58849 - Internal Aerodynamic Performance Evaluation of a Double Entrance S-Duct Intake at High Subsonic Conditions 

Intakes, or inlets, are an essential element of aircraft propulsion systems. Aircraft with fuselage-embedded engines require intake ducts with bends to direct oncoming air into the engine. The design of an intake varies from one aircraft to another depending upon the engine location, airframe configuration, observability demands and flight speed. Intake ducts, because of geometric and aerodynamic constraints of aircraft structure, may have multiple bends, wall curvatures, cross-section transitions, area diffusion and certain length.  Consequently, they typically experience flow separation, losses, total pressure distortion, and swirling flow near their exits. All are detrimental to the performance of an aircraft engine. Distortion and swirl impose additional unsteady loading on the front stages of compression systems and reduce engine operational capacity and stability margins. In some aircraft, double-entrance ducts are used to meet geometric constraints and maintain the required airflow. Performance losses for simple S-duct diffusers are further aggravated by double entrance and its ensuing curvatures in multiple planes. The present paper experimentally investigates aerodynamic performance of a bifurcated Y-duct with S-bends in both horizontal and vertical planes. The test article was configured with two identical trapezoidal entrances with low profile and frontal area. The two intake ducts merged downstream and transitioned to a circular exit, a configuration typical of single-engine air vehicles with two side intakes. The important geometric parameters were an exit-to-entrance area ratio of 1.64. The intake duct was 3D printed in high-density plastic. The performance of the intake was evaluated at inlet Ma = 0.63 by measuring the surface static pressure along the four streamwise rows of pressure taps at the top, bottom, and two sides of the intake and total pressure and velocity vector using 5-hole probe across the exit plane of the intake duct, the aerodynamic interface plane (AIP).  Static pressure recovery was evaluated along the convoluted duct diffuser and total pressure loss was measured across the cross-section at the AIP. The total pressure distribution at the AIP was further used to determine the radial and circumferential distortion coefficients and the velocity vector was used to obtain the swirl-angle distribution and average swirl index at the exit plane. This work will promote improved design techniques for double-entrance S-duct intakes and will be useful for predicting the performance of aero-engines and air vehicles at high-subsonic flight conditions.