Numerical study on Multiple Pure Tones in a thin annular duct with impact of inflow distortion

Abstract

The next generation of ultra high bypass ratio (UHBR) turbofan engines may have to face new acoustic challenges related to inflow distortions caused by the short nacelle geometry. During take-off and cut-back conditions the flow regime at the fan blades may reach supersonic conditions. Thus a shock pattern is developing on the rotor blades and propagates upstream through the engine inlet and interacts with the mean inflow distortion. This paper presents numerical investigations performed with the DLR's computational Fluid dynamics (CFD) solver TRACE using a simplified quasi-3D approach. First, the applicability of the approach for the considered problem is shown. Afterwards the influence of the inflow distortion on a shock pattern, as it would be created by ideally equal blades is investigated by a Harmonic Balance (HB) method computation in the frequency domain. Real fans often show small variabilities in stagger angles due to manufacturing process and a specific blade ordering may be chosen. A stagger blade variation following a double cosine is addressed to investigate the effect of this specific blade patterning. For equal blades rearrangement, the obtained results reveal the expected scattering into the two adjacent modes caused by the inflow distortion of azimuthal mode order m0 = 1. In addition it is shown that the new modes are subject to further scattering. For non-identical staggered rotor blades it is shown, that the pressure variation associated to the double cosine structure clearly dominates the inlet region. Far upstream of the rotor blade leading edge the modal spectrum is dominated by modes of order m < B.