A single flexible tube in a rigid array as a model for fluidelastic instability in tube bundles

A mathematical model for fluid damping controlled instability of tubes presented in this paper is based on the unsteady flow theory. Motion dependent fluid forces are measured in a water channel. From the measured fluid forces, fluid stiffness and fluid damping coefficients, are calculated as a function of reduced flow velocity, oscillation amplitude, and Reynolds number. Once these coefficients are known, the mathematical model can be applied to predict structural instability due to fluid damping. Many cases are considered: single tube, twin tubes, tube row, triangular array, and square arrays. The results show the instability regions based on the fluid damping coefficients and provide the answers to a series of questions on fluid elastic instability of tube arrays in crossflow.

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Fluid-damping-controlled instability of tubes in crossflow

Surface pressure survey in a parallel triangular tube array

The effect of grid generated turbulence on the fluidelastic instability response in parallel triangular tube array

Unsteady Reynolds averaged Navier-Stokes simulation of the post-critical flow around a closely spaced group of silos

Experiments and computations of a loosely supported tube in a rigid bundle subjected to single-phase flow

A new method for the generation of representative time-domain turbulence excitations

The modal identification of dynamical systems under operational conditions, when subjected to wide-band unmeasured excitations, is today a viable alternative to more traditional modal identification approaches based on processing sets of measured FRFs or impulse responses. Among current techniques for performing operational modal identification, the so-called blind identification methods are the subject of considerable investigation. In particular, the SOBI (Second-Order Blind Identification) method was found to be quite efficient. SOBI was originally developed for systems with normal modes. To address systems with complex modes, various extension approaches have been proposed, in particular: (a) Using a first-order state-space formulation for the system dynamics; (b) Building complex analytic signals from the measured responses using the Hilbert transform. In this paper we further explore the latter option, which is conceptually interesting while preserving the model order and size. Focus is on applicability of the SOBI technique for extracting the modal responses from analytic signals built from a set of vibratory responses. Aspects of the theoretical formulation for complex SOBI using the Hilbert transform are clarified and a convenient computational procedure for obtaining the complex cross-correlation response matrix is developed. We show that the correlation matrix of the analytic responses can be computed through a straightforward Hilbert transform of the standard real correlation matrix typically obtained from measurements. Then, based on numerical simulations of a physical multi-modal system subjected to distribute random excitation, we assert the quality of the identified modal matrix and modal responses extracted using both the standard and the complex SOBI techniques. To perform such analysis, a simple and feasible physical device is proposed, which enables controlled levels of the modeshapes complexity, without introducing significant modal damping even for strongly complex modes.

Laurent Borsoi

Papers

Experiments and computations of a loosely supported tube in a rigid bundle subjected to single-phase flow

A new method for the generation of representative time-domain turbulence excitations

On using the Hilbert transform for blind identification of systems with complex modes