‘Paradoxical’ mechanics under fluid flow

Interest in the fluid loading of engineering structures has been stimulated by acute technological problems arising in the exloitation of off-shore oil fields. Here slender riser pipes, and complete tethered-buoyant platforms must be designed against potentially dangerous instabilities. We show here how conventional structural theorems can be not only violated, but actually reversed under fluid loading due to its essentially non-conservative character. Under increasing flow, for example, the stiffness of an elastic structure can increase, pass through infinity, and become negative. Surprisingly, the negative stiffness domain is stable, but can be destabilized by the addition of a constraint. Experiments on a hanging articulated pipe conveying fluid nicely confirm the theory.