A comparative study on optimization of multiple essentially nonlinear isolators attached to a pipe conveying fluid Mostafa Khazaee, S.E. Khadem ⇑ , Amin Moslemi, Ali Abdollahi Department of Mechanical Engineering, Tarbiat Modares University, Tehran P.O. Box 14115-177, Iran article info Article history: Received 1 April 2019 Received in revised form 11 October 2019 Accepted 13 October 2019 Available online xxxx Keywords: Nonlinear energy sink Stochastic optimization Multi-objective optimization Pipe conveying fluid Parallel NESs Series NESs abstract In recent years, nonlinear energy sinks (NES) have been recognized as a promising method for passive vibration control of pipes conveying fluid due to their advantages over the clas- sic tuned mass damper (TMD). Nevertheless, it has been shown that their performance is highly sensitive to uncertainties, and they are only effective in a narrow range of flow velocities and external excitations. Therefore, uncertainties in both design and aleatory parameters must be considered in their optimal design. For the first time, this article inves- tigates stochastic optimization of multiple NESs configured in parallel and series configu- rations attached to a simply supported pipe conveying fluid. The main goal of this optimization approach is to overcome the inherent sensitivity of NES performance and increase the efficiency range of that. The optimization problem is based on a multi- objective model, with the aim of maximizing the expected value of the NES efficiency esti- mated from the monte-carlo (MC) simulations, as well as minimizing the corresponding standard deviation. Stiffness property, damping, mass ratio, and location of NES are consid- ered as random design parameters. The distributions of NESs efficiency for stochastic based optimizations are compared to deterministic optima. The results demonstrate that the stochastic optima are much more robust than the deterministic one, and is able to deal with various sources of uncertainties particularly in initial excitation and flow velocity. It is observed that there is an optimal number for NESs in both configurations, above which the performance would not be improved and maybe reduced by increasing the number of them. Also, comparing the present work to other similar works, yields a satisfying result. Ó 2019 Elsevier Ltd. All rights reserved. 1. Introduction Pipes conveying fluid have various applications in a wide range of engineering fields, such as fluid transportation facili- ties, municipal water supply systems, risers, and heat exchangers. Interactions of pipes conveying fluid with external exci- tations and the motion of the inside fluid cause undesirable vibrations and dynamic instabilities that can lead to large deformations, fatigue, and noise [1–7]. Various ways for classifying vibrations of pipes conveying fluid have been proposed in the literature. Based on the source of excitation, pipes vibrations can be classified as instability induced vibration (IIE), extraneously induced vibration (EIE), and movement induced vibration (MIE) [5]. It is crucial to find a technique to eliminate these undesired vibrations. The majority of researches have proposed the active vibration control for vibration suppression https://doi.org/10.1016/j.ymssp.2019.106442 0888-3270/Ó 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: mostafa.khazaee@modares.ac.ir (M. Khazaee), Khadem@modares.ac.ir (S.E. Khadem), amin_moslemi@modares.ac.ir (A. Moslemi), a.abdolahi@modares.ac.ir (A. Abdollahi). Mechanical Systems and Signal Processing xxx (xxxx) xxx Contents lists available at ScienceDirect Mechanical Systems and Signal Processing journal homepage: www.elsevier.com/locate/ymssp Please cite this article as: M. Khazaee, S. E. Khadem, A. Moslemi et al., A comparative study on optimization of multiple essentially non- linear isolators attached to a pipe conveying fluid, Mechanical Systems and Signal Processing, https://doi.org/10.1016/j. ymssp.2019.106442