Experiments on the planar lateral vibration of a vertical fluid-conveying pipe with an end mass

This paper experimentally examines lateral vibrations of a cantilevered pipe that is hung vertically with an end mass and conveys fluid with a flow velocity that is entirely constant. It is well known from linear stability theory that when the velocity of a fluid exceeds a certain value, a certain characteristic mode of pipe vibration becomes unstable. Moreover, higher modes of pipe vibration become unstable simultaneously with increasing flow velocities. We are interested in the case in which two-frequency oscillation occurs in the pipe involving the above two unstable modes at higher flow velocities. This phenomenon will be called mixed mode flutter. This phenomenon for a cantilevered pipe is essentially different from the cases of a pined-pined or clamped-clamped pipe with two marginal modes. In these cases, only the higher mode of the pipe vibration becomes unstable while the lower mode becomes stable at much higher flow velocities. Experiments were conducted with a silicon rubber pipe conveying water. The planar behaviors of the vertical fluid-conveying pipe with an end mass were observed using an image processing system that is based on the images captured by two CCD cameras. As a result, lower mode flutter, unstable mixed-mode flutter and also higher mode flutter were experimentally confirmed for the cantilevered pipe. Furthermore, the above phenomena are discussed using the basic equation of nonlinear planar lateral vibration of a vertical fluid-conveying pipe with an end mass from the viewpoint of nonlinear dynamics.