Forcing categorization and identification for fluid-structure interaction

Flow-induced structural vibration is a complicated and often undesirable phenomena. There are a variety of physical mechanisms that lead to vibration, which often makes cause and effect relationships difficult to determine. Here we identify three main categories of aerodynamic forcing mechanisms. The first is external forcing related to upstream disturbances, such as wind gusts or wakes of upstream objects. The second mechanism is termed ``self forcing'', and is related to unsteady aerodynamics caused by the flow over the object itself. Examples include vortex shedding, separated flow, and turbulence. The third mechanism is motion-dependent forcing, in which the unsteady forces are caused specifically by the motion of the structure. The latter mechanism is often termed ``flutter'' and is characterized by negative aerodynamic damping. Example experimental measurements from an axial compressor will be used to demonstrate forcing identification: the temporal response characteristics of blade vibration inform characterization of the causal forcing mechanism.