G.L. Kuiper, A.V. Metrikine


Experimental investigation of dynamic stability of a cantilever pipe aspirating fluid


International Journal of Fluids and Structures


Summary. The dynamic stability of a submerged cantilever pipe conveying fluid upwards is considered as one of the unresolved issues in the area of fluid-structure interaction. There is a contradiction between theoretical predictions and experiments. Reported experiments did not show any instability, while theory predicts instability beyond a critical fluid velocity. Recently, several papers appeared, improving the theoretical modeling of the cantilever pipe conveying fluid. All theories predict unstable behaviour, either oscillatory instability or static instability, denoted here as flutter and divergence, respectively. To investigate whether the previous experimental setups did not allow to observe the instability or the pipe aspirating water is unconditionally stable, a new test setup was built which could attain a higher internal fluid velocity than the predicted critical velocities. A cantilever pipe of about 5 m length was partly submerged in water. The free open end of the pipe was in the water, whereas the fixed end was above the waterline. The experiments clearly showed that the cantilever pipe aspirating water becomes unstable by flutter beyond a critical velocity of water convection through the pipe. Below this velocity the pipe is stable, whereas above it, the pipe shows a complex motion that consists of two alternating types of motion. The first type is a nearly periodic orbital motion with the amplitude of a few pipe diameters and the second one is a quasi-chaotic motion with very small amplitude. Increasing the internal fluid velocity results in a larger amplitude of the orbital motion.