SECOND-ORDER FREQUENCY-INDEPENDENT MODELING OF EXPERIMENTAL FLUIDELASTIC FORCES

The importance of fluidelastic forces in flow-excited vibrations is crucial, in view of their damaging potential. Flow-coupling coefficients are often experimentally obtained from vibration experiments, performed within a limited experimental frequency range. For any given flow velocity, these coefficients are typically frequency-dependent. This is not only awkward for attempting physical interpretations, but also leads to numerical difficulties when performing time-domain computations. In this work, we address this problem by assuming that the measured fluidelastic forces encapsulate "hidden" (non-measured) dynamics of the coupled flow. This leads to the possibility of modeling the flow-structure coupled dynamics through conventional ODEs with constant parameters. The substructure analysis of such a model, augmented with a set of "hidden" flow variables, highlights an inevitability of the frequency-dependence found in the measured flow forces, when these are condensed at the few measured degrees of freedom. The formulation thus obtained clearly suggests the mathematical structure of the measured fluidelastic forces. Then, inspired by work in the fields of soil-structure interaction and viscoelasticity, we proceed by identifying a computationally convenient and physically meaningful second-order flow-coupling matrix model. Finally, the developed concepts and procedures are applied with success to experimental results obtained at CEA, for the fluidelastic interaction forces acting on a flexible tube within a rigid bundle subjected to cross-flow, although the problem addressed embraces a much wider range of applications. The proposed flow modeling and identification approach shows significant potential in practical applications, with many definite advantages.