Vortex-induced vibrations of a cantilevered blunt plate: POD of TR-PIV measurements and structural modal analysis

Vortex-induced vibrations sustained by immersed bodies may induce early structural fatigue and acoustic noise radiation. The present study consists of an experimental characterization of the vortex shedding and induced vibrations of a cantilevered blunt rectangular aluminium plate of chord to thickness ratio 16.7, immersed in a uniform water flow in the hydrodynamic tunnel of the French Naval Academy Research Institute. Experiences have been conducted at zero degrees incidence for Reynolds numbers Rec (based on chord length) ranging from 2.5 x 105 to 9.5 x 105, leading to turbulent wake conditions. The hydrodynamic properties of the wake have been evaluated by statistical analysis, Proper Orthogonal Decomposition (POD) and vortex core identification of Time Resolved Particle Image Velocimetry fields. The structural response of the plate has been examined by laser vibrometry through modal analysis. The fluid-structure interactions have been analysed for three different vibration regimes: out of resonance, lock-off resonance and lock-in resonance with the 1st twisting mode. The features of the first nine POD modes were analysed according to the different vibration regimes to understand the physics proper to the near wake. In the out of resonance regime, the strength of the primary Karman vortices is preferentially controlled by the Strouhal and Reynolds numbers. The thickness based Strouhal number is 0.195. A secondary hydrodynamic excitation source of characteristic Strouhal number 0.227 coexists with the primary Karman vortex shedding mode. At the lock-off resonance, this secondary excitation source is identified as a second Karman vortex shedding mode which is coupled with the natural frequency. A local maximum of vibration magnitude is attained for this regime. The coexistence of these two Karman modes is responsible for the early occurrence of resonance. The lock-in resonance occurs when the primary vortex shedding mode locks with the natural frequency. It is characterized by an enlargement of the wake and by a reinforcement of the primary Karman vortices strength.(c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

The present study experimentally characterized the vortex shedding and induced vibrations of a rectangular aluminium plate immersed in water flow. The hydrodynamic properties of the wake were evaluated and the structural response of the plate was examined. Three different vibration regimes were analyzed, and the features of the first nine Proper Orthogonal Decomposition (POD) modes were studied. The study identified the control factors of the primary Karman vortices and the coexistence of different vortex shedding modes in different vibration regimes.