Rotor/stator interaction noise prediction in C-shaped duct with lifting surface method

To comply with the rigorous constraints imposed on nacelle length in Ultra-High Bypass Ratio (UHBR) engines, bifurcations that were conventionally positioned downstream of the fan stage are integrated into the outlet guide vanes (OGVs). Consequently, the rearward fan noise is strongly affected by the bifurcations, as the azimuthal uniformity of OGV is disrupted and the aft-fan duct is partitioned into two C-shaped ducts. This impact is investigated using lifting surface method, assuming that the duct is infinitely long with a constant C-shaped cross-section, and the flow is uniform neglecting the distortion caused by bifurcations. Through the incorporation of a tailored Green's function, the bifurcations are modeled as sound reflection boundaries. Subsequently, the unsteady loading on the vanes and the rearward sound power are calculated and compared against those in the annular duct. Analysis reveals that multiple azimuthal modes are scattered, resulting in a hump-shaped mode distribution. The radial mode distribution is found to be intimately related to the radial obliquity of rotor wake. The unsteady loading on different vanes shows diverse yet similar distribution after reducing the difference in the inter-blade phase angle (IBPA). These findings should contribute to the noise prediction and acoustic design of UHBR fan featured by integrated OGV and C-shaped aft-fan duct.