SUMMARY

Turbulent mixing enhancement has received a great deal of attention in the fluid mechanics community in the last few decades. Generally speaking, mixing enhancement involves exciting natural instabilities in the shear layer resulting in increased entrainment of surrounding fluid and more rapid mixing compared to the unexcited case. There are many applications where mixing enhancement is desirable and among aerospace technologies one of the most obvious choices is jet engines. Benefits of implementing mixing enhancement on a jet engine include: improved combustion efficiency, reduced noise, and reduced plume temperature resulting in lower heat signature and thermal loading on parts. The last of these benefits is the focus of the current work. Techniques for implementing jet mixing enhancement are typically classified as either active or passive. Examples of passive techniques include non-circular nozzles and mechanical tabs mounted at the nozzle exit. Most of the early studies on active enhancement used acoustic actuators because they are highly controllable and well suited to laboratory scale experiments with clean jets. Unfortunately these actuators often do not work well with the higher Reynolds number turbulent flows typically encountered in industry since the turbulent fluctuations overwhelm the small amplitude perturbations. In more recent years some researchers have begun to study pulse-fluidic actuators, which have shown great promise in delivering effective mixing enhancement for full scale realistic jet flows. The current work focuses on mixing enhancement of an axisymmetric jet via high amplitude fluidic pulses applied at the nozzle exit. The work consists of small scale “clean jet” experiments, small scale micro-turbine engine experiments, and full scale laboratory simulated core exhaust experiments using actuators designed to fit within the engine nacelle of the C-17 cargo aircraft. The objectives of the small scale experiments were to discover the critical pulse-fluidic jet mixing parameters and to understand the physical mechanisms. The other experiments demonstrate the effectiveness of pulse-fluidic jet mixing on more realistic flows and compare the results with the more fundamental small scale results. Additional work was done to optimize, in real time, mixing on a micro-turbine using an evolution strategy.