Jet mixing enhancement by high amplitude pulse-fluidic actuation

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 the increased dispersion of the fluid that makes up a flow. For a jet this means increased entrainment of surrounding fluid and more rapid spreading over the unforced 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. The last of these benefits is the focus of the current work.; 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 a full scale aircraft.; The small scale clean jet experiments show that mixing enhancement compared to the unforced case is likely due to a combination of mechanisms. The first mechanism is the growth of shear layer instabilities, similar to what occurs with an acoustically excited jet except that in this case the forcing is highly nonlinear. The result of the instability is a frequency "bucket" with an optimal forcing frequency. The second mechanism is the generation of counter rotating vortex pairs similar to those generated by mechanical tabs.; In this case the penetration depth determines the extent to which this mechanism acts. This mechanism is therefore more important as the pulsing amplitude is increased. Finally, for a jet that is being forced in the antisymmetric mode the alternate back and forth vectoring of the jet in the near field coupled with the other mixing mechanisms plays a small role as well. However, since actuator to jet momentum ratios are on the order of 1%, the effect is minimal. The key mixing parameters were found to be the actuator to jet momentum ratio (amplitude) and the pulsing frequency, where the optimal frequency depends on the amplitude. The importance of phase, offset, duty cycle, and geometric configuration were also explored.; The experiments on the jet engine and full scale simulated core nozzle demonstrated that pulse fluidic mixing enhancement was effective on realistic flows. The same parameters that were important for the cleaner small scale experiments were found to be important for the more realistic cases as well. This suggests that the same mixing mechanisms are at work. Additional work was done to optimize, in real time, mixing on the small jet engine using an evolution strategy.