Dependence of Post-Stable Fluidelastic Behavior on the Degrees of Freedom of a Tube Bundle

Abstract

Several experimental studies of fluidelastic instability in tube bundles have revealed the existence of post-stable hysteresis behavior. The objective of the present study was to determine the dependence of this behavior on the number of degrees of freedom of the bundle. We tested two different bundle geometries in air cross-flow. We controlled both the number of flexible tubes in the bundle and, using highly asymmetric beams, the number of degrees of freedom per flexible tube. We also varied tube mass within the mass-damping parameter range of mδ/pd2 = 1·9?23, and we varied the number of tube rows upstream of the flexible tubes of interest.We found that a dominant tube typically controls the fluidelastic instability and post-stable behavior of a fully flexible bundle. Furthermore, a single-degree-of-freedom system (a tube constrained to move transverse to the flow) can display similar post-stable behavior as that of the entire bundle. Thus, for the arrays investigated, the fluidelastic mechanism requires neither fluid coupling between tubes nor coupling between streamwise and transverse motion of a single tube to generate hysteresis. However, the exact details of the post-stable behavior (e.g., hysteresis effects, limit cycle amplitudes) depend quite strongly on array misalignment and the number of upstream tube rows. Amplitude-dependent dampimg measurements reported here suggest that finite limit cycles result from fluidelastic, not structural, nonlinearity. Also, transient excitation tests indicate that turbulence may be substantially more likely than previously thought to excite instability of an array operating just inside a hysteresis region. For broad hysteresis regions, such reductions in critical velocity are as significant to heat-exchange design as array pitch and pattern effects. The present work suggests that a nonlinear, single-degree-of-freedom model would be a good first step towards their prediction.