Pierre Argoul

Nonlinear argumental oscillators: A few examples of modulation via spatial position

Under certain conditions, an oscillator can enter a stable regime when submitted to an external harmonic force whose frequency is far from the natural frequency of the oscillator. This may happen when the external force acts on the oscillator in a way which depends on the oscillators spatial position. This phenomenon is called “argumental oscillation”. In this paper, six argumental oscillators are described and modeled, and experimental results are given and compared to numerical simulations based on the models. A polar Van der Pol representation, with embedded time indications, is used to allow a precise comparison. The pendulums are modeled as Duffing oscillators. The six models are based on various pendulums excited by spatially localized magnetic-field sources consisting of wire coils. Each pendulum receives the excitation via a steel element, or a permanent magnet, fitted at the tip of the pendulums rod. The spatial localization induces another nonlinearity besides the Duffing nonlinearity. A control system allowing a real-time Van der Pol representation of the motion is presented. Attractors are brought out from experimental results.

Crowd-Structure Interaction in Laterally Vibrating Footbridges: Comparison of Two Fully Coupled Approaches

Two models that deal with the crowd-structure interaction have been developed. The first is a 1D continuous model and the other is a 2D discrete one. In this paper, a summary of the formulation of these two models is presented. Both approaches used to represent the pedestrian-structure coupling phenomenon are detailed and compared. We start by introducing the partial and ordinary differential equations that govern the dynamics of both the continuous and the discrete models. First, the equation of dynamics of the footbridge for the case of lateral vibrations is recalled. Then, the Kuramoto phase equation is implemented for describing the coupling between the pedestrians and the laterally moving deck of a footbridge. Results obtained from numerical simulations are presented and compared with available experimental data.

Pedestrian Trajectories and Collisions in Crowd Motion

In several biological systems, crowding can occur (ants, bees, birds, fishes, people, etc.). In human societies, crowding affects several aspects of our lives. Human crowds are considered to be one of the most complex systems. Their dynamics, resulting from individual interactions and giving rise to fascinating phenomena, is often difficult to understand and hard to model, which intrigues experts from different fields. The technological advancement, especially in computer performance, has allowed to model and reproduce pedestrian movement more realistically. Nowadays, researchers from different disciplines, such as social sciences and bio-mechanics, are better equipped to examine and understand the crowd movement and pedestrian interactions. Professionals from architects and transport planners to fire engineers and security advisors are also interested in crowd models that allows them to optimize the design and operation of a facility. In the phase of modelling pedestrian and crowd movement, the role of mechanics is prime. Using the principles of mechanics, the necessary equations of new models are derived so that scientists are able to simulate pedestrians’ cognitive and behavioral mechanisms as observed by psychologists and sociologists. Finally, it is worth mentioning that by only using the equations from collision theory, several collective phenomena were well reproduced.

Experimental Study on a Scaled Model of Offshore Wind Turbine on Monopile Foundation

Offshore wind turbines are slender structures with sensitive dynamics, strongly influenced by soil-structure interaction. The structure is subjected to cyclic and dynamic loads with frequencies close to the first natural frequency of the offshore wind turbine. To avoid resonance phenomenon due to external excitations, it is essential to precisely evaluate the initial first natural frequency of the wind turbine and its long term evolution. The present work deals with the design and analysis of a scaled model of an offshore wind turbine with monopile foundation. This study is aimed at experimental evaluation of the initial first natural frequency of this scaled model, followed by the comparison of experimental results with those obtained from the existing analytical models.

Experimental Analysis of a Tuned Mass Damper with Eddy Currents Damping Effect

A Tuned Mass Damper (TMD) is a structural passive control device fixed on a structure and composed of a linear oscillator which natural frequency is tuned to that of the structure, or to the dominant resonance frequency. In this paper, an experimental TMD with adjustable stiffness and eddy current damping is proposed. The first step is to check if the dynamical properties of the proposed TMD are constant during the dynamic test and for different values of stiffness and damping. Therefore, the instantaneous modal parameters are evaluated by applying the continuous wavelet transform on the experimental data. Then, the TMD is set with optimal parameters and used to control vibrations of a frame scale model. The structure response with and without the TMD is evaluated from the experimental measurements in case of a shock applied to the top floor.

Free Oscillations of a Beam With a Local Non Linearity: Comparison of Mechanical Modeling and Experiments by Means of Wavelet Analysis

This paper deals with the analysis and modeling of a linear clamped beam with a local non-linearity. The non linear effects are due to large displacements of the thin beam clamped at one end and screwed to the main beam at the other end. This set-up was built and instrumented at the University of Liege. First, assuming that the thin beam can be replaced by nonlinear springs, a dynamic sub-structuring method is used to obtain the governing non linear differential equations of the system. Free decay responses are then numerically computed and compared to those obtained experimentally. To make the comparison, the wavelet tool is then used and the wavelet processing of free decay mechanical signals is presented in detail. Instantaneous amplitudes and frequencies are extracted from the wavelet analysis of free decay responses of the system, for the first three fundamental response components and their associated super-harmonics. Numerical and experimental results are presented and compared.Copyright