A system of 36 impinging microjets was implemented on a Mach=0.9 round jet, and the noise reduction was studied as a function of the microinjection mass flux, the number of microjets blowing, the layout of the blowing microjets, and the microjet diameter. Depending on the microinjection flow parameters, the global jet-noise reduction varied from 0 to 1.8 dB, showing some non-monotonic behaviors due to the change between subsonic and supersonic regimes of the microjets. The study of the layout of the microjets shows that the noise reduction decreases when the microjets are too close to each other and that some configurations of microjet pairs could be favourable, which can be linked to the flow structures induced by the microjets. Spectral analyses pointed out different mechanisms involved in the control, with a highfrequency regeneration for high injections, a local behavior of the control at the mid-frequencies and a global behavior at the low-frequencies.

This experimental study investigates the control of flow in a short diffuser with a 2 x 45 degrees divergence angle, using wall synthetic jets. Measurements are made by particle image velocimetry. Velocity profiles, velocity fields, and Vorticity maps show that the flow, initially separated as a free jet, undergoes a global excitation which creates periodic oscillating structures producing large fluid motions in the vicinity of the wall. This results in an increased mixing of the primary separated jet with the surrounding fluid. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.

Wake visualizations and wall pressure measurements have been conducted on a 2-D circular cylinder submitted to a zero-mass-flow oscillating jet, at Reynolds numbers about 10. Jets with relative momentum flux C^ of the order of 10' are efficient for a large range of azimuthal positions (up to 120°), both on natural laminar separation and tripped transitioned separation. Jets force the boundary layer transition, deviate the separating structures towards the wall, and, as a result, generate appreciable lift force. The drag is modified in a more complex way.

We present in this paper an active anechoidal termination composed of a porous material, whose absorbent properties are improved by a secondary source. A simple model of acoustic propagation in a porous medium leads to optimal absorption criteria, which are verified by absorption coefficient measurements under normal incidence at the end of a Kundt duct. A secondary source is placed at the backward interface of the material, and imposes zero pressure by destructive interferences, which is a necessary condition for the maximum absorption. Several control set-ups are tested and provide the anechoism for broadband excitations.

The authors consider a characterization method and an adaptive time algorithm used for the evaluation and optimization, respectively, of the required IIR (infinite impulse response) filter coefficients in an active noise control system. A broadband noise attenuation of up to 20 dB was experimentally achieved by these methods, using a controller consisting of an ADSP 2100 digital signal processor. The approach leads to integration of the feedback control within the controller.< >