engleski

Applications of Harmonic Balance Method in Periodic Flows

The Harmonic Balance method for periodic flows is presented in this work. The Harmonic Balance method deploys Fourier series expansion based on the assumption that variables are periodic and can be accurately represented by a mean value and first n harmonics. This procedure yields a number of coupled steady state equations, replacing the single transient equation. The number of equations depends on the number of harmonics chosen: for n harmonics 2n+1 equations are solved. The coupling between equations is achieved through source terms which account for the time derivative term in transient equation. This paper deals with two distinct applications of the Harmonic Balance method: turbomachinery and waves on free surface. The turbomachinery test case presented is ERCOFTAC centrifugal pump consisting of rotor and stator. Due to presence of different frequencies in rotor and stator, the multiple frequency approach is used. Harmonic Balance method is compared against conventional transient solver, comparing efficiency, head and torque in several time instants. Pressure contours on rotor blade are compared to present the local accuracy of the method. The second test case deals with regular surface wave propagation with current. For the mildly steep wave imposed, the validation is performed comparing the surface elevation at the middle of domain. The Harmonic Balance surface elevation and transient surface elevation are compared in frequency domain using mean value and first harmonic. The method is implemented in foam-extend, a community driven fork of the open source CFD software, OpenFOAM.

harmonic balance method; periodic flows; openFoam; fourier expansion; turbomachinery

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