Research Papers

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,
Göttingen 37073, Germany
e-mail: Franziska.Eichner@dlr.de

Joachim Belz

Institute for Aeroelasticity DLR,
German Aerospace Center,
Bunsenstr. 10,
Göttingen 37073, Germany
e-mail: Joachim.Belz@dlr.de

1Corresponding author.

Manuscript received June 26, 2018; final manuscript received August 21, 2018; published online October 16, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(3), 031019 (Oct 16, 2018) (8 pages) Paper No: GTP-18-1360; 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 carbon fiber reinforced polymer (CFRP) fans, an accurate calculation of vibration amplitudes is required. Forced response analyses were performed for blade row interaction and boundary layer ingestion (BLI). The resonance points considered were identified in the Campbell diagram. Forced response amplitudes were calculated using a modal approach and the results are compared to the widely used energy method. For the unsteady simulations, a time-based linearization of the unsteady Reynolds average Navier–Stokes equations were 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 modeshapes as well as modes with similar natural frequencies were identified as critical cases. The behavior 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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Fig. 1

Counter-rotating integrated shrouded propfan—CRISP2

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Fig. 2

Forced response methodology: modal approach

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Fig. 3

Forced response methodology: energy method

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Fig. 4

Forced response amplitude through energy method

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Fig. 5

Generic BLI: (a) flow-Condition at Inlet and (b) fourier decomposition of BLI

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Fig. 6

Computational fluid dynamics mesh

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Fig. 7

Campbell diagram of the first rotor

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Fig. 8

Deformation due to blade row interaction: (a) 82% rotational speed, (b) 88% rotational speed, (c) 92% rotational speed, and (d) 102% rotational speed

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Fig. 9

Blade row interaction: (a) aerodynamic excitation and (b) aerodynamic damping

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Fig. 10

Deformation due to BLI: (a) 50% rotational speed, EO 02; (b) 52% rotational speed, EO 08; (c) 55% rotational speed, EO 05; and (d) 75% rotational speed, EO 04

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Fig. 11

Boundary layer ingestion: (a) aerodynamic excitation and (b) aerodynamic damping



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