A New Nonlinear Time-Domain Flutter Analysis Approach for Distorted Flows

Abstract

This paper aims to establish an efficient and accurate Computational Fluid Dynamic (CFD) method for calculating the flutter stability of fan blades in the presence of inlet distortion due to crosswind. Due to the asymmetry of the flow and interaction of fan and distortion, this type of analysis will require a whole assembly computational model. Therefore, reducing the computational cost needed to obtain accurate aerodynamic damping values is paramount. Mode-tracking free-flutter analyses are computationally expensive and troublesome due to the presence of many frequencies in the response signals and, hence, require long physical times to converge the aerodynamic damping. Moreover, the unsteady flow field associated with the distortion can be more significant than the one due to flutter, giving rise to additional numerical difficulties. On the other hand, the energy method approach is commonly performed for a single Nodal Diameter (ND) at a time, requiring many computations to establish the least stable ND. This paper proposes an alternative approach based on the multi-ND energy method. The underlying hypothesis is that the unsteady pressure scales linearly with the blade displacements, and the scattering of circumferential modes due to the distortion is negligible. The general method presented here is general, and can be used to compute the aerodynamic damping for other types of distortion (such as downstream distortions due to pylon and structure).