Numerical Study of Low Engine Order Excitations Due to Manufacturing Variability Part I: Characterization of the Geometric Scatter and Its Effect on Forced Response

Abstract

The geometric scatter in the surfaces of blades and vanes due to manufacturing variability has an impact on the dynamics (structural mistuning) that has been widely studied, but also on the aerodynamics. This geometric variability changes the fluid field, generating disturbances not directly related to the airfoil count. This work focuses on the aerodynamic forcing generated by this manufacturing variability on the adjacent rows. In this paper, deviations from the nominal geometry are obtained from a set of scanned parts. The average manufactured geometry is computed through the point-by-point deviations between the scanned parts and the nominal geometry. Then, principal component analysis (PCA) is applied to the deviations from the average geometry to find a way to describe them in a compact and simple manner. The cold geometries are reconstructed using the mean geometry and the geometric modes; afterwards, a cold-to-hot process is applied to obtain the hot geometry, and finally the resulting aerodynamics are obtained using an in-house computational fluid dynamics (CFD) solver. The loss of the spatial periodicity of the flow due to the geometric variations generates an aerodynamic forcing with a wave number different from the airfoil count. The described method is applied to a representative row constituted by packets of vanes, and two forced response cases are generated. In the first one, the average packet geometry is used to analyze the aerodynamic forcing associated with the geometric variation of the airfoils inside the vane packet. The second one is a conceptual study carried out under some simplified assumptions in order to relate the geometric variability to random low engine order (LEO) disturbances, using the geometric modes obtained from the PCA analysis. In both cases, the induced vibrations on the adjacent rotor row are comparable to the excitation due to the nominal blade-passing.