Use of High-Speed Microjets for Active Separation Control in Diffusers

Abstract

Inlets to aircraft propulsion systems must supply flow to the compressor with minimal pressure loss, flow distortion, or unsteadiness. Flow separation in internal flows such as inlets and ducts in aircraft propulsion systems and external flows such as over aircraft wings is undesirable because it reduces the overall system performance. An experimental investigation is described that was carried out to study the feasibility of using high-speed microjets, supersonic for most cases, to control boundary-layer separation in an adverse pressure gradient. The geometry used is a simple diverging Stratford ramp equipped with arrays of 400-μm-diam microjets. Measurements include detailed surface flow visualizations, mean surface pressure distributions, and velocity field measurements using particle image velocimetry. The results clearly indicate that by activating these microjets the separated flow regions were eliminated. This led to a significant increase in the momentum of the flow near the surface where the gain in momentum was at least an order of magnitude higher than the momentum injected by the microjets. Given the simplicity of the system and its low mass flow requirements, combined with the benefits achieved by this approach, microjets appear to be promising actuators for efficient separation control for internal and external flow applications.