Vibration of Support Excited in Coaxial Flow

The dynamic response of slender tubes in coaxial flow with forced motion of the tube support structure is investigated. The steady-state response of fixed-free tubes was studied for various flow velocities of water and levels of base excitation. The data shows: (1) fluid flow acts as an excitation mechanism at low levels of base excitation, (2) for sufficient base excitation, the fluid flow acts as a damping mechanism, and (3) for sufficiently high levels of base excitation, the response is bounded by the case of zero flow velocity. When fluid flow acts as a damping mechanism, it is shown that the peak response can be described as the product of a linear function of base excitation and a nonlinear function of flow velocity. It is found that the Euler-beam equation with viscous damping may be used to describe the response for the first two modes where the damping parameter is a linear function of the flow velocity. (Shaker equipment limited the experimental studies to the first two modes.) The damping due to coaxial flow of water was found to be almost two hundred times that in air for the highest flow velocity used.