Existence of large scale unsteady flow structures manifested in contra-rotating vortex pairs has been previously identified in rotor disc cavities. The non-axisymmetric nature with a unknown number of vortices presents a computational challenge as a full 360° circumferential domain will be needed, requiring significant computational resources. A novel circumferential spatial Fourier spectral technique is adopted in the present work to facilitate efficient computational predictions of the non-axisymmetric flows. Given that the flow non-uniformities in the circumferential direction are of large length scales, only a few circumferential Fourier harmonics would be needed, resulting in a drastic reduction in number of circumferential mesh points to be required. The modeling formulations and implementation aspects will be described. Computational examples will be presented to demonstrate the validity and effectiveness of the present modeling approach. The computational results show that the non-axisymmetric flow patterns in terms of the number of vortex pairs are sensitive to small scale external disturbances. It is also indicated that the occurrence of a non-axisymmetric flow might be captured by the present Fourier solution with even one harmonic.

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