Vibration Generation Mechanism Of Two Tandem Identical Cylinders

Research about flow-induced vibration has gained greater development in the world in the past decades, the lack of control of flow-induced vibration may reduce life of engineering equipment, chimney equipment in steelworks, heat exchange tubes, and even graphite rod in nuclear reactor.Two identical cylinders could be chosen as the simplest case to facilitate the study of mechanism for flow-induced vibration. Two identical cylinders in tandem arrangement with both ends spring-mounted, which can only vibrate in the cross-flow direction. L/D varied from 1.2, 1.5, 2.0, 2.5 and 3.0, which is changed by the replace the position of upstream cylinder,Two laser vibrometer are used to measure the amplitude of two cylinders and the phase angle between them. The amplitude increased as the reduced velocity increased. After vortex resonance, and the phase angle between the displacement of two cylinders have shown relationship with the change of L/D. Pressure scanner and laser vibrometer are used synchronously to measure the time history of displacement and pressure. When the amplitude of downstream cylinder is small, the added mass and flow-induced damping are positive, which indicate the flow prevent the cylinder from vibration due to the upper and lower shear layers separate from upstream cylinder reattach on the surface of downstream cylinder respectively. While when the violent vibration happen, those two parameters become negative, the flow prompt the cylinder to vibrate,or to say the flow make the downstream cylinder vibrate more easily. Combined with the results of Particle Image Velocimetry(PIV), both shearlayers from upstream cylinder alternately wiggle along two different sides of the downstream cylinder, providing the lift for the movement and excite the vibration.