Subsonic jet noise reduction by fluidic control: The interaction region and the global effect

A microjet arrangement comprising both penetration (or immersion) and convergence (jets oriented such that two jets of a pair interact with one another) is used to control a subsonic turbulent jet with a view to noise reduction. The acoustic effect of the so-called fluidevron system is comparable to chevrons and nonconverging microjets as far as the noise reduction is concerned. Detailed experimental measurements are performed for a main jet with Mach and Reynolds numbers of 0.3 and $310000$⁠, respectively. A direct numerical simulation study is performed for a model, plane mixing-layer problem using the immersed-boundary method, in order to help understand the topological features of the fluidevron–mixing-layer interaction. In terms of modifications produced in the flow, two relatively distinct regions are identified: the near-nozzle region, $0<(x/D)<1$⁠, where the dynamics are dominated by the fluidevron–main-jet interaction, and the region $(x/D)>1$⁠, where the jet recovers many of the uncontrolled-jet flow characteristics, but with globally reduced turbulence levels and a longer potential core. The flow structure produced in the near-nozzle region is found to comprise an ejection of fluid from the main jet; the ejection process leads to very high fluctuation levels. This highly turbulent fluid, on being reassimilated by the mixing-layer downstream of the interaction point, has a spectacular local impact on turbulent kinetic energy production and on the entrainment: the former is reduced by 70%, and the latter boosted by 30% over the range $0.2<(x/D)<3$⁠. The impact of the flow control on the integral scales of the turbulence is assessed, as these are central to acoustic-analogy-based source models. A significant reduction is found in the radial integral scales, and these are then weighted by the local fluctuation energy in order to assess the impact of the control on the source mechanisms of the flow (considered in the context of Lighthill’s formulation of the problem). Considerable reductions are shown between the base line and controlled flows in terms of these energy-weighted space scales.