Application of the Modal Approach for Prediction of Forced Response Amplitudes for Fan Blades

[+] Author and Article Information
Franziska Eichner

Institute for Aeroelasticity, DLR, German Aerospace Center, Bunsenstr. 10, 37073 Göttingen, Germany

Joachim Belz

Institute for Aeroelasticity, DLR, German Aerospace Center, Bunsenstr. 10, 37073 Göttingen, Germany

1Corresponding author.

ASME doi:10.1115/1.4041453 History: Received June 26, 2018; Revised August 21, 2018


Forced response is the main reason for high cycle fatigue in turbomachinery. Not all resonance points in the operating range can be avoided especially for low order excitation. For highly flexible CFRP fans an accurate calculation of vibration amplitudes is required. Forced response analyses were performed for blade row interaction and boundary layer ingestion. The resonance points considered were identified in the Campbell diagram. Forced response amplitudes were calculated using a modal approach and results are compared to the widely used energy method. For the unsteady simulations a time-linearised URANS method was applied. If only the resonant mode was considered the forced response amplitude from the modal approach was confirmed with the energy method. Thereby forced response due to BLI showed higher vibration amplitudes than for blade row interaction. The impact of modes which are not in resonant to the total deformation were investigated by using the modal approach, which so far, only considers one excitation order. A doubling of vibrational amplitude was shown in the case of blade row interaction for higher rotational speeds. The first and third modeshape as well as modes with similar natural frequencies were identified as critical cases. The behaviour in the vicinity of resonance shows high vibration amplitudes over a larger frequency range. This is also valid for high modes with many nodal diameters, which have a greater risk of critical strain.

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