Vortex-induced vibration effect on fatigue life estimate of turbine blades

An analysis of a turbine blade fatigue life that includes the physics of fluid-structure interaction on the high cycle fatigue (HCF) life estimate of turbine blades is carried out. The rotor wake excitation is modeled by rows of Karman vortices superimposed on an inviscid uniform flow. The vortex-induced vibration problem is modeled by a linear cascade composed of five turbine blades and the coupled Euler and structural dynamics equations are numerically solved using a time-marching boundary element technique. The analysis can be applied to any blade geometries; it is not limited to the blade geometry considered here. Two major design parameters have been identified; the ratio of blade spacing to blade chord length s/c of the stator, and the normalized frequency parameter c/d which is related to the wake passing frequency of the rotor. For a rigid cascade, it is found that aerodynamic resonance prevails at the resonant c/d values corresponding to an isolated blade while s/c is responsible for the level of the aerodynamic response. If the central blades were elastic, the parameter s/c plays a different role in the fluid-structure interaction problem. With a c/d that could lead to structural resonance for an isolated blade, changing s/c would stabilize the aerodynamic and structural response of the elastic blade in a cascade. On the contrary, an improper choice of s/c might turn the elastic blade response into structural resonance even though the oncoming c/d is non-resonant. The results of the nonlinear effects of c/d and s/c could be used together with the Campbell diagram to obtain an improved HCF design of rotor-stator pair.