A microjet arrangement comprising both penetration ͑or immersion͒ and convergence ͑jets oriented such that two jets of a pair interact with one another͒ 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 310 000, 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 Ͻ ͑x / D͒ Ͻ 1, where the dynamics are dominated by the fluidevron-main-jet interaction, and the region ͑x / D͒ Ͼ 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 0.2Ͻ ͑x / D͒ Ͻ 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 problem͒. Considerable reductions are shown between the base line and controlled flows in terms of these energy-weighted space scales.
With the development of so-called 'electric' aircraft, the integration of jet pumps to various systems of the engines could offer significant gains (mass, reliability,...) over the technologies currently used. Since jet pumps may directly contribute to ramp noise, an accurate predictive tool is therefore desired to propose adapted noise reduction solutions in an industrial context. A direct computation of the unsteady turbulent flow being too expensive, the main idea is to compute acoustic sources from a stochastic velocity field and to inject it in Euler's equations to model near-field acoustic propagation. A Kirchoff analogy is then used to reduce calculation cost in the far field. In following previous works by Bailly and Juvé (AIAA Paper 99-1872) and Billson et al. (AIAA paper 2003-3282), a combined approach to generate a stochastic velocity field is presented and validated in the present work. This methodology is based on the sweeping hypothesis-or the fact that small scale turbulent structures are advected by energy containing eddies. The validation study is done on aerodynamic quantities of a cold free jet at Mach number M=0.72. The capability of the method to reproduce space-time velocity correlations in the shear layer is shown. The model is then tested on a cold free jet configuration to predict radiated acoustic levels using a volumic Lighthill solver.
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