Missions

The aim of this project is to carry out an in-depth experimental study of the influence of surrounding turbulence on aerodynamic instabilities affecting pendulum structures. The study will cover both model systems, such as a simple pendulum placed in a wind tunnel, and realistic geometries for applications such as cable transportation.

Activités

The simple pendulum remains one of the most fundamental systems studied in physics. Indeed, the harmonic oscillator is commonly used as a model to illustrate a wide variety of mechanisms in all branches of physics. However, despite this popularity, subtle behaviors remain to be discovered and explored when a pendulum is strongly coupled to fluid mechanics. In a previous work (Obligado et al., Journal of Fluid Mechanics (2013)), for example, we showed that the equilibrium of a pendulum disk against a flow exhibits bi-stability and hysteresis. This particular behavior results from the specifics of the pendulum's aerodynamic coupling (via drag and lift forces) with the surrounding flow, while at the same time making the pendulum a simple generic model for bi-stable stochastic phenomena. In this spirit, we have, for example, shown that spontaneous transitions can occur between multi-stable branches, which link the statistics of universal extreme events to the dynamics of the pendulum wake (Gayout et al., Phys. Rev. Lett. (2021)).

The present project aims to further explore the role of surrounding turbulence on the dynamics and multi-stability of the pendulum in the wind. Turbulence can indeed have ambivalent effects: on the one hand, it can amplify pendulum fluctuations (thus favoring the transition from one stable potential well to another) and, on the other hand, it can radically alter the overall energy landscape of the multi-stable system and eventually suppress multi-stability. To this end, experiments will be carried out in the wind tunnel of the Laboratoire de Physique de l'ENS de Lyon, which has recently been equipped with an active grid, enabling the intensity of the turbulence to be adjusted. Initially, the angular dynamics of the pendulum will be explored while systematically varying the turbulence intensity, with the aim of elucidating the impact of turbulent fluctuations on the dynamics of the pendulum in each of the stable branches, as well as the modification of the hysteretic behavior. This investigation will then be complemented by high-speed imaging diagnostics of the flow dynamics (in particular the pendulum wake), using time-resolved PIV and Lagrangian Partilce Tracking, to search for aerodynamic signatures of pendulum behavior. The versatility of the active grid will also enable us to explore new phenomena, such as the role of spatial and temporal modulation of turbulence on pendulum dynamics, and multi-stability, which can lead to instabilities when flow frequencies synchronize with the pendulum.
Beyond the fundamental aspects of these studies, where the pendulum is considered as a multi-stable stochastic system forced by complex correlated noise (turbulence), the results of this work are of great practical relevance to aerodynamic pendulum systems, such as urban and transportation systems.

Compétences

The candidate must have a sound knowledge of fluid mechanics and aerodynamics, as well as mastery of metrologies such as PIV or PTV.

Contexte de travail

The work will take place in the Physics laboratory of the Ecole Normale Superieure de Lyon (LPENSL). This group contains both turbulence experimentalists and theorists and numericians. This complementarity of approaches is generally very fruitful.
In addition, the LPENSL is a general physics laboratory, with many skills.
This specific project is part of a broader collaboration funded by ANR, including the laboratoire PPrime (Poitiers), the LHEEA (Nantes), the CSTB and 3 partners from cable car industry (POMA, MND and Dopplemayr).

Contraintes et risques

None