Experimental studies on steady microjet arrays in supersonic crossflow

Effective flow control actuators for high speed flows are essential for improved performance of military and civilian aircrafts. Passive, active, and hybrid flow control techniques have been employed to attenuate some of the adverse effects inherently associated with supersonic flows. One such technique involves the using of secondary small scale jets, as a means of flow control. This dissertation explores the concept of using microjet arrays to generate oblique shocks of varying strength, the basic characterization of the microjet-generated shocks, and implementation of these arrays in a canonical shock wave - boundary layer interaction (SBLI) generated by a compression ramp in supersonic crossflow.;Jets in supersonic crossflow are known to produce a three-dimensional bow-shock structure due to the blockage of the flow. Multiple jets in a linear array interact with both one another and the incoming supersonic flow. A parametric study was carried out to analyze the effect of microjet injection in a supersonic crossflow. The microjet orifice diameter and spacing within an array was varied and their impact on the three-dimensional flow field was examined in detail. Spanwise linear arrays of high-momentum microjets are used to generate either single or multiple oblique shocks in a supersonic crossflow. The shocks generated using microjets can be tailored to be either parallel, or coalescing depending upon the application. The flow field was studied using shadowgraph, density field measurements, and velocity field measurements. The results obtained indicate that the microjet-generated shock angle increases linearly with the momentum coefficient for the range of tested conditions. The strength of the microjet-generated shocks in terms of the density or velocity variations across the shock is found to be constant along the inviscid portion of the shock. Properties of microjet-generated shocks were compared with a ramp shock. The results demonstrated that the behavior of microjet-generated shocks is very similar to the ramp shock and microjet-generated shocks provide an equivalent turning of the flow as compared to a physical ramp.;Jet injection in supersonic crossflow is known to create a pair of counter-rotating vortex pairs (CVPs) and generate streamwise vorticity. The streamwise CVPs generated by the each microjet in an array remain coherent until about 20 diameters from the point of injection for arrays with larger spanwise separation distances between the micro-orifices. However, as the center-to-center spacing is reduced the CVPs interact with one another and dissipate rapidly leading to reduced vorticity generation. The flow structure for the array with reduced center-to-center spacing approaches to that of a two-dimensional shock oblique shock. A simplified analysis of the vorticity transport in the flow field showed that the dominating mechanisms of vorticity generation/transport are expansion and vortex stretching. The contribution due to the baroclinic torque less than the expansion effects. A conceptual model for the interaction of the microjets within an array and with the incoming crossflow is presented.;Understanding of the basic properties of the microjet array interaction with the supersonic crossflow is useful in understanding the fundamentals of this ubiquitous flow which can be also used for designing microjet-based actuators for various flows. Such a high-momentum microjet array actuator is used upstream of an unswept 24 deg. compression corner to control the shock wave-boundary layer interaction in a Mach 2 flow. The wall pressure fluctuations in the interaction region are reduced by approximately 50% and the flow near the compression corner appears to be energized with control, based on the unsteady surface pressure measurements. The pressure spectra show that microjet control results in a redistribution of energy on the wall and the ramp surfaces.