Holographic vibration measurement and numerical modelling of immersed structures

Abstract

Holographical measurement of structures immersed in water may bring a useful contribution to the study of fluid-structures interaction and also to the numerical models of such mechanical components. Among several cases of particular interest, this paper focuses on turbomachinery and plates which are basical elements of transducers and underwater machinery. The paper presents two series of measurements. One concerns a uniform plate, for which eigenfrequencies and eigenmodes are strongly affected by the level of immersion. Experimental results are correlated with those obtained for the numerical model, and allow updating of the model. The second example is that of a propeller. The experimental results made possible the validation of a method for the vibroacoustic analysis of structures having symmetry properties. It represents an extension of the method of the linear representations of finite symmetry groups to problems of coupling fluid-structure. This approach, while keeping the quality of the approximations, leads to a significant reduction of the number of degrees of freedom, with a maximal reduction for the so-called repetitive structures. For the propeller, this approach will allow to model its vibration in immersed state and improve the model by using the interferometric measurements.