EXPERIMENTAL INVESTIGATION OF LOW MODE NUMBER CYLINDERS SUBJECTED TO VORTEXINDUCED VIBRATIONS

Abstract

The excitation of three low-mode number, flexible cylinders in uniform-flow is investigated to determine effects of structural mode shape on vortex-induced vibrations. Experiments are performed in a re-circulating flow channel and in a small flow visualization tank using object tracking and digital particle image velocimetry (DPIV) to measure the excitation of the cylinder, to estimate forces acting on the structure, and to observe the wake of the structure under the observed body motions. Previous research has focused on understanding the effect of in-line to cross-flow natural frequency ratio on the excitation of the structure in an attempt to model the excitation of multiple structural modes on long, flexible bodies. The current research investigates the impact of structural mode shape on this relationship by holding the inline to cross-flow natural frequency constant and attempting to excite a specific structural mode shape. Understanding the effects of mode excitation will improve the ability to predict fatigue life in structures which may exhibit combined irregular and regular oscillations by understanding how mode shape affects the vibrating response. In this series of experiments, three cylinders are tested. The in-line to cross-flow natural frequency is kept constant with a ratio of 2:1, while the modes associated with these frequencies are varied: first mode inline to first mode cross-flow, second mode in-line to first mode cross-flow, and third mode in-line to first mode cross-flow. It is found that it is difficult to isolate the effect of mode shape on the vibration of the structure as the cylinder can easily be excited with different combinations of modes. It is shown that for the low-mode number beams tested, there are three primary responses possible for the cylinder when trying to excite the first mode in the cross-flow direction and first mode in the in-line direction (cylinder 1), the first mode in the cross-flow direction and the second mode in the in-line direction (cylinder 2); and the first mode in the cross-flow direction and third mode in the in-line direction (cylinder 3). Cylinder 1 response consists of an expected excitation of the first mode cross-flow with an excitation of the first mode frequency in in-line direction. Regular and periodic cylinder oscillations obtained along the length of the cylinder. Cylinder 2 response consists of an expected excitation of the first mode cross-flow with an excitation of the second mode frequency in in-line direction; however, the structural shape resembles the first mode