Vorticity and Turbulence Properties of Microjet Arrays for Active Flow Control

Jet to crossflow velocity ratio and lateral jet-to-jet spacing are two important parameters which have a direct impact on separation control efficacy for fluidic jet in crossflow actuators (JICF). The influence these parameters have on the control of a separated flow field have been examined in this study. Two component Particle Image Velocimetry (PIV) and three component Stereo PIV techniques have been used to measure the global streamwise velocity field and cross plane velocity fields near the microjet injection location, respectively. Three microjet array configurations were evaluated; 6.25, 12.5, and 25 diameter jet-to-jet spacings. Jet velocity to crossflow velocity ratios of 2, 5, and 10 were examined at each array configuration. A highly adverse pressure gradient imposed on a flat plate provided a suitable flow field for examining actuator effectiveness (Reynolds number = 46,400 (1.47 m/s)). The smallest jet-to-jet spacing of 6.25d is characterized by high levels of Turbulent Kinetic Energy (TKE) just downstream of injection accompanied by a rapid loss of coherent streamwise vortices. As the jet-to-jet spacing increases, mutual vortex interaction decreases and coherent streamwise vortices can be identified further downstream of injection. A drop in TKE levels is also seen as jet spacing increases due to decreased jet-to-jet interaction. Jet to velocity ratios of 2 and 10 are effective in drastically reducing or eliminating the separation bubble while a velocity ratio of 5 is the least effective. Two different mechanisms for boundary layer re-energization are seen; momentum transfer from large scale counter-rotating vortices (CVPs) and momentum flux through turbulent transport. Although both of these mechanisms are present for all blowing configurations, one is more dominant depending on the spatial actuator configuration or jet to crossflow velocity ratio.