The present investigation examines a simple fluid-structure interaction problem, which is represented by the unsteady response of an airfoil/blade to a Karman vortex street in an inviscid uniform flow. Two different cases were examined; one with a rigid airfoil/blade, where the structural stiffness is infinite, another with an elastic blade. In both cases, the flow remains attached to the airfoil/blade surface. A time-marching technique solving the Euler equations and a two-degree-of-freedom structural dynamic model is used to examine the interactions between the fluid and the structure. The interactions between the convected vortices and the structure modify the shed wake whose energy, in turn, feeds into the forces and moments acting on the structure. For a rigid airfoil/blade, it is found that the amplitude of the aerodynamic response is not proportional to the density of the oncoming vortex street, but depends on c/d , the ratio of the chord length (c) to the axial spacing (d) of the convected vortices. When the number of vortices per unit length is increased, the amplitudes of the aerodynamic response increase and then decrease even though the density of the vorticity keeps increasing and so is the energy of the excitation wake. Maxima are observed at c/d=0·5, 1·5 and 2·5. This behaviour is analogous to the structural resonance phenomenon and is labeled "aerodynamic resonance". The existence of such an "aerodynamic resonance" is important to turbomachinery applications where the blade is elastic, the flow is unsteady and the shed vortices from the previous row are convected downstream by the mean flow. Thus, "aerodynamic resonance" alone or in conjunction with structural resonance could impact negatively on the fatigue life of turbine blades and their combined effects should be accounted for in blade design. A preliminary attempt to assess this impact has been carried out. It is found that the relative fatigue life of a blade could be reduced by four orders of magnitude as a result.
ASJC Scopus subject areas
- Mechanical Engineering