The impact of inlet boundary layer thickness on the unsteady aerodynamics of S-duct intakes

The need to reduce aero-engine emissions and direct operating costsis driving the civil aerospace sectortowards considering more integrated propulsion systems. Many of the proposed novel aircraft architectures employ convoluted intakes for either the aero-engine or propulsion system. These intakes are characterized by unsteady distortion that can hinder the performance and operability of the propulsion system. This work assessesthe impact of the inlet boundary layer on the unsteady aerodynamics of an S-duct intake using time-resolved particle image velocimetry at the aerodynamic interface plane.An increase in the boundary layer thickness at the intake inlet increasesthe flow unsteadiness on the swirl angle by up to 40% relativeto the baseline case. The azimuthal orientation of the inlet boundary layer modifies the intensity and topology of the most frequent swirl distortion pattern. For a relatively thick inlet boundary layer, the reduction of the dominant frequencies associated withthe unsteady swirl angle is postulated to be beneficial for the engine stability. Overall, this works gives guidelines for the integration between the intake and the engine across the range of potential inlet operating conditions.