Influence of Axial Gaps on Forced Response of Different Mode Shapes in a Low-Pressure Compressor

Forced-response vibrations present an ongoing challenge in compressor design. This is especially apparent when axial-gap variations are considered: narrower gaps come with the benefit of a lower component weight. From the standpoint of aeroelasticity, however, this typically causes a stronger wake-induced excitation. Conversely, recent results have demonstrated an increased excitation of certain mode shapes with increasing axial gap. This paper investigates how the forced response of different mode shapes is affected by operating-point-related and design-related changes of the excitation source. Extensive time-linearised simulations are conducted at various off-design operating points with different mode shapes. Fourier-transformed gust boundary conditions are considered as the excitation source. The reduced frequency is identified as the determining factor for the sensitivity of the aerodynamic work with respect to axial-gap variations. Three reduced-frequency regimes are identified: moderate sensitivity for low reduced frequencies, low sensitivity for moderate reduced frequencies, and high sensitivity for high reduced frequencies. The cases with an inverse behaviour of the aerodynamic work with respect to the axial-gap variation are shown to occur in the low-sensitivity regime. It is demonstrated that they are a result of changes in the interaction of the incoming wakes with the rotor potential field.