Experimental investigation of fluidelastic instability of a rotated square array subjected to two-phase and air cross-flow

Abstract

This paper reports laboratory experiments to investigate the fluidelastic instability of a rotated square array with a pitch ratio of 1.64 subjected to cross-flow. A series of air–water two-phase flow, as well as air flow, experiments were conducted. Test done for a range of array flexibilities yielded valuable insight into the array dynamics over a wide range of flow void fractions from 40% to 97%. The study revealed the existence of transverse fluidelastic instability for 97% void fraction, with variable vibration strength based on the number of flexible tubes, and stable behaviour for lower void fractions. Generally, a significant increase in fluid damping was noted in the transverse and streamwise directions in two-phase flow. In air flow, a series of experimental tests showed complex fluidelastic behaviour for this array. Fluidelastic instability was found in air flow tests in the streamwise direction only, while divergence, associated with negative fluid stiffness force, was observed in the transverse direction. Dynamic instability in air required coupling between adjacent tubes; confirming the stiffness controlled instability mechanism. For air flow, fluidelastic behaviour was strongly dependent on tube location within the array.