All Years Seminars
[INMA] 2005-12-08 (14.00 pm) : The use of iterative linear system solvers for vibro and aero-acoustic simulations
At Auditoire Euler
Speaker :
Karl Meerbergen (Free Field Technologies (LLN))
Abstract :
Vibrations and more particularly, acoustics, are often modelized by finite
elements. This gives rise to large sparse algebraic linear systems of
equations. Traditionally, direct methods are used for reasons of robustness.
The major drawbacks of direct methods are the memory consumption and
execution times for 3D models. Iterative methods are often much cheaper, but
are not guaranteed to converge. In this talk, we present numerical results
for industrial applications that show how much we can gain with commonly
used preconditioned Krylov methods.
[INMA] 2005-12-06 (14.00 pm) : Colloquium CESAME/MAPA : Optimization on Manifolds
At Auditoire Euler
Speaker :
Absil, Pierre-Antoine
Abstract :
It is well known that several matrix computation problems
can be recast as finding local or global optimizers of a cost function
whose domain is not a linear space, but can nevertheless be endowed
with a manifold structure (that is, it admits local coordinate systems
that are "compatible"). We will see how this manifold structure can be
exploited in order to define and analyse optimization algorithms such
as the Newton method and the conjugate gradients method. In particular,
we will see how these optimization methods, when applied to computing
eigenvalues of a symmetric matrix, lead to competitive numerical
algorithms that also shed a new light on existing algorithms.
[INMA] 2005-11-29 (14.00 pm) : Grip force control during active collisions in different gravitational fields
At Auditoire Euler
Speaker :
White, Olivier
Abstract :
Studies have shown that grip force, normal to each finger-object
contact surface is adjusted in anticipation of load forces, acting
tangentially to the contact surface when lifting and moving hand-held objects.
Prediction may not be correct and in many cases, predictive feedforward control
may be supplemented by feedback control. A problem is then to dissociate the two
forms of control in the behavioural record. One method is to study self-produced
collisions in which the time period over which the load is applied is too short to
allow a feedback loop to operate.
Changes in gravity present a significant challenge to dexterous
object manipulation. The aim of the present study was to analyse
grip force control in repetitive collisions during parabolic flight
to determine the role played by gravity in prediction of collision load
forces.
Seven subjects participated in the study. Each subject was secured by
a seat belt on a chair with an instrumented manipulandum grasped between
the thumb and the index finger of the right hand. Subjects performed a
targeted tapping task with the manipulandum while dynamics of precision
grip and kinematics of the upper limb were recorded. Subjects briefly
tapped the manipulandum randomly on surfaces placed above and below the
hands neutral position.
In normal gravity, subjects were able to produce similar impact forces in
both directions but they produced larger forces downwards in micro- and
hypergravity. In contrast, they applied more grip force upwards than downwards,
leading to an inefficient grip-to-load force ratio.
During the transport of the object towards the target, the acceleration
contributed either positively or negatively to the inertial forces felt by
the fingers and grip force was adequately adjusted. However, while acceleration
contributed equivalently for both directions in microgravity, grip force was
still higher upwards. In contrast to previous studies, the grip-load force
modulation disappeared and was not learned at the term if the experiment.
We suggest that a continuous internal model of altered gravity field and
collisions is under building in the central nervous system.
[INMA] 2005-11-22 (15.15 pm) : Eléments finis discontinus adaptatifs de haut-ordre appliqués au modèle des eaux peu profondes
At Auditoire Euler
Speaker :
Paul-Emile Bernard (CESAME)
Abstract :
[INMA] 2005-11-22 (14.00 pm) : Self-Excited Vibrations of Drilling Systems with Drag Bits, a consequence of a regenerative effect
At Auditoire Euler
Speaker :
Christophe Germay (ULg)
Abstract :
Self-excited stick-slip oscillations of a rotary drilling system with a
drag bit are commonly modeled by reducing the bit-rock interface to an equivalent
velocity weakening law. A novel attempt in the comprehension of the self-excited
nature of the vibrations was formulated recently. It relies on a detailed
description of both the cutting processes and frictional contact mobilized at
bit-rock interface. The nature of this boundary condition complicates greatly
the model and its analysis as the axial and torsional modes of vibrations are
coupled through the boundary conditions via a state-dependent delay (regenerative
effect) and a rate-independent discontinuous friction. Nevertheless, this new
approach offers new perspectives in the comprehension of the mechanisms responsible
for the self-excitation undergone by the drilling system.
Seminars
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