Title: Sources of unsteady flow in subsonic aircraft inlets
Abstract: The paper presents the results of a study of turbulent flow typically found in subsonic inlets. Space-time correlations measured at the inlet-engine interface plane were used to represent the structure of the turbulent flow. The results have application in the analysis of rotor noise and performance. It was observed that turbulence which originated outside and was convected through the inlet had elongated shapes with stream correlation lengths much longer than those in transverse directions. However, when inlet originated turbulence was measured, the lengths were about the same in each direction. Thus, the contraction effect on the flow through the inlet strongly affects the shape of the turbulent eddies. The basis for the observations were high-frequency response total pressure transducers mounted on rakes in the inlet near the fan plane. The inlets used were representative of current technology in subsonic aircraft. 15-in.-diameter models were used in a 9 × 9-ft wind tunnel. Flow through the inlets was established using a downstream engine. Mach numbers studied were representative of the full-scale inlet. Conditions represented in the results are static, crosswind, and angle of attack.