoubes [Team leader, INRIA, Senior Researcher, HDR]

• Thibaut Metivet [INRIA, Researcher]

• Victor Romero Gramegna [INRIA, ISFP]

PhD Students

• Emile Hohnadel [ENS DE LYON]

• Jean Jouve [ENS RENNES]

• Alexandre Teixeira Da Silva [INRIA]

Technical Staff

• Octave Crespel [ENSIMAG, Engineer, from Sep 2023]

Interns and Apprentices

• Adrien Decosse [ENS Rennes, Intern, from May 2023 until Jul 2023]

Administrative Assistant

• Julia Di Toro [INRIA]

Visiting Scientist

• Rahul Narain [INRIA, from Oct 2023]

2 Overall objectives

ELAN is a young research team of Inria and Laboratoire Jean Kuntzmann (UMR 5224), with an original
positioning across Computer Graphics and Computational Mechanics. The team is focussed on the
design of predictive, robust, efficient, and controllable numerical models for capturing the shape and
motion of visually rich mechanical phenomena, such as the buckling of an elastic ribbon, the flowing of
sand, or the entangling of large fiber assemblies. Target applications encompass the digital entertainment
industry (e.g., feature film animation, special effects), as well as virtual prototyping for the mechanical
engineering industry (e.g., direct and inverse design of textiles and metamaterials, sport performance
optimisation, cosmetology); though very different, these two application fields require predictive and
scalable models for capturing complex mechanical phenomena at the macroscopic scale. An orthogonal
objective is the improvement of our understanding of natural physical and biological processes involving
slender structures and frictional contact, through active collaborations with soft matter physicists. To
achieve its goals, the team strives to master as finely as possible the entire modeling pipeline, involving a
pluridisciplinary combination of scientific skills across Mechanics and Physics, Applied Mathematics,
and Computer Science.

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3 Research program

Thanks to an original and transverse positioning across Computer Graphics and Computational Mechan-
ics, complemented by tight connections with physicists, our goal is to tackle some challenging numerical
modelling issues related to complex macroscopic phenomena characterised by a nonlinear mechan-
ical behaviour and rich geometrical deformations. One major ambition of the ELAN team is to favour
interactions between all the relevant communities, with two objectives: 1/ significantly improve our
understanding and modelling capabilities of complex mechanical phenomena, in tight connection with
physicists, and 2/ better anticipate practical solutions for the wide diversity of exciting applications to
come in the near future. We propose in particular to focus on three research axes, detailed below.

3.1 Discrete modelling of slender elastic structures

For the last 15 years, we have investigated new discrete models for solving the Kirchhoff dynamic equa-
tions for thin elastic rods [20, 21, 24]. All our models share a curvature-based spatial discretisation,
allowing them to capture inextensibility of the rod intrinsically, without the need for adding any kinematic
constraint. Moreover, elastic forces boil down to linear terms in the dynamic equations, making them
well-suited for implicit integration. Interestingly, our discretisation methodology can be interpreted
from two different points-of-views. From the finite-elements point-of-view, our strain-based discrete
schemes can be seen as discontinuous Galerkin methods of zero and first orders. From the multibody
system dynamics point of view, our discrete models can be interpreted as deformable Lagrangian systems
in finite dimension, for which a dedicated community has started to grow recently [47]. We note that
adopting the multibody system dynamics point of view helped us formulate a linear-time integration
scheme [19], which had only be investigated in the case of multibody rigid bodies dynamics so far.

High-order spatial discretisation schemes for rods, ribbons and shells Our goal is to investigate similar
high-order modelling strategies for surfaces, in particular for the case of inextensible ribbons and shells.
Elastic ribbons have been scarcely studied in the past, but they are nowadays drawing more and more the
attention from physicists [34, 45]. Their numerical modelling remains an open challenge. In contrast to
ribbons, a huge litterature exists for shells, both from a theoretical and numerical viewpoints (see, e.g.,
[39, 25]). However, no real consensus has been obtained so far about a unified nonlinear shell theory
able to support large displacements. In [23] we have started building an inextensible shell patch by
taking as degrees of freedom the curvatures of its mid-surface, expressed in the local frame. As in the
super-helix model, we show that when taking curvatures uniform over the element, each term of the
equations of motion may be computed in closed-form; besides, the geometry of the element corresponds
to a cylinder patch at each time step. Compared to the 1D (rod) case however, some difficulties arise
in the 2D (plate/shell) case, where compatibility conditions are to be treated carefully. In [2] we have
proposed a new, curvature-based discretisation for a developable ribbon (i.e., a narrow plate), which we
plan to extend for building an inextensible plate model.

Numerical continuation of rod equilibria in the presence of unilateral constraints
In Alejandro
Blumentals’ PhD thesis [22], we have adopted an optimal control point of view on the static problem of
thin elastic rods, and we have shown that direct discretisation methods 1 are particularly well-suited for
dealing with scenarios involving both bilateral and unilateral constraints (such as contact). We would
like to investigate how our formulations extend to continuation problems, where the goal is to follow a
certain branch of equilibria when the rod is subject to some varying constraints (such as one fixed end
being applied a constant rotation). To the best of our knowledge, classical continuation methods used for
rods [35] are not able to deal with non-persistent or sliding contact.

3.2 Discrete and continuous modelling of frictional contact

Most popular approaches in Computer Graphics and Mechanical Engineering consist in assuming that
the objects in contact are locally compliant, allowing them to slightly penetrate each other. This is the

1Within this optimal control framework, our previous curvature-based methods can actually be interpreted as a special case of

direct single shooting methods.

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Inria Annual Report 2023

principle of penalty-based methods (or molecular dynamics), which consists in adding mutual repulsive
forces of the form k f (δ), where δ is the penetration depth detected at current time step [26, 44]. Though
simple to implement and computationally efficient, the penalty-based method often fails to prevent
excessive penetration of the contacting objects, which may prove fatal in the case of thin objects as
those may just end up traversing each other. One solution might be to set the stiffness factor k to a large
enough value, however this causes the introduction of parasitical high frequencies and calls for very small
integration steps [18]. Penalty-based approaches are thus generally not satisfying for ensuring robust
contact handling.

In the same vein, the friction law between solid objects, or within a yield-stress fluid (used to model
foam, sand, or cement, which, unlike water, cannot flow beyond a certain threshold), is commonly
modeled using a regularised friction law (sometimes even with simple viscous forces), for the sake of
simplicity and numerical tractability (see e.g., [46, 37]). Such a model cannot capture the threshold
effect that characterises friction between contacting solids or within a yield-stress fluid. The nonsmooth
transition between sticking and sliding is however responsible for significant visual features, such as
the complex patterns resting on the outer surface of hair, the stable formation of sand piles, or typical
stick-slip instabilities occurring during motion.

The search for a realistic, robust and stable frictional contact method encouraged us to depart from
those, and instead to focus on rigid contact models coupled to the exact nonsmooth Coulomb law for
friction (and respectively, to the exact nonsmooth Drucker-Prager law in the case of a fluid), which better
integrate the effects of frictional contact at the macroscopic scale. This motivation was the sense of the
hiring of F. Bertails-Descoubes in 2007 in the Inria/LJK BIPOP team, specialised in nonsmooth mechanics
and related convex optimisation methods. In the line of F. Bertails-Descoubes’s work performed in the
BIPOP team, the ELAN team keeps on including some active research on the finding of robust frictional
contact algorithms specialised for slender deformable structures.

Optimised algorithms for large nodal systems in frictional contact
In the fibre assembly case, the
resulting mass matrix M is block-diagonal, so that the Delassus operator can be computed in an efficient
way by leveraging sparse-block computations [28]. This justifies solving the reduced discrete frictional
contact problem where primary unknowns are forces, as usually advocated in nonsmooth mechanics [42].
For cloth however, where primal variables (nodal velocities of the cloth mesh) are all interconnected via
elasticity through implicit forces, the method developed above is computationally inefficient. Indeed,
the matrix M (only block-sparse, but not block-diagonal) is costly to invert for large systems and its
inverse is dense. Recently, we have leveraged the fact that generalised velocities of the system are 3D
velocities, which simplifies the discrete contact problem when contacts occur at the nodes. Combined
with a multiresolution strategy, we have devised an algorithm able to capture exact Coulomb friction
constraints at contact, while retaining computational efficiency [43]. This work also supports cloth
self-contact and cloth multilayering. How to enrich the interaction model with, e.g., cohesion, remains
an open question. The experimental validation of our frictional contact model is also one of our goals in
the medium run.

Continuum modelling of granular and fibrous media Though we have recently made progress on
the continuum formulation and solving of granular materials in Gilles Daviet’s PhD thesis [27, 30, 29],
we are still far from a continuum description of a macroscopic dry fibrous medium such as hair. One
key ingredient that we have not been considering in our previous models is the influence of air inside
divided materials. Typically, air plays a considerable role in hair motion. To advance in that direction, we
have started to look at a diphasic fluid representation of granular matter, where a Newtonian fluid and
the solid phase are fully coupled, while the nonsmooth Drucker-Prager rheology for the solid phase is
enforced implicitly [31]. This first approach could be a starting point for modelling immersed granulars
in a liquid, or ash clouds, for instance.

A long path then remains to be achieved, if one wants to take into account long fibres instead of
isotropic grains in the solid phase. How to couple the fibre elasticity with our current formulation remains
a challenging problem.

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3.3 Inverse design of slender elastic structures [ERC GEM ]

With the considerable advance of automatic image-based capture in Computer Vision and Computer
Graphics these latest years, it becomes now affordable to acquire quickly and precisely the full 3D
geometry of many mechanical objects featuring intricate shapes. Yet, while more and more geometrical
data get collected and shared among the communities, there is currently very little study about how
to infer the underlying mechanical properties of the captured objects merely from their geometrical
configurations.

An important challenge consists in developing a non-invasive method for inferring the mechanical
properties of complex objects from a minimal set of geometrical poses, in order to predict their dynamics.
In contrast to classical inverse reconstruction methods, our claim is that 1/ the mere geometrical shape
of physical objects reveals a lot about their underlying mechanical properties and 2/ this property can
be fully leveraged for a wide range of objects featuring rich geometrical configurations, such as slender
structures subject to contact and friction (e.g., folded cloth or twined filaments).

In addition to significant advances in fast image-based measurement of diverse mechanical materials
stemming from physics, biology, or manufacturing, this research is expected in the long run to ease
considerably the design of physically realistic virtual worlds, as well as to boost the creation of dynamic
human doubles.

To achieve this goal, we shall develop an original inverse modelling strategy based upon the following

research topics:

Design of well-suited discrete models for slender structures We believe that the quality of the up-
stream, reference physics-based model is essential to the effective connection between geometry and
mechanics. Typically, such a model should properly account for the nonlinearities due to large displace-
ments of the structures, as well as to the nonsmooth effects typical of contact and friction.

It should also be parametrised and discretised in such a way that inversion gets simplified math-
ematically, possibly avoiding the huge cost of large and nonconvex optimisation. In that sense, unlike
concurrent methods which impose inverse methods to be compatible with a generic physics-based model,
we instead advocate the design of specific physics-based models which are tailored for the inversion
process.

More precisely, from our experience on fibre modelling, we believe that reduced Lagrangian models,
based on a minimal set of coordinates and physical parameters (as opposed to maximal coordinates
models such as mass-springs), are particularly well-suited for inversion and physical interpretation
of geometrical data [33, 32]. Furthermore, choosing a high-order coordinate system (e.g., curvatures
instead of angles) allows for a precise handling of curved boundaries and contact geometry, as well as
the simplification of constitutive laws (which are transformed into a linear equation in the case of rods).
We are currently investigating high-order discretisation schemes for elastic ribbons and developable
shells [23, 2].

Static inversion of physical objects from geometrical poses We believe that pure static inversion may
by itself reveal many insights regarding a range of parameters such as the undeformed configuration of
the object, some material parameters or contact forces.

The typical settings that we consider is composed of, on the one hand, a reference mechanical
model of the object of interest, and on the other hand a single or a series of complete geometrical poses
corresponding each to a static equilibrium. The core challenge consists in analyzing theoretically and
practically the amount of information that can be gained from one or several geometrical poses, and to
understand how the fundamental under-determinacy of the inverse problem can be reduced, for each
unknown quantity (parameter or force) at play. Both the equilibrium condition and the stability criterion
of the equilibrium are leveraged towards this goal. On the theoretical side, we have recently shown that a
given 3D curve always matches the centerline of an isotropic suspended Kirchhoff rod at equilibrium
under gravity, and that the natural configuration of the rod is unique once material parameters (mass,
Young modulus) are fixed [1]. On the practical side, we have recently devised a robust algorithm to find a
valid natural configuration for a discrete shell to match a given surface under gravity and frictional contact
forces [4]. Unlike rods however, shells can have multiple inverse (natural) configurations. Choosing
among the multiple solutions based on some selection criteria is an open challenge. Another open issue,

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Inria Annual Report 2023

in all cases, is the theoretical characterisation of material parameters allowing the equilibrium to be
stable.

Dynamic inversion of physical objects from geometrical poses To refine the solution subspaces
searched for in the static case and estimate dynamic parameters (e.g., some damping coefficients), a
dynamic inversion process accounting for the motion of the object of interest is necessary.

In contrast to the static case where we can afford to rely on exact geometrical poses, our analysis in
the dynamic case will have to take into account the imperfect quality of input data with possible missing
parts or outliers. One interesting challenge will be to combine our high-order discretised physics-based
model together with the acquisition process in order to refine both the parameter estimation and the
geometrical acquisition. Our pluridisciplinary work [6] gives encouraging results regarding the ability to
recover material parameters and friction coefficient from merely visual observations of elastic bodies in
motion.

Experimental validation with respect to real data The goal will be to confront the theories developed
above to real experiments. Compared to the statics, the dynamic case will be particularly involving as
it will be highly dependent on the quality of input data as well as the accuracy of the motion predicted
by our physics-based simulators. Such experiments will not only serve to refine our direct and inverse
models, but will also be leveraged to improve the 3D geometrical acquisition of moving objects. Besides,
once validation will be performed, we shall work on the setting up of new non-invasive measurement
protocols to acquire physical parameters of slender structures from a minimal amount of geometrical con-
figurations. Our recent publication on validation benchmarks [7] represents a first important milestone
towards this research direction.

4 Application domains

4.1 Mechanical Engineering

Many physicists and mathematicians have strived for centuries to understand the principles governing
those complex mechanical phenomena, providing a number of continuous models for slender structures,
granular matter, and frictional contact. In the XXth century, industrial applications such as process
automatization and new ways of transportation have boosted the fields of Mechanical Engineering and
Computer-Aided Design, where material strength, reliability of mechanisms, and safety, standed for the
main priorities. Instead, large displacements of structures, buckling, tearing, or entanglement, and even
dynamics, were long considered as undesirable behaviors, thus restraining the search for corresponding
numerical models.

Only recently, the engineering industry has shown some new and growing interest into the modeling
of dynamic phenomena prone to large displacements, contact and friction. For instance, the cosmetology
industry is more and more interested in understanding the nonlinear deformation of hair and skin, with
the help of simulation. Likewise, auto and aircraft manufacturers are facing new challenges involving
buckling or entanglement of thin structures such as carbon or optical fibers; they clearly lack predictive,
robust and efficient numerical tools for simulating and optimizing their new manufacturing process,
which share many common features with the large-scale simulation scenarii traditionally studied in
Computer Graphics applications.

4.2 Computer Graphics

In contrast, Computer Graphics, which has emerged in the 60’s with the advent of modern computers,
was from the very beginning eager to capture such peculiar phenomena, with the sole aim to produce
spectacular images and create astonishing stories. At the origin, Computer Graphics thus drastically
departed from other scientific fields. Everyday-life phenomena such as cloth buckling, paper tearing, or
hair fluttering in the wind, mostly ignored by other scientists at that time, became actual topics of interest,
involving a large set of new research directions to be explored, both in terms of modelling and simulation.
Nowadays, although the image production still remains the core activity of the Computer Graphics

Project ELAN

7

community, more and more research studies are directed through the virtual and real prototyping of
mechanical systems, notably driven by a myriad of new applications in the virtual try on industry (e.g.,
hairstyling and garment fitting). Furthermore, the advent of additive fabrication is currently boosting
research in the free design of new mechanisms or systems for various applications, from architecture
design and fabrication of metamaterials to the creation of new locomotion modes in robotics. Some obvi-
ous common interests and approaches are thus emerging between Computer Graphics and Mechanical
Engineering, yet the two communities remain desperately compartmentalized.

4.3 Soft Matter Physics

From the physics-based viewpoint, since a few decades a new generation of physicists became interested
again in the understanding of such visually fascinating phenomena, and started investigating the tight
links between geometry and elasticity 2. Common objects such as folded or torn paper, twined plants,
coiled honey threads, or human hair have thus regained some popularity among the community in
Nonlinear Physics 3. In consequence, phenomena of interest have become remarkably close to those
of Computer Graphics, since scientists in both places share the common goal to model complex and
integrated mechanical phenomena at the macroscopic scale. Of course, the goals and employed methodo-
logies differ substantially from one community to the other, but showcase some evident complementarity:
while computer scientists are eager to learn and understand new physical models, physicists get more and
more interested in the numerical tools, in which they perceive not only a means to confirm predictions
afterwards, but also a support for testing new hypothesis and exploring scenarios that would be too
cumbersome or even impossible to investigate experimentally. Besides, numerical exploration starts
becoming a valuable tool for getting insights into the search for analytic solutions, thus fully participating
to the modeling stage and physical understanding. However, physicists may be limited to a blind usage of
numerical black boxes, which may furthermore not be dedicated to their specific needs. According to us,
promoting a science of modeling in numerical physics would thus be a promising and rich avenue for the
two research fields. Unfortunately, very scarce cooperation currently exists between the two communities,
and large networks of collaboration still need to be set up.

5 Social and environmental responsibility

5.1 Footprint of research activities

The ELAN team is environment-sensitive. Since its creation in 2017, 100% of its research staff moves daily
from home to the lab using soft transportation means (biking, public transportation). Intercontinental
missions are limited while train is the preferred mode of transportation in Europe.

5.2 Impact of research results

A large part of the research conducted in the team is of fundamental level. Direct applications lie in
numerical arts, cloth design, sports, and environmental studies, all of these being of limited negative
impact for the environment. Collaborations with industry leading specially harmful activities to the
environment are avoided.

6 Highlights of the year

6.1 Awards

Our 2011 paper entitled "A hybrid iterative solver for robustly capturing Coulomb friction in hair dynam-
ics" [28] has been awarded a Test of Time Award at Siggraph Asia 2023 in Sydney.

2In France this new trend was particularly stimulated by the work of Yves Pomeau, who convinced many young scientists to

study the nonlinear physics of common objects such as paper, plants, or hair [36].

3It is however amusing to observe that research in these areas is quite successful in obtaining the IG Nobel prize [16, 40], thus

still being considered as an exotic research topic by physicists.

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Inria Annual Report 2023

This paper introduced the first nonsmooth algorithm able to capture dry friction in flexible fibre
assemblies in a fast and robust manner. The method was inspired by the French Moreau school on
nonsmooth mechanics pioneered in the 80’s, which was essentially applied to granular materials and
rigid bodies at that time. The frictional contact algorithm was implemented in the free so-bogus library
and was used in several FX movies (including The Hobbit: An unexpected journey and Superman: Man of
steel) for simulating hair and fur realistically.

7 New software, platforms, open data

7.1 New software

7.1.1 Feel++

Keywords: High order finite elements, Discontinuous Galerkin, High-Performance Computing

Functional Description: Feel++ is a high-performance C++ library for the resolution of general vari-
ational formulations, including continuous and discontinuous Galerkin methods, finite element
or spectral element methods, reduced basis formulations, etc. It features a high-level domain
specific embedded language (DSEL) for Galerkin methods, space dimension-agnostic computation
kernels and seamless and automatic parallelism. It also includes applicative toolboxes to solve
physics problems in fluid mechanics, solid mechanics, thermal conduction, and the corresponding
multi-physics coupling.

Release Contributions: - Support of distance-based contact forces between immersed bodies - BVH

implementation for contact pruning - Various improvements in expression support

URL: https://docs.feelpp.org/home/index.html

Contact: Thibaut Metivet

Partners: Université de Strasbourg, UGA, Inria

7.1.2 Sand6

Keywords: Granular matter, Frictional contact, Drücker-Prager rheology

Scientific Description: sand6 is a software to simulate the dynamics of granular matter using a con-
tinuum approach accounting for non-smooth flow rules. It is based on the nonsmooth Material
Point Method described in [DBD16a] and is currently maintained and developed in the team for
various aspects related to the modeling of frictional contact in continuous systems.

Functional Description: Simulation of granular matter as a continuum media

Release Contributions: This software contains a C++ implementation of the algorithms described in the
2016 ACM SIGGRAPH paper entitled "A Semi-Implicit Material Point Method for the Continuum
Simulation of Granular Materials" by Gilles Daviet and Florence Bertails-Descoubes. It is currently
maintained and further developed by Thibaut Métivet. It was notably updated and physically valid-
ated in the context of transient granular collapses in the paper [Rousseau G, Métivet T, Rousseau
H, Daviet G, Bertails-Descoubes F. Revisiting the role of friction coefficients in granular collapses:
confrontation of 3-D non-smooth simulations with experiments. Journal of Fluid Mechanics. 2023
Nov,975:A14.].

URL: https://gitlab.inria.fr/elan-public-code/sand6

Publications: hal-01310189, hal-03845323

Contact: Thibaut Metivet

Participants: Thibaut Metivet, Florence Bertails Descoubes, Gilles Daviet

Project ELAN

7.1.3 MERCI

9

Name: Energy Minimisation of Curvature-based numerical models for Inextensible Ribbons

Keywords: Thin elastic rod, Thin elastic ribbon, Physical simulation

Scientific Description: MERCI is a C++/lua software for computing the statics of thin elastic ribbons
discretised with curvature-based elements. It is based on the super-ribbon model described in
[Charrondière et al. 2020, Charrondière et al. 2024], and relies on the free [IPOPT](https://coin-
or.github.io/Ipopt/) optimisation software (coinor project) for the static solver. The ribbon can
be clamped at one or both ends, and even closed. Contact is treated by contraints with planes.
Once the setup is defined, the equilibrium of the ribbon under the specified boundary conditions,
external forces, and constraints, is computed. MERCI can be used as a C++ library, or via its lua
interface.

Reference code of the PhD thesis:

Raphaël Charrondière, "Modélisation numérique de rubans par éléments en courbures", 2021,
Université Grenoble Alpes, https://hal.inria.fr/tel-03545017v2

and of the following papers:

R. Charrondière, F. Bertails-Descoubes, S. Neukirch, V. Romero, "Numerical modeling of inextens-
ible elastic ribbons with curvature-based elements", Computer Methods in Applied Mechanics and
Engineering 364, June 2020, p. 1–32, [doi:10.1016/j.cma.2020.112922], [hal-02515877].

R. Charrondière, S. Neukirch, F. Bertails-Descoubes, "MERCI: Mixed curvature-based elements for
computing equilibria of thin elastic ribbons", to appear in 2024.

Functional Description: MERCI is a C++/lua software for computing the statics of thin elastic ribbons
discretised with curvature-based elements. It is based on the super-ribbon model described in
[Charrondière et al. 2020, Charrondière et al. 2024], and relies on the free [IPOPT](https://coin-
or.github.io/Ipopt/) optimisation software (coinor project) for the static solver. The ribbon can
be clamped at one or both ends, and even closed. Contact is treated by contraints with planes.
Once the setup is defined, the equilibrium of the ribbon under the specified boundary conditions,
external forces, and constraints, is computed. MERCI can be used as a C++ library, or via its lua
interface.

Reference code of the PhD thesis:

Raphaël Charrondière, "Modélisation numérique de rubans par éléments en courbures", 2021,
Université Grenoble Alpes, https://hal.inria.fr/tel-03545017v2

and of the following papers:

R. Charrondière, F. Bertails-Descoubes, S. Neukirch, V. Romero, "Numerical modeling of inextens-
ible elastic ribbons with curvature-based elements", Computer Methods in Applied Mechanics and
Engineering 364, June 2020, p. 1–32, [doi:10.1016/j.cma.2020.112922], [hal-02515877].

R. Charrondière, S. Neukirch, F. Bertails-Descoubes, "MERCI: Mixed curvature-based elements for
computing equilibria of thin elastic ribbons", to appear in 2024.

Publication: hal-02515877

Authors: Raphael Charrondiere, Raphael Charrondiere, Raphael Charrondiere, Florence Descoubes,

Sébastien Neukirch

Contact: Florence Bertails Descoubes

Partners: Sorbonne Université, UGA

10

7.1.4 ElanFab

Inria Annual Report 2023

Keywords: Experimental mechanics, Experimental design, Thin elastic ribbon, Thin elastic rod, Thin

elastic shell, Frictional contact

Functional Description: Experimental platform of the ELAN team.

The aim of this platform is to experimentally explore the mechanics and geometry of highly
deformable elastic objects of low dimensions (rods, fibers, plates, shells).

The platform allow us to fabricate with controlled materials and geometries elastic objects. By
means of state of the art manufacturing techniques we are able to make curved elastic objects, with
a controlled target geometry. For the moment we use elastomeric materials to remain in the elastic
regime, however we are interested in exploring new materials to include viscous and plastic effects.

Our platform has a modular mechanical testing device that allow load and tensile testing in multiple
configurations for a wide range of force magnitudes, from 1e-3 to 100 Newtons. In this setup we
have tested highly compliant, as well as, very stiff materials, for example we study the tensile
response of feathers and elastic knotted rods.

The platform is constantly undergoing new improvements to allow us to obtain geometrical inform-
ation by means of a combination of image analysis and computer vision techniques. Multiple views
are obtained by using multiple cameras and mirrors or by using one single camera that moves in a
highly controlled manner. Furthermore we are implementing the use of a semi fast camera to study
dynamic phenomena of complex elastic objects assemblies. We are also implementing structured
light into our setup to improve the accuracy of our measurements.

Author: Victor Romero Gramegna

Contact: Victor Romero Gramegna

7.1.5 so-bogus

Keywords: Frictional contact, Constraint solving

Scientific Description: The so-bogus library, based on the bogus library, implements several methods
(analytical solver, Gauss-Seidel solver, root-finding solver, etc.) for solving Signorini-Coulomb
problems in 2D and 3D. It serves as the reference code for the paper "A Hybrid Iterative Solver for
Robustly Capturing Coulomb Friction in Hair Dynamics", Daviet et al. 2011, ACM Transactions on
Graphics (SIGGRAPH Asia 2011).

Functional Description: A fast and robust solver for capturing frictional contact between many Lag-
rangian systems with exact Coulomb friction. Reference code for the paper "A Hybrid Iterative
Solver for Robustly Capturing Coulomb Friction in Hair Dynamics", Daviet et al. 2011, ACM
Transactions on Graphics (SIGGRAPH Asia 2011).

The so-bogus software is currently maintained and further developed by Thibaut Métivet and
Florence Bertails-Descoubes.

URL: https://gitlab.inria.fr/elan-public-code/so-bogus

Publication: hal-00647497

Contact: Thibaut Metivet

Participants: Florence Descoubes, Thibaut Metivet, Gilles Daviet

Project ELAN

7.1.6 meche2D

Name: meche2D

11

Keywords: Physical simulation, Thin elastic rod, Frictional contact

Scientific Description: Meche2D is a library for coupling 2D elastic fibers with frictional contact. It relies

on the supercircle code for fibers, and on so-bogus for frictional contact.

Functional Description: Meche2D is a library for coupling 2D elastic fibers with frictional contact. It
relies on the supercircle code for fibers, and on so-bogus for frictional contact. New since 2020-2023:
fibers can be clamped or unclamped, and contact detection is exact between two circular arcs.

Publication: https://hal.science/hal-03996830v2

Contact: Florence Bertails Descoubes

Participants: Florence Bertails Descoubes, Emile Hohnadel, Thibaut Metivet

7.2 New platforms

7.2.1 Elanfab 7.1.4

Participants:

Victor Romero.

The aim of this platform is to experimentally explore the mechanics and geometry of highly deform-

able elastic objects of low dimensions (rods, fibers, plates, shells).

The platform allow us to fabricate with controlled materials and geometries elastic objects. By means
of state of the art manufacturing techniques we are able to make curved elastic objects, with a controlled
target geometry. For the moment we use elastomeric materials to remain in the elastic regime, however
we are interested in exploring new materials to include viscous and plastic effects.

Our platform has a modular mechanical testing device that allows load and tensile testing in multiple
configurations for a wide range of force magnitudes, from 1e-3 to 100 Newtons. In this setup we have
tested highly compliant, as well as, very stiff materials, for example we study the tensile response of
feathers 8.6 and elastic knotted rods 8.7.

The platform is constantly undergoing new improvements to allow us to obtain geometrical informa-
tion by means of a combination of image analysis and computer vision techniques. Multiple views are
obtained by using multiple cameras and mirrors or by using one single camera that moves in a highly
controlled manner. Furthermore we are implementing the use of a semi-fast camera to study dynamic
phenomena of complex elastic objects assemblies. We are also implementing structured light into our
setup to improve the accuracy of our measurements.

8 New results

8.1 Nonsmooth simulations of 3D Drucker-Prager granular flows and validation

against experimental column collapses

Participants:

Thibaut Métivet, Florence Bertails-Descoubes.

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

12

Inria Annual Report 2023

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.

8.2 Randomly stacked open-cylindrical shells as a functional mechanical device

Participants:

Émile Hohnadel, Thibaut Métivet, Florence Bertails-Descoubes.

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].

8.3 Hydrodynamic model for fish locomotion

Participants:

Thibaut Métivet.

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.

8.4 Mixed elements for a curvature-based ribbon model

Participants:

Florence Bertails-Descoubes.

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 independ-
ently, 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

Project ELAN

13

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 [17] 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.

8.5 High-order contact detection between fibres

Participants: Octave Crespel, Émile Hohnadel, Thibaut Métivet, Florence Bertails-

Descoubes.

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.

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

Participants:

Jouve Jean, Romero Victor, Florence Bertails-Descoubes.

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.

8.7 Untangling the physics of self-locking in tight knots

Participants:

Alexandre Teixeira Da Silva, Thibaut Métivet, Victor Romero,
Florence Bertails-Descoubes.

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 be-
haviour of tight knots, with a particular focus on the respective roles of elasticity and friction in the

14

Inria Annual Report 2023

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.

8.8 Frictional three-point bending test: disentangling the role of friction through

real and numerical experiments

Participants:

Émile Hohnadel, Thibaut Métivet, Florence Bertails-Descoubes.

In collaboration with Joël Marthelot, Ignacio Andrade-Silva and Olivier Pouliquen (IUSTI, Aix Mar-
seille 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.

9 Partnerships and cooperations

9.1 International research visitors

9.1.1 Visits of international scientists

Rahul Narain

Status: Assistant Professor

Institution of origin: Indian Institute of Technology Delhi

Country: India

Dates: October 9 - December 22, 2023

Context of the visit : Scientific collaboration on our feather modelling project

Mobility program/type of mobility: Inria Invited Researcher Program

9.1.2 International collaborations

• Scientific collaboration with Tomohiko Sano (Sigma Lab, Japan) on the analysis of random stacks

of open elastic rings

• Scientific collaboration with Rahul Narain (IIT Delhi, India) and Theodore Kim (Yale, United States)

on the numerical modelling of feathers

9.2 European initiatives

9.2.1 H2020 projects

THREAD THREAD project on cordis.europa.eu

Title: Joint Training on Numerical Modelling of Highly Flexible Structures for Industrial Applications

Duration: From October 1, 2019 to March 31, 2024

Project ELAN

Partners:

15

• University of Liège (ULIEGE), Belgium

• Ecole Nationale Supérieure d’Arts et Métiers (ENSAM), France

• Friedrich-Alexander-Universitaet Erlangen-Nuernberg (FAU), Germany

• Martin-Luther-Universitat Halle-Wittenberg (MLU), Germany

• Indian Institute of Technology Delhi

• Fraunhofer Gesellschaft Zur Forderung der Angewandten Forschung ev (Fraunhofer), Ger-

many

• Sveuciliste u Rijeci, Gardevinski Fakultet u Rijeci (University of Rijeka, Faculty of civil engin-

eering Unirifce), Croatia

• C3M DOO, Center za Racunalnistvo Vmehaniki Kontinuuma - Modeliranje in Trzenje (C3M

DOO), Slovenia

• Universitaet Innsbruck (UIBK), Austria

• Centrale-Supelec, France

• Norges Teknisk-Naturvitenskapelige Universitet NTNU (NTNU), Norway

• Univerza v Ljubjani (UL), Slovenia

• Universidad de Sevilla, Spain

Inria contact: Florence DESCOUBES

Coordinator:

Summary: Virtual prototyping is a cornerstone in modern product development cycles: It accelerates
the design process, reduces costs and improves product performance and quality. Highly flexible
slender structures like yarns, cables, hoses or ropes are essential parts of high-performance en-
gineering systems. The complex response of such structures in real operational conditions is far
beyond the capabilities of current virtual prototyping tools.

There is a pressing need for a new generation of young scientists capable of solving fundamental
problems related to slender structures and transferring results to applications. THREAD addresses
the mechanical modelling, mathematical formulations and numerical methods for highly flexible
slender structures. It brings mechanical engineers and mathematicians together around major
challenges in industrial applications and open-source simulation software development. It estab-
lishes an innovative modelling chain starting from detailed 3D modelling and experimental work
to build validated 1D nonlinear rod models, which are then brought to a system-level simulation
thanks to the outstanding numerical properties of the developed algorithms. This holistic approach
combines advanced concepts in experimental and theoretical structural mechanics, non-smooth
dynamics, computational geometry, discretisation methods and geometric numerical integration
and will enable the next generation of virtual prototyping.

The ESRs will receive comprehensive local and network-wide training covering state-of-the-art
research topics as well as valuable transferable skills. They will benefit from close cooperation with
twelve industrial partner organisations implementing a comprehensive programme of research
secondments and contributing their experience. As a main objective of THREAD, interdisciplinary
and inter-sectoral training boosts the career development of young researchers and supports them
to solve future challenges.

9.3 National initiatives
FISHSIF ANR Project

Participants:

Thibaut Métivet.

16

Inria Annual Report 2023

Title: FISHSIF: Fish In Silico with Hydrodynamics and Social Forces

Duration: 01/10/2021 - 31/03/2025

Summary: The FISHSIF project has received a three-year funding from the ANR (Agence Nationale pour
la Recherche). The goal of this project is to introduce dynamical cognition models within full
hydrodynamic simulations in order to understand the role played by social or flow interactions
in the organisation and behaviour of schools of fish. The project will be led in a collaboration
between the ELAN team, the Laboratoire Interdisciplinaire de Physique (LIPhy, UGA/CNRS) and
the Laboratoire de Psychologie et NeuroCognition (LPNC, UGA/CNRS).

Partners:

• ELAN Inria project-team

• Laboratoire Interdisciplinaire de Physique (LIPhy), Université Grenoble-Alpes (UGA)

• Laboratoire de Psychologie et NeuroCognition (LPNC), Université Grenoble-Alpes (UGA)

National collaborations

• Long-term collaboration with Arnaud Lazarus and Sébastien Neukirch (Institut Jean le Rond

d’Alembert, Sorbonne Université).

• Long-term collaboration with Christophe Prud’homme and Vincent Chabannes (Université de

Strasbourg and Centre de modélisation et de simulation de Strasbourg).

• Collaboration with Olivier Pouliquen and Joël Marthelot (IUSTI, Aix-Marseille University).

• Collaboration with Philippe Peyla, Aurélie Dupont (LIPhy, UGA/CNRS) and Christian Graff (LPNC,

UGA/CNRS) within the FISHSIF project.

Regional collaborations

• Collaboration with Mélina Skouras (Inria/LJK - Anima team).

• Collaboration with Stefanie Wuhrer and Jean-Sébastien Franco (Inria/LJK - Morphéo team).

.

10 Dissemination

10.1 Promoting scientific activities

10.1.1 Scientific events: selection

Member of the conference program committees Florence Bertails-Descoubes was member of the ACM
Siggraph Technical Papers Program Committee in 2023.

10.1.2 Journal

Member of the editorial boards Since 2021, Florence Bertails-Descoubes is Associate Editor of ACM
Transactions on Graphics.

Reviewer - reviewing activities

• Siggraph (ACM, accepted papers published in Transactions on Graphics)

• Siggraph Asia (ACM, accepted papers published in Transactions on Graphics)

• Journal of the Mechanics and Physics of Solids (JMPS, Elsevier)

Project ELAN

10.1.3 Invited talks

17

• Septembre 2023: Invited lecture [10] at the international conference on High Flexible Solid Struc-

tures (Florence Bertails-Descoubes)

10.1.4 Research administration

Ph.D. Award Committees

• Florence Bertails-Descoubes was member of the international ACM Siggraph Ph.D. award in 2023.

• Florence Bertails-Descoubes was member of the national GdR IG-RV Ph.D. award in 2023.

National Selection Committees Florence Bertails-Descoubes was member of the 2023 Jury d’Admission
for the national concours "Chargées and Chargés de Recherche de Classe Normale".

Local Selection Committees Florence Bertails-Descoubes was member of the 2023 Maître de Con-
férence Jury "MCF 0194" at Grenoble INP and Université Grenoble Alpes, and of the 2023 Maître de
Conférence Jury "MCF 0163" at Université Grenoble Alpes.

10.2 Teaching - Supervision - Juries

10.2.1 Teaching

Licence

• Émile Hohnadel: Langages et automates, 27h, L2, Université Grenoble Alpes

• Jean Jouve: Méthodes numériques de base, 26h, Ensimag 1A, Grenoble INP.

• Victor Romero: Electromagnétisme et optique pour la chimie, PHY405, 33h, DLST, Université

Grenoble Alpes, Grenoble.

Master

• Thibaut Métivet, Florence Bertails-Descoubes and Jean Jouve: Mécanique Numérique, 36h, En-

simag 3A, Grenoble INP.

10.2.2 Supervision

Post-doctorate

• Bruno Ventéjou: 01/05/2022-, supervised by Thibaut Métivet and Philippe Peyla (LIPhy, UGA)

Ph.D.

• Alexandre Teixeira : 01/02/2021-, supervised by Thibaut Métivet, Florence Bertails-Descoubes and

Mélina Skouras (Anima, Inria GRA)

• Émile Hohnadel : 01/09/2021-, supervised by Florence Bertails-Descoubes and Thibaut Métivet

• Jean Jouve : 01/09/2021-, supervised by Victor Romero and Florence Bertails-Descoubes

Internship

• Adrien Decosse : 25/05/2023-14/07/2023, supervised by Thibaut Métivet and Florence Bertails-

Descoubes

18

10.2.3 Juries

Inria Annual Report 2023

Florence Bertails-Descoubes was member of the Ph.D committee of Ke Wu (as an examiner), March 31st
2023, at Centrale Lille, of Pierre Zins (as a president), April 25th 2023, at Inria Grenoble, and of Maxime
Calka (as a president), June 8th 2023, at TIMC Grenoble.

10.3 Popularization

In October 2023, Octave Crespel participated in La Fête de la Science organised at Inria Grenoble.

11 Scientific production

11.1 Major publications

[1] F. Bertails-Descoubes, A. Derouet-Jourdan, V. Romero and A. Lazarus. ‘Inverse design of an isotropic
suspended Kirchhoff rod: theoretical and numerical results on the uniqueness of the natural shape’.
In: Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 474.2212
(Apr. 2018), pp. 1–26. DOI: 10.1098/rspa.2017.0837. URL: https://hal.inria.fr/hal-0182
7887.

[2] R. Charrondière, F. Bertails-Descoubes, S. Neukirch and V. Romero. ‘Numerical modeling of inex-
tensible elastic ribbons with curvature-based elements’. In: Computer Methods in Applied Mech-
anics and Engineering 364 (1st June 2020), pp. 1–32. DOI: 10.1016/j.cma.2020.112922. URL:
https://hal.inria.fr/hal-02515877.

[3]

J. Li, G. Daviet, R. Narain, F. Bertails-Descoubes, M. Overby, G. Brown and L. Boissieux. ‘An Im-
plicit Frictional Contact Solver for Adaptive Cloth Simulation’. In: ACM Transactions on Graphics.
Proceedings SIGGRAPH 2018 37.4 (Aug. 2018), pp. 1–15. DOI: 10.1145/3197517.3201308. URL:
https://hal.inria.fr/hal-01834705.

[4] M. Ly, R. Casati, F. Bertails-Descoubes, M. Skouras and L. Boissieux. ‘Inverse Elastic Shell Design
with Contact and Friction’. In: ACM Transactions on Graphics 37.6 (Nov. 2018), pp. 1–16. DOI:
10.1145/3272127.3275036. URL: https://hal.inria.fr/hal-01883655.

[5] T. Metivet, A. Sengers, M. Ismail and E. Maitre. ‘Diffusion-redistanciation schemes for 2D and
3D constrained Willmore flow: application to the equilibrium shapes of vesicles’. In: Journal of
Computational Physics 436 (2021), p. 110288. DOI: 10.1016/j.jcp.2021.110288. URL: https:
//hal.archives-ouvertes.fr/hal-02905870.

[6] A.-H. Rasheed, V. Romero, F. Bertails-Descoubes, S. Wuhrer, J.-S. Franco and A. Lazarus. ‘A Visual
Approach to Measure Cloth-Body and Cloth-Cloth Friction’. In: IEEE Transactions on Pattern
Analysis and Machine Intelligence (July 2021). DOI: 10.1109/TPAMI.2021.3097547. URL: https:
//hal.inria.fr/hal-03285624.

[7] V. Romero, M. Ly, A.-H. Rasheed, R. Charrondière, A. Lazarus, S. Neukirch and F. Bertails-Descoubes.
‘Physical validation of simulators in Computer Graphics: A new framework dedicated to slender
elastic structures and frictional contact’. In: ACM Transactions on Graphics 40.4 (1st Aug. 2021),
Article 66: 1–19. DOI: 10.1145/3450626.3459931. URL: https://hal.inria.fr/hal-0321745
9.

11.2 Publications of the year

International journals

[8] G. Rousseau, T. Métivet, H. Rousseau, G. Daviet and F. Bertails-Descoubes. ‘Revisiting the role
of friction coefficients in granular collapses: confrontation of 3-D non-smooth simulations with
experiments’. In: Journal of Fluid Mechanics (15th Nov. 2023). DOI: 10.1017/jfm.2023.835. URL:
https://inria.hal.science/hal-03845323.

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[9] T. G. Sano, E. Hohnadel, T. Kawata, T. Métivet and F. Bertails-Descoubes. ‘Randomly stacked open
cylindrical shells as functional mechanical energy absorber’. In: Communications Materials 4.59
(25th Aug. 2023). DOI: 10.1038/s43246-023-00383-2. URL: https://hal.science/hal-0399
6830.

International peer-reviewed conferences

[10] F. Bertails-Descoubes. ‘Simulating fibre assemblies: from Hollywood illusions to physical predic-
tions’. In: HFSS 2023 – International Conference on Highly Flexible Slender Structures. Rijeka,
Croatia, 2023, pp. 1–3. URL: https://inria.hal.science/hal-04265998.

[11] E. Hohnadel, O. Crespel, T. Métivet and F. Bertails-Descoubes. ‘Accurate contact detection and
response in fibre assemblies with friction’. In: HFSS 2023 – International Conference on Highly
Flexible Slender Structures. Rijeka, Croatia, 2023, pp. 1–1. URL: https://hal.science/hal-043
88542.

[12] T. Métivet, E. Hohnadel, T. G. Sano, T. Kawata and F. Bertails-Descoubes. ‘Investigating the compac-
tion of open ring stacks through real and numerical experiments’. In: HFSS 2023 - International
Conference on Highly Flexible Slender Structures. Rijeka, Croatia, 2023, pp. 1–2. URL: https://ha
l.science/hal-04225083.

[13] A. Teixeira da Silva, T. Métivet, V. Romero, M. Skouras and F. Bertails-Descoubes. ‘Untangling the
physics of self-locking in tight knots’. In: HFSS 2023 - International Conference on Highly Flexible
Slender Structures. Rijeka, Croatia, 2023, pp. 1–2. URL: https://inria.hal.science/hal-0438
8669.

Conferences without proceedings

[14] B. Ventéjou, T. Métivet, A. Dupont, C. Graff and P. Peyla. ‘A hydrodynamic toy model for fish
locomotion’. In: DFD 2023 - 76th Annual Meeting of the Division of Fluid Dynamics. Washinghton
DC, United States, 19th Nov. 2023. URL: https://hal.science/hal-04254582.

Reports & preprints

[15] O. Crespel, E. Hohnadel, T. Métivet and F. Bertails-Descoubes. Contact detection between curved
fibres: high order makes a difference. 27th Dec. 2023. URL: https://hal.science/hal-04364565.

11.3 Cited publications

[16] B. Audoly and S. Neukirch. ‘Fragmentation of Rods by Cascading Cracks: Why Spaghetti Does Not

Break in Half’. In: Physical Review Letters 95.9 (2005), p. 095505.

[17] B. Audoly and S. Neukirch. ‘A one-dimensional model for elastic ribbons: A little stretching makes
a big difference’. In: Journal of the Mechanics and Physics of Solids 153 (2021), p. 104457. DOI:
10.1016/j.jmps.2021.104457.

[18] D. Baraff. ‘Analytical methods for dynamic simulation of non-penetrating rigid bodies’. In: Com-
puter Graphics Proceedings (Proc. ACM SIGGRAPH’89 ). New York, NY, USA: ACM, 1989, pp. 223–
232.

[19] F. Bertails. ‘Linear Time Super-Helices’. In: Computer Graphics Forum (Proc. Eurographics’09)
28.2 (Apr. 2009). URL: http://www-ljk.imag.fr/Publications/Basilic/com.lmc.publi
.PUBLI_Article@1203901df78_1d3cdaa/.

[20] F. Bertails, B. Audoly, M.-P. Cani, B. Querleux, F. Leroy and J.-L. Lévêque. ‘Modélisation de coiffures
naturelles à partir des propriétés physiques du cheveu’. In: Journées Francophones d’Informatique
Graphique (AFIG). AFIG / EG-France. Strasbourg, France, Nov. 2005.

[21] F. Bertails-Descoubes. ‘Super-Clothoids’. In: Computer Graphics Forum (Proc. Eurographics’12)
31.2pt2 (2012), pp. 509–518. DOI: 10.1111/j.1467-8659.2012.03030.x. URL: http://www.inr
ialpes.fr/bipop/people/bertails/Papiers/superClothoids.html.

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Inria Annual Report 2023

[22] A. Blumentals. ‘Numerical modelling of thin elastic solids in contact’. PhD thesis. Université de

Grenoble Alpes, July 2017.

[23] A. Blumentals, F. Bertails-Descoubes and R. Casati. ‘Dynamics of a developable shell with uniform
curvatures’. In: The 4th Joint International Conference on Multibody System Dynamics. Montréal,
Canada, May 2016. URL: https://hal.inria.fr/hal-01311559.

[24] R. Casati and F. Bertails-Descoubes. ‘Super Space Clothoids’. In: ACM Transactions on Graphics
(Proc. ACM SIGGRAPH’13) 32.4 (July 2013), 48:1–48:12. DOI: 10.1145/2461912.2461962. URL:
http://doi.acm.org/10.1145/2461912.2461962.

[25] D. Chapelle and K. Bathe. The Finite Element Analysis of Shells - Fundamentals - Second Edition.
Computational Fluid and Solid Mechanics. Springer, 2011, p. 410. DOI: 10.1007/978-3-642-164
08-8.

[26] P. Cundall. ‘A computer model for simulating progressive large scale movements of blocky rock
systems. In Proceedings of the Symposium of the International Society of Rock Mechanics’. In:
Proceedings of the Symposium of the International Society of Rock Mechanics. Vol. 1. 1971, pp. 132–
150.

[27] G. Daviet. ‘Modèles et algorithmes pour la simulation du contact frottant dans les matériaux
complexes : application aux milieux fibreux et granulaires’. PhD thesis. Grenoble Alpes Universités,
Dec. 2016.

[28] G. Daviet, F. Bertails-Descoubes and L. Boissieux. ‘A hybrid iterative solver for robustly capturing
Coulomb friction in hair dynamics’. In: ACM Transactions on Graphics (Proc. ACM SIGGRAPH
Asia’11) 30 (6 2011), 139:1–139:12. URL: http://www.inrialpes.fr/bipop/people/bertails
/Papiers/hybridIterativeSolverHairDynamicsSiggraphAsia2011.html.

[29] G. Daviet and F. Bertails-Descoubes. ‘A semi-implicit material point method for the continuum
simulation of granular materials’. In: ACM Transactions on Graphics. SIGGRAPH ’16 Technical
Papers 35.4 (July 2016), p. 13. DOI: 10.1145/2897824.2925877. URL: https://hal.inria.fr/h
al-01310189.

[30] G. Daviet and F. Bertails-Descoubes. ‘Nonsmooth simulation of dense granular flows with pressure-
dependent yield stress’. In: Journal of Non-Newtonian Fluid Mechanics 234 (Apr. 2016), pp. 15–35.
DOI: 10.1016/j.jnnfm.2016.04.006. URL: https://hal.inria.fr/hal-01236488.

[31] G. Daviet and F. Bertails-Descoubes. ‘Simulation of Drucker–Prager granular flows inside Newto-
nian fluids’. working paper or preprint. Feb. 2017. URL: https://hal.inria.fr/hal-01458951.

[32] A. Derouet-Jourdan, F. Bertails-Descoubes, G. Daviet and J. Thollot. ‘Inverse Dynamic Hair Model-
ing with Frictional Contact’. In: ACM Trans. Graph. 32.6 (Nov. 2013), 159:1–159:10. DOI: 10.1145/2
508363.2508398. URL: http://doi.acm.org/10.1145/2508363.2508398.

[33] A. Derouet-Jourdan, F. Bertails-Descoubes and J. Thollot. ‘Stable Inverse Dynamic Curves’. In:
ACM Transactions on Graphics (Proc. ACM SIGGRAPH Asia’10 ) 29 (6 Dec. 2010), 137:1–137:10. DOI:
http://doi.acm.org/10.1145/1882261.1866159. URL: http://doi.acm.org/10.1145/18
82261.1866159.

[34] M. A. Dias and B. Audoly. ‘“Wunderlich, Meet Kirchhoff”: A General and Unified Description of
Elastic Ribbons and Thin Rods’. In: Journal of Elasticity 119.1 (Apr. 2015), pp. 49–66. DOI: 10.1007
/s10659-014-9487-0. URL: https://doi.org/10.1007/s10659-014-9487-0.

[35] E. J. Doedel, R. C. Pfaffenroth, A. R. Chambodut, T. F. Fairgrieve, Y. A. Kuznetsov, B. E. Oldeman,
B. Sandstede and X. Wang. AUTO 2000: Continuation and Bifurcation Software for Ordinary Differ-
ential Equations (with HomCont). Mar. 2006.

[36] ESPCI. Rencontre en l’honneur de Yves Pomeau, octobre 2016. https://www.sfpnet.fr/rencontre-
celebrant-la-medaille-boltzmann-d-yves-pomeau. ESPCI, 2016. URL: https://www.sfpnet.fr
/rencontre-celebrant-la-medaille-boltzmann-d-yves-pomeau.

[37]

I. Frigaard and C. Nouar. ‘On the usage of viscosity regularisation methods for visco-plastic fluid
flow computation’. In: Journal of Non-Newtonian Fluid Mechanics 127.1 (Apr. 2005), pp. 1–26. DOI:
10.1016/j.jnnfm.2005.01.003.

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[38] M. Gazzola, M. Argentina and L. Mahadevan. ‘Scaling macroscopic aquatic locomotion’. In: Nature

Physics 10.10 (2014), pp. 758–761.

[39] A. L. Gol’Denveizer. Theory of Elastic Thin Shells. Pergamon Press, 1961.

[40] R. E. Goldstein, P. B. Warren and R. C. Ball. ‘Shape of a Ponytail and the Statistical Physics of Hair
Fiber Bundles’. In: Phys. Rev. Lett. 108 (7 Feb. 2012), p. 078101. DOI: 10.1103/PhysRevLett.108
.078101. URL: https://link.aps.org/doi/10.1103/PhysRevLett.108.078101.

[41] É. Hohnadel, J. Marthelot, I. Andrade-Silva, T. Métivet, O. Pouliquen and F. Bertails-Descoubes.
‘Frictional three-point bending test: disentangling the role of friction through real and numerical
experiments’. In: European Solid Mechanics Conference ESMC 2022. Galway, Ireland, July 2022. URL:
https://inria.hal.science/hal-03960300.

[42] M. Jean. ‘The Non Smooth Contact Dynamics Method’. In: Computer Methods in Applied Mechanics
and Engineering 177 (1999). Ed. by J. Martins and A. Klarbring. Special issue on computational
modeling of contact and friction, J.A.C. Martins and A. Klarbring, editors, pp. 235–257.

[43]

J. Li, G. Daviet, R. Narain, F. Bertails-Descoubes, M. Overby, G. Brown and L. Boissieux. ‘An Im-
plicit Frictional Contact Solver for Adaptive Cloth Simulation’. In: ACM Transactions on Graphics.
Proceedings SIGGRAPH 2018 37.4 (Aug. 2018), pp. 1–15. DOI: 10.1145/3197517.3201308. URL:
https://hal.inria.fr/hal-01834705.

[44] M. Moore and J. Wilhelms. ‘Collision detection and response for computer animation’. In: Computer

Graphics Proceedings (Proc. ACM SIGGRAPH’88 ). 1988, pp. 289–298.

[45] D. Moulton, P. Grandgeorge and S. Neukirch. ‘Stable elastic knots with no self-contact’. In: Journal
of the Mechanics and Physics of Solids 116 (2018), pp. 33–53. DOI: 10.1016/j.jmps.2018.03.019.
URL: http://www.sciencedirect.com/science/article/pii/S0022509617310104.

[46]

J. Spillmann and M. Teschner. ‘An Adaptive Contact Model for the Robust Simulation of Knots’. In:
Computer Graphics Forum 27.2 (2008). Proc. Eurographics’08.

[47] H. Sugiyama, J. Gertsmayr and A. Mikkola. ‘Flexible Multibody Dynamics — Essential for Accurate
Modeling in Multibody System Dynamics’. In: Journal of Computational Nonlinear Dynamics 9 (1
Nov. 2013).
<scratchpad>
- Main topic: Research Centre ELAN at Inria, focusing on numerical modeling of complex mechanical phenomena
- Key points:
  • Discrete modeling of slender elastic structures
  • Discrete and continuous modeling of frictional contact
  • Inverse design of slender elastic structures
  • Application domains: Mechanical Engineering, Computer Graphics, Soft Matter Physics
  • Social and environmental responsibility
- Structure: The document is divided into sections, including Project-Team, Activity Report, Contents, Research Program, etc.
- Potential artifacts to ignore: HTML tags, links, and other web-related elements
</scratchpad>

<final_summary>
The ELAN research centre at Inria focuses on developing numerical models for capturing complex mechanical phenomena, such as the buckling of slender elastic structures and the flowing of sand. The team's research program includes discrete modeling of slender elastic structures, discrete and continuous modeling of frictional contact, and inverse design of slender elastic structures. The centre's work has applications in Mechanical Engineering, Computer Graphics, and Soft Matter Physics, and the team is committed to social and environmental responsibility. The ELAN centre aims to improve our understanding of natural physical and biological processes involving slender structures and frictional contact, and to develop predictive, robust, and efficient numerical models for capturing complex mechanical phenomena at the macroscopic scale.
The ELAN research centre at Inria focuses on developing numerical models for capturing complex mechanical phenomena, such as the buckling of slender elastic structures and the flowing of sand. The team's research program includes discrete modeling of slender elastic structures, discrete and continuous modeling of frictional contact, and inverse design of slender elastic structures. The centre's work has applications in Mechanical Engineering, Computer Graphics, and Soft Matter Physics, and the team is committed to social and environmental responsibility. The ELAN centre aims to improve our understanding of natural physical and biological processes involving slender structures and frictional contact, and to develop predictive, robust, and efficient numerical models for capturing complex mechanical phenomena at the macroscopic scale.