Nozzle-Exit Boundary-Layer Effects on a Controlled Supersonic Jet

摘要

The effect of nozzle-exit boundary-layer thickness on the evolution of a round Mach 1.3, Reynolds number 1x106 jet is examined without and with control. For the latter, the flapping mode is considered at the preferred column mode frequency (Strouhal number=0.3). The boundary-layer thickness is varied from a very small value to 25% of the diameter. For the no-control cases, 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 by other researchers at Mach 0.9. However, the subsequent growth toward the centerline is faster for the thicker boundary layers. With flapping mode control, increasing the thickness of the boundary layer has different effects on the flapping and nonflapping planes. The rapid spreading of the jet observed on the flapping plane with thin boundary layers is greatly diminished as the nozzle-exit layer is thickened. Conversely, the rate of spreading on the nonflapping plane is increased. Likewise, the characteristic vortical rings observed with thin layers in experiment and simulations become less prominent with thicker layers, indicating reduced control authority. Thus, increasing the boundary-layer thickness reduces the differences between controlled and uncontrolled cases.