20 janvier 2020

Seminaire Laeticia Pernod

Laeticia Pernod a sountenu sa thèse de doctorat à le Laboratoire d’Hydrodynamique, d’Énergétique et d’Environnement Atmosphérique (LHEEA) Ecole Centrale de Nantes et a été PostDoc Researcher au Laboratoire de Mécanique des Structures et des Systèmes Couplés (LMSSC) et au Conservatoire national des arts et métiers (Cnam) de 01/2019 à 12/2019
Joint numerical and experimental study of fluid-structure interactions on composite marine lifting surfaces and passive flow-induced vibration damping using resonant piezoelectric shunts

Applications for marine lifting surfaces are progressively expanding and gaining economic importance in a growing number of maritime engineering fields. Marine lifting surfaces are indeed used as hydrofoils for high-speed ships, stabilizers, rudders, marine propellers and turbine blades. However, when subjected to hydrodynamic flows, these submerged structures may undergo strong fluid-structure interactions such as flow-induced vibrations (FIV), with almost always unwanted and harmful consequences. Indeed, flow-induced vibrations may trigger a sharp increase in the vibration amplitude when there is a coincidence between a natural frequency of the structure and a hydrodynamic excitation frequency. Such high-amplitude vibrations lead to shorter life cycles due to structural fatigue, as well as a reduced acoustic discretion. Moreover, within the main objective of CO2 emissions reduction, the key design of lifting surfaces clearly remains in the development of lighter structures, but also in the modification of the structural properties to increase the performances and to reduce the possible flow-induced vibrations mentioned previously. In particular, the development of composite marine propellers for surface ship and submarines has been the subject of increasing researches in the last decade, because of their inherent bend-twist coupling property that enables the propeller to passively adapt to the incoming flow. Nonetheless, these highly flexible structures are much more prone to strong fluid-structure interactions and the development of harmful hydrodynamic instabilities, such as FIV.

Therefore, vibrations control and damping solutions, as well as accurate understanding and prediction of the hydroelastic response of marine lifting surfaces and composite marine lifting surfaces, in particular using high-fidelity fluid-structure coupled numerical methods, are critical to many maritime applications, in order to improve both hydrodynamic and structural dynamic performances, as well as to ensure structural safety.

This presentation will consider the two aspects of this problem, first focusing on the fluid-structure interaction and hydroelastic response of a composite hydrofoil using an innovative joint experimental and high-fidelity fluid-structure coupled numerical method, and then dealing with the passive vibration damping of hydrofoils using resonant piezoelectric shunts, both experimentally and numerically. Concerning the first study, the main novelties are, first, the use of a state-of-the-art strain measurement technique, via a fully-distributed-optical fiber sensor directly embedded within the composite plies. This method allows for a finer representation of the structural deformations under hydrodynamic loading. Second, a tightly-coupled high-fidelity fluid-structure interaction numerical model taking into account the turbulent effects in the flow and the ply-by-ply modelling of the composite, is compared to the experimental results. A composite profile is specifically designed as a trapezoidal hydrofoil and is tested for moderate Reynolds number and pre-stall and post-stall incidences. High-speed imaging of the hydrofoil tip and vibrometer measurements are carried out to determine the experimental tip displacements and hydrofoil’s vibrations. The numerical and experimental results show a very strong hydroeleastic response, with a structural resonance even for low Reynolds numbers due to the high flexibility of the structure. Strong coupling of the fluid and the structure, with lock-in of the von Kármán vortex-shedding to the structure for small incidences, and an excitation of the structure by leading-edge vortex-shedding for higher incidences, are also observed. Leveraging the experimental protocol developed in our first research, the second study provides a first experimental prototype for the application of vibration damping using a resonant piezoelectric shunt in water. The structure is first tested under hydrodynamic flows for various Reynolds numbers to investigate its flow-induced vibrations. It shows a significant lock-in phenomenon between the von Kármán vortex shedding and the first torsional mode. This allows to determine the natural frequency of interest to test the control solution. Second, experimental and numerical modal analyses are carried out to determine the open and short circuit natural frequencies in order to compute the piezoelectric coupling factor. Indeed, the latter is related to the expected performance of the passive vibration damping strategy. Third, the values for the resistive and inductive components of the RL-shunt are inferred from the coupling factor and the natural frequencies. Finally, the control solution is tested under various hydrodynamic flows, and shows a reduction of the RMS values.

20 janvier 2020, 13h3014h30
Bâtiment M6