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Experimental results are presented on the active control of a backward-facing step flow revealing some features observed in combustion chambers. These experiments were performed under nonreactive conditions, and the effect of the pressure fluctuations induced by the unsteady heat release was simulated by an external acoustic excitation produced by two loudspeakers placed far downstream from the step. This excitation signal was delivered by a generator or by a hot-wire probe placed in the shear layer. The latter configuration reproduces some aspects of the actual coupling that have been observed with combustion instabilities. This preexcited flow was then controlled using six synthetic jets located on the vertical side of the step. The control loop uses an recursive least-mean-square autoadaptive algorithm applied to a signal from either a microphone or a hot-wire probe placed downstream of the step edge. The influence of the external acoustic excitation on the flow organization and the effect of the control loop are described. Without excitation, the control algorithm is not able to reduce the natural unsteady phenomena. However, it does damp the shear-layer pressure or velocity fluctuations intensified by an external excitation and bring the flow close to its natural feature. The efficiency of the control depends on the frequency of the external excitation, the signal used in the control loop, and the phase shift between the velocity and the pressure signals in the shear layer. NomenclatureA e = input voltage for the external excitation, V Fr = efficiency parameter f d = natural roll-up frequency of the shear layer, Hz f c = external excitation frequency, Hz G energy = gain parameter H m− f ( f ) = transfer function between pressure and velocity signals, Pa · s/m h = step height, m P excitation ( f ) = autospectrum of the pressure signal measured with the external excitation, Pa P total ( f ) = autospectrum of the pressure signal measured with the flow submitted to an external excitation, Pa RMS 1 = rms of the velocity or pressure signal measured without flow, Pa or m/s RMS 2 = rms of the velocity or pressure signal measured with the flow, with the external excitation switched on and with the control loop, Pa or m/s RMS 3 = rms of the velocity or pressure signal measured with the flow with both the external excitation and the control loop switched off, Pa or m/s RMS 4 = rms of the velocity or pressure signal measured in the shear, Pa or m/s Sr h = Strouhal number calculated from the step height Sr θ = Strouhal number calculated from the momentum thickness U e = freestream velocity, m/s V ( f ) = autospectrum of the velocity signal measured in the shear layer, m/s

For several years, aeronautical industries have wished to improve internal acoustical comfort. In order to make it, they need metrological tools which are able to help them to spot acoustical sources and the associated path in a specific frequency range (i.e., for helicopters’ internal noise: 1000–5000 Hz). Two major source’ localization’ tools exist: holography and beamforming, but these two techniques are based on a free field’s hypothesis. So, problems appear when these techniques are used in a reverberant medium. This paper deals with the study and the comparison of holography and beamforming results in an enclosed area. To complete the study, intensimetry is also implemented to have information on the energy propagation. In order to test the performances of each method, two reflecting panels are put at right angles to create a reverberant environment, in an anechoic chamber. We seek to locate loudspeakers clamped in one panel, in the presence of parasite loudspeakers located on the other one. Then, a parametrical study is led: localization and number of sources, coherent or noncoherent sources. Thus, using limitations, precautions to take, and a base of comparison three methods are put forward. Finally, some envisaged solutions to limit problems of reflections (signal processing, overturning, etc.) are presented.

Two hydrophones are mounted side by side with a separation distance of 20 mm for the frequency range 1–10 kHz. In an anechoic chamber, the probe is validated with a microphonic intensity probe for acoustic power measurements of a reference sound source. The results are compared and agree very well. Directional characteristics of the probe are carried out. Then, an experimental setup is developed in a small cavity (2 × 1 × 0.8 m3) filled with water. A hydrophone as projector is put in the middle of the tank. The characterization of this test facility is done in terms of the time response between the different boundaries and the receiver. The sound power measurements obtained by an hydrophone associated with a gating system to simulate free-field and those obtained by the hydrophonic probe are compared. Sound source localizations are also investigated. The results are in good agreement in spite of the small bounded water tank. This technique seems to be very interesting to study material characteristics like transmission or absorption and source localization.