Boundary Layer Control and Wall-Pressure Fluctuations in a Serpentine Inlet

In this thesis, the benefits of boundary layer control (BLC) in improving aerodynamic performance and engine stability were examined in a compact, serpentine inlet exhibiting flow separation. A 1/14-scale turbofan engine simulator provided the flow through the inlet. The inlet's mass flow was measured to be 759 scfm (0.939 lbm/s) with an average throat Mach number of 0.23 when the simulator speed was 40 krpm. Boundary layer suction, blowing, and their combination were used to minimize the inlet's flow separation. The effectiveness of the suction alone and the blowing alone was shown to be approximately equivalent, and the effectiveness of the combined use of both was seen to be better than either one by itself. With blowing and suction flowrates around 1% of the simulator's core flow, the inlet's distortion was lowered by 40.5% (from 1.55% to 0.922%) while the pressure recovery was raised by 9.7% (from 87.2% to 95.6%). With its reduction in distortion, BLC was shown to allow the simulator to steadily operate in a range that would have otherwise been unstable. Minimizing the flow separation within the inlet was shown to directly relate to measurements from flush-mounted microphones along the inlet wall: as the exit distortion decreased the microphone spectrum also decreased in magnitude. The strong relationship between the aerodynamic profiles and the microphone signal suggests that microphones may be used in an active flow control scheme, where the BLC effort can be tailored for different engine operating conditions. Unfortunately, the sensing scheme used in this experiment showed the microphone signal to continue to decrease even when the separation is overly compensated; therefore refinements must be made before it would be practical in a real application.