Unsteady separation point injection for pressure recovery improvement in high subsonic diffusers

Abstract

Serpentine inlet ducts on modem tactical aircraft distort the inlet flow and decrease pressure recovery at the aerodynamic interface plane (AIP). Current inlet designs are more aggressive, increasing distortion and decreasing pressure recovery at the AIP. Often the flow separates from the wall of the diffuser, creating most of the distortion and pressure loss in the inlet. Diffuser separation experiments were conducted at high subsonic cruise conditions in a 2D test section. Periodic injection tangential to the flow at the separation point improved downstream pressure recovery. The injection also increased static pressure measured at the test section walls in the separated region. Flow visualization tests indicated that the separation shrinks as the injection mass flow increases. Pressure recovery also increased as injection mass flow increased. The unsteady component of the injection flow remained constant with injection mass flow, indicating that the steady component of the injection enhanced control of the separation. The preliminary conclusion is that the average velocity of the injection flow should be at least equivalent to the velocity of the core flow to maximize pressure recovery. Experiments were also conducted in a one-sixth scale tactical aircraft diffuser at cruise conditions (3.1 lb/sec, maximum M = 0.65). Periodic injection at the separation point improved the pressure recovery at the AIP. The improvement in pressure recovery at the AIP was limited to the area of pressure loss due to the separation in the diffuser. The diffuser has strong secondary flows that also cause losses at the AIP. These secondary flows prevented the injection from restoring pressure recovery as well as it had in the 2D test section. Higher injection mass flows than in the 2D case were required to achieve the same degree of improvement in pressure recovery at the AIP.