Control of supersonic impinging jets using microjets

Supersonic impinging jet flows are dominated by the presence of a feedback loop between the flow and acoustic fields, which leads to many adverse phenomena such as highly unsteady loads on the nearby surfaces, a drastic increase of noise level, and a dramatic lift loss in hover. It has been demonstrated by Alvi et al. (2000, 2003) and Shih et al. (2001) that arrays of microjets near the nozzle exit can be used to disrupt the feedback loop, thus reducing these undesired effects. However, the effectiveness of the control was found to be strongly dependent on the ground plane distances and jet operating conditions.; Current research was carried out at the same facility used by Alvi et al. (2000, 2003) and Shih et al. (2001). However, this investigation focuses on understanding the physical mechanism behind microjet control, and, as a result, devising optimum control strategies. For this purpose, a comprehensive parametric study was carried out. The test matrix chosen included microjet operating pressure, microjet angle, microjet space, the use of micro-tabs instead of microjets and the spatial distribution of microjets relative to the main jet. Marked improvement in the reduction of flow unsteadiness and related adverse effects was achieved in the current study.; Planar and three-dimensional velocity field measurements were made using Particle Image Velocimetry (PIV). The results obtained from these detailed velocity field measurements were found to be consistent with acoustic data and were further examined to explore the physical mechanisms behind microjet control. The velocity/vorticity measurements clearly reveal that the activation of microjets introduces strong streamwise vorticity in the form of well-organized, counter-rotating pairs while concomitantly significantly reducing azimuthal vorticity. Such experimental evidence suggests that the generation of these streamwise vortices is the result of the vorticity tilting and stretching mechanisms initiated when the microjets interact with primary shear layer instabilities. The emergence of these longitudinal structures weakens the large-scale axisymmetric structures in the jet shear layer while also introducing stronger three-dimensionality into the flow. Both these factors lead to a weakening of the feedback loop and accounts for the reduction of flow unsteadiness. These results also show a significant reduction in the near-field turbulent intensities with the activation of microjets, which is consistent with the reduction of the near-field OASPL.; Remarkably, all these effects are achieved by using a mass flow rate less than 0.5% of the primary jet mass flux.