The requirements for advanced aircraft engine components lead to designs that are more lightweight and efficient, yet more susceptible to excessive vibration, complex dynamic behavior, and uncertain durability and reliability. This complex nature of the dynamic behavior also leads to thicker blade designs; hence, increased fuel burn, increased noise, potentially reduced engine life, and increased maintenance costs. As part of the NASA Aeronautics Research Fixed Wing (FW) Project, Glenn Research Center has been investigating potential technologies that support the FW goals for lighter, quieter, and more efficient aircraft.
One of the challenge areas in the development of highly efficient and lighter aircraft engines is that high-performance rotating blades are subject to high cycle fatigue (HCF) limitations as a result of high vibratory stresses. Excessive vibration of turbomachinery blades requires damping treatments to mitigate excessive vibration levels that cause HCF problems. Designing damping treatments for rotating blades in an extreme engine environment is a difficult task with various factors such as high temperatures and centrifugal accelerations. Several damping methods have been investigated by researchers for use in aircraft engine blades. Piezoelectric damping has also been explored as a potential solution for damping treatment. These efforts reported incremental reduction in resonant response utilizing various techniques; however, their investigations were only non-spinning demonstrations.
The present innovation attempts to fill this void by developing and demonstrating the active piezoelectric damping treatment on aircraft engine rotating fan blades. Specifically, the objective was to develop multiphysics piezoelectric finite element modeling methodology for harmonic forced vibration response analysis coupled with shunted piezoelectric circuits under rotating conditions. ANSYS Multiphysics software was utilized to incorporate specific modeling techniques in this effort, along with experimental test verification processes to validate this numerical simulation methodology. This new method was proved as a feasible and cost-effective virtual solution approach able to predict active piezoelectric damping of the rotating engine blades with excellent computational efficiency and accuracy.
A model of a rotating composite fan blade was formulated using a prestressed modal analysis method prior to conducting the harmonic forced rotating blade vibration formulation. A blade spin rig experimental test was performed at speeds from zero rpm (revolution per minute) to 5000 rpm to verify the mathematical formulation processes. To capture the true blade behavior in this rotational condition, a large strain (or finite strain) method was developed to take into account the blade shape and stiffness changes under high centrifugal rotation. Rigid-body effects (e.g. large rotation) were also taken into account that separate displacements due to rigid-body motion and those associated with the small strains. Spin softening (or stress stiffening) of the rotating blade structure due to the stress state was also taken into account in this process. The equation of motion corresponding to the piezoelectric actuator and the blade structure was assembled, which includes the degrees of freedom of the piezoelectric actuator (voltages and displacements) and the degrees of freedom of the blade structure (displacements).
The piezoelectric circuit element simulates electric circuit components that can be directly connected to the piezoelectric finite element domain for the simulation of circuit-fed piezoelectric dampers for vibration control under the influence of harmonic (sinusoidally varying) forces, currents, displacements, and voltages. This circuit element was used to model resistor (R) and inductor (L) elements connected to each electrode of the piezoelectric patch. The harmonic response analysis solves time-dependent equations of motion for structures undergoing steady-state vibration. A finite element model of the entire blade model, including the airfoil and “dovetail” attachment was used. Subsequently, finite element modeling techniques were coupled with the piezoelectric element for predicting forced vibration blade response to blade rotations, and developed with a harmonically-varying blade base excitation verified with the GRC magnetic bearing blade spin rig test while the blades rotate.