Limitation of offshore windturbines Oscillations using a traCK NES

Ecole de l'air et de l'espace; France; Sur place; EUR 60 000; Plein temps

Résumé du poste

A research institution in France is seeking a postdoctoral researcher to work on Nonlinear Energy Sinks (NES) for offshore wind turbines. The ideal candidate will have a PhD in structural dynamics, proficiency in programming and analytical analysis, and the ability to publish findings. This role provides an opportunity to contribute to significant research in vibration control and resilience in mechanical systems. The position offers a salary of €60,000 gross per year and benefits including a subsidized restaurant and cultural activities.

Prestations

Subsidized on-site restaurant

Public transportation benefits

Cultural activities

Qualifications

• Candidate must have a PhD in structural dynamics or a related field.
• Proficiency in English and/or French is required.
• Experience in analytical analysis is preferred.

Responsabilités

• Develop code to design a track NES with controlled properties.
• Validate the effectiveness of track NES using a wind turbine model.
• Publish research findings in peer-reviewed journals.

Connaissances

Analytical analysis

Nonlinear vibrations

Programming (Python)

Scientific writing

Formation

PhD in structural dynamics or related field

Outils

Python

Description du poste

Offer Description

Scientific context and objectives

Energy production is a critical challenge for modern society. In France, offshore wind power stands out as one of the most viable solutions, given its high production capacity, the number of exploitable sites, and the relatively rapid deployment of wind farms [1]. As wind turbines grow in size to maximise efficiency, their increasingly slender structures become more susceptible to vibrations. Effective vibration control is essential to prevent stress levels that could disrupt operations, trigger shutdowns, or, worst of all, shorten the structure’s lifespan. It is therefore necessary to develop more efficient vibration absorbers to protect wind turbines from the many stresses they endure [2].

Wind turbines are complex systems, subject to excitation from wind and waves. Their structural properties (mass and stiffness) are influenced by aero‑hydrodynamic interactions, rotational speed, material or structural wear, and, in the case of monopile or fixed‑bottom turbines, interactions with the seabed [3, 4]. These properties can change over time, altering the natural frequencies at which the turbine is most sensitive [4, 5]. This renders conventional vibration absorbers ineffective, as they are tuned to fixed frequencies. While active absorption methods can adapt to such changes, their reliance on external power sources is a significant limitation.

Nonlinear Energy Sinks (NES) are essentially nonlinear vibration absorbers which can tune themselves to a wide range of frequencies, making them inherently robust against changes in the primary structure’s natural frequencies [6, 7]. When the NES resonance frequency approaches one of the structure’s, a locking occurs, triggering an irreversible energy transfer from the structure to the NES, thereby mitigating vibrations. Optimising NES performance, particularly for offshore wind turbines, is an active and evolving area of research [5, 7].

Innovative NES designs are currently under investigation, including track NES, which consist of a mass rolling on two tracks (see Fig. 2a, b). The rolling tracks’ geometry dictates the path of the centre of mass, effectively defining the NES stiffness through the action of gravity. This design thus offers a practical way to control and adjust the stiffness of the NES. These promising devices absorb energy from the primary system (in this case, the wind turbine) through pure translational motion. While the absorption performance of these devices scales with mass, much like conventional absorbers, practical constraints limit their physical mass. The design freedom is more on the shape of the rolling tracks. The LOCKNES project will therefore focus its effort on the improvement of the rolling tracks to improve the NES performance, drawing inspiration from other absorbers from the literature [9].

Methodology

The study will begin with a simplified system consisting in a wind turbine model represented as a single degree‑of‑freedom (dof) oscillator equipped with a track NES. Using this 2‑dof model, the following developments are planned:

• Development of a Python code to calculate the rolling tracks’ geometry, ensuring the NES’s centre of mass follows a prescribed kinematic motion.
• Analytical investigation of the system’s dynamics, employing NES‑specific tools and an existing formal calculus code for perturbation analysis.
• Establishment of guidelines for the path and rotation law from the analytical results that optimise the NES efficiency.
• Implementation of a finite difference Python code to validate the analytical results against numerical ones.

In the second phase, the effectiveness of the track NES will be validated using a realistic offshore wind turbine model. This phase will involve:

• Modelling the wind turbine and its excitation, drawing on relevant literature for inspiration.
• Creation of a finite element model of the wind turbine, with the NES incorporated as an additional dof.
• Assessment of the track NES’s performance under realistic operating conditions.

Contributions

The postdoctoral researcher is expected to make the following contributions to the project:

• Development of a code to design a track NES with controlled mass and stiffness properties.
• Establishment of guidelines to dimensionalise a track NES.
• Evaluation of a track NES’s effectiveness using a realistic wind turbine model.
• Publication of research work in a peer‑reviewed journal and presentation in a conference.

Through this project, the candidate will have the opportunity to develop expertise in:

• Kinematic computation to design a system with a prescribed motion.
• Analytical tools for the study of (nonlinear) dynamical systems.
• Finite Element Analysis.

Research Environment

Host Institution Founded in 2004, GeM is a Joint Research Unit of Nantes Université, Centrale Nantes, and the CNRS (UMR‑6183). It brings together all the expertise in civil engineering, materials mechanics and processes, and modelling and simulation in structural mechanics from the Nantes Saint‑Nazaire metropolitan area within a single laboratory.

Team and Collaborations The postdoctoral researcher will join the laboratory’s structural dynamics research unit, a team comprising 14 permanent researchers, including nonlinear dynamics specialists Vincent Mahe and Mathias Legrand, as well as 6 PhD and postdoctoral researchers. This project shares strong interests with ongoing work at the Ecole de l’Air et de l’Espace (EAE) on the design of nonlinear absorbers and isolators applied to the vibration control of aerospace structures [10]. As part of this project, a collaboration is planned to build on the expertise of Benjamin Chouvion, particularly regarding the underlying physical phenomena and optimisation techniques for NES design [10, 11].

Location The campus of Centrale Nantes is located along the Erdre river, close from the centre of Nantes, and is easily accessible with common transportation or by bike. The campus comprises about 3500 persons, including about 2000 students, and offers several cultural activities (sports and others). Nantes is a pleasant dynamic city with a rich cultural scene, a nice historical district and many green spaces along its rivers and parks.

A one‑week stay at the EAE, in Salon de Provence, is planned as part of the project.

References

[1] ADEME. Transition ecologique, le souffle venu des territoires. ADEME Magazine, April 2024.

[2] H. Zuo, K. Bi, and H. Hao. A state‑of‑the‑art review on the vibration mitigation of wind turbines. Renewable and Sustainable Energy Reviews, 121:109710, April 2020.

[3] M. Ghozlane and F. Najar. Nonlinear analysis of a floating offshore wind turbine with internal resonances. Nonlinear Dynamics, 112(3):1729–1757, February 2024.

[4] P. Alkhoury, M. Aït‑Ahmed, A.-H. Soubra, and V. Rey. Vibration reduction of monopile‑supported offshore wind turbines based on finite element structural analysis and active control. Ocean Engineering, 263:112234, November 2022.

[5] P.-O. Mattei and R. Cote. Optimization of a dynamic absorber with nonlinear stiffness and damping for the vibration control of a floating offshore wind turbine toy model. Journal of Theoretical, Computational and Applied Mechanics, page 10123, May 2023.

[6] A. F. Vakakis, O. V. Gendelman, L. A. Bergman, A. Mojahed, and M. Gzal. Nonlinear targeted energy transfer: State of the art and new perspectives. Nonlinear Dynamics, 108(2):711–741, April 2022.

[7] H. Zuo and S. Zhu. Development of novel track nonlinear energy sinks for seismic performance improvement of offshore wind turbine towers. Mechanical Systems and Signal Processing, 172:108975, June 2022.

[8] J. Wang, C. Zhang, H. Li, and Z. Liu. Experimental and numerical studies of a novel track bistable nonlinear energy sink with improved energy robustness for structural response mitigation. Engineering Structures, 237:112184, June 2021.

[9] V. Mahé, A. Renault, A. Grolet, H. Mahé, and O. Thomas. Experimental investigation of the direct and subharmonic responses of a new design of centrifugal pendulum vibration absorber. Mechanism and Machine Theory, 188:105401, 2023.

[10] K. Dhital and B. Chouvion. Passive Aeroelastic Control of a Near‑Ground Airfoil with a Nonlinear Vibration Absorber. Aerospace, 11(12):1043, December 2024.

[11] B. Chouvion. A wave approach to show the existence of detached resonant curves in the frequency response of a beam with an attached nonlinear energy sink. Mechanics Research Communications, 95:16–22, January 2019.

Where to apply

E‑mail benjamin.chouvion@ecole‑air.fr

Requirements

Research Field Engineering – Mechanical engineering Education Level: Master Degree or equivalent

Skills/Qualifications

The candidate should fulfil the following requirements:

• PhD in structural dynamics or a related field.
• Proficiency in English and/or French.

The following skills would be an asset:

• Experience in analytical analysis.
• Experience in nonlinear vibrations.
• Familiarity with programming tools, preferably Python.
• Ability to write scientific papers for journal publication.

Salary: 60 000€ gross per year
Contract duration: 12 months
Start date: between 01/01/2026 and 01/09/2026
Benefits: subsidised on‑site restaurant, public transportation, cultural activities

Selection process

Applicants should submit the following materials:

• Academic CV.
• Your best journal paper, report, or presentation (according to you).
• Names and contact information of two academic references.
• Any additional supporting materials (e.g., publications, reports, letters of recommendation).

Applications are being accepted immediately and will be considered in the weeks following receipt.