9. Active Control of Flexible Cylinders Undergoing Vortex-Induced Vibrations Using Piezo Stripe Actuators

In this study, piezo stripe actuators were used to control the vibrations of a flexible cylinder undergoing underwater flow-induced vibrations. The piezo actuators were attached at the anti-nodes of a rectangular plastic beam and urethane rubber was used to mold the test model to have a circular cylinder shape. Forced base oscillation experiments were first carried out in air to characterize the system and piezo responses. Experiments were then performed in a recirculating water channel with the flexible cylinder in a uniform free stream, where the cylinder undergoes self-excitation due to vortex shedding in the wake and forced excitation due to the piezo actuators. The actuators were oriented to apply an excitation only in the in-line direction of the flow. In the tests, two separate cases were investigated. In the first case, the piezo actuators were activated at a flow speed corresponding to a flow-induced response with a spatial mode change, causing the cylinder to be excited with a higher mode, leading to a significantly smaller amplitude response than without the piezo actuation (vibration suppression). In the second case, piezo actuators were activated at a flow speed corresponding to a significant flow-induced amplitude increase. The interaction of the piezo forcing and the forced response of the cylinder results in a jump to the higher amplitude response regime (vibration enhancement). This study presents two important observations: (1) it is possible that piezo actuators can trip the response frequency to force the cylinder to oscillate with a different mode thus reducing the total response amplitude significantly, (2) it is also possible to prematurely increase the response amplitude for a particular flow speed (i.e. jumping from a lower branch to upper branch response).