Numerical simulation of transonic fan under circumferential inlet distortion using harmonic balance method

Simulating the unsteady flow of transonic fan under circumferential inlet distortion using conventional dual time stepping method would need multiple blade passages or the whole annulus in order to apply the direct periodic condition. Also, the initial transients of the solution cost most of the computation time to decay until it reaches a periodic steady state. This makes unsteady solutions extremely time consuming and hampers its application in fan blading design process. A fast calculation method is needed to estimate the impact of the circumferential inlet distortion on the fan aerodynamic performance. Therefore, the computation feasibility and the computation speed for the unsteady calculation of the fan under circumferential inlet distortion using harmonic balance method which adopts the phase shift condition were studied. Both the sine wave form and the square wave form inlet total pressure distortion were considered. For the sine wave form inlet distortion, only a small numbers of harmonics were needed to obtain comparable results to the dual time stepping method and a maximum factor of 48 speed-up could be achieved. As for the square wave form inlet distortion, more harmonics should be adopted to get accurate results and only a factor of 7 speed-up was achieved. The results demonstrate that the harmonic balance method can apply to the unsteady calculation of transonic fan under circumferential inlet distortion effectively and reduce the computation time significantly.