Controlling the development of coherent structures in high speed jets and the resultant near field

This work uses Large-Eddy Simulations to examine the effect of actuator parameters and jet exit properties on the evolution of coherent structures and their impact on the near-acoustic field without and with control. For the controlled cases, Localized Arc Filament Plasma Actuators (LAFPAs) are considered, and modeled with a simple heating approach that successfully reproduces the main observations and trends of experiments.;A parametric study is first conducted, using the flapping mode (m = +/-1), to investigate the sensitivity of the results to various actuator parameters including: actuator model temperature, actuator duty cycle, and excitation frequency. It is shown by considering a Mach 1.3 jet at Reynolds number of 1 x 106 that the response of the jet is relatively insensitive to actuator model temperature within the limits of the experimentally measured temperature values. Furthermore, duty cycles in the range of 20%--90% were observed to be effective in reproducing the characteristic coherent structures of the flapping mode.;Next, jet flow parameters were explored to determine the control authority under different operating conditions. To begin, the effect of the laminar nozzle exit boundary layer thickness was examined by varying its value from essentially uniform flow to 25% of the diameter. In the absence of control, the distance between the nozzle lip and the initial appearance of breakdown is proportional to the boundary-layer thickness, which is consistent with theory and previous results obtained by other researchers at Mach 0.9.;The second flow parameter studied was the effect of Reynolds number on a Mach 1.3 jet controlled by the flapping mode at an excitation Strouhal number of 0.3. The higher Reynolds number (Re=1,100,000) jet exhibited reduced control authority compared to the Re=100,000 jet. Like the effect of increasing the nozzle exit boundary layer thickness, increasing the Reynolds number cause a reduction in spreading on the flapping plane and an increase on the non-flapping plane. Therefore, these thicker layers and higher Reynolds number jets may require actuators with a higher energy input (i.e. higher duty cycle, higher actuator temperature, more actuators) to ensure the excitation of the flow instability.;The final parameter studied is the effect of Mach number on the development and decay of large scale structures for no-control and control cases for Mach 0.9 and Mach 1.3 jets. For this exercise, the axisymmetric mode (m=0) was considered at excitation frequencies of St=0.05, 0.15, and 0.25, with emphasis on the evolution of coherent structures and their effects on the resultant near field pressure map. Without control, the two jets have similar shear layer growth until the end of the potential core length of the subsonic case, at which point the subsonic jet spreads at a higher rate. For the controlled cases, relatively larger streamwise hairpin vortices have been noted for the subsonic cases than the supersonic cases resulting in stronger entrainment of the ambient fluid. This increased entrainment in the subsonic cases causes a reduction in the normalized convective velocity resulting in similar normalized values to that of the supersonic cases. As the excitation frequency is increased, more hairpin vortices are present and the normalized convective velocity is reduced for both subsonic and supersonic cases. (Abstract shortened by ProQuest.).