Michel Sunyach

Jet-Noise Reduction by Impinging Microjets: An Acoustic Investigation Testing Microjet Parameters

The effects of microjets on the aerodynamic characteristics of a Mach 0.9 high-Reynolds axisymmetric jet are investigated and are interpreted in the light of previous acoustic results. These measurements are obtained by means of Stereoscopic Particle Image Velocimetry in planes normal to the jet axis. Three parameters of the microjets system are varied: the outgoing mass flux per microjet, the number of microjets and their layout in the azimuth of the main jet. The aerodynamic results indicate a strong correlation between the maximum level of turbulence just behind the nozzle exit and the high-frequency noise, previously shown to potentially balance the acoustic benefits obtained for lower frequencies. The maximum level of turbulence measured at the longitudinal position corresponding to half the potential core length is also highly correlated to the jet noise reduction, which is highlighted by the similar evolution of these two quantities regarding the mass flux per microjet and the number of microjets. For low values of the number of microjets, the microjets are shown to act independently, and their contributions to the turbulence reduction are retrieved far downstream the impinging point without any noticeable azimuthal diffusion.

Changing lift and drag by jet oscillation : experiments on a circular cylinder with turbulent separation

Abstract Oscillating jet actuators have been implemented and tested on a circular cylinder. Their action on the separation of turbulent boundary layers is investigated using complementary approaches. Wall pressure distribution shows that a large lift is generated, at the expense, however, of a slightly increased drag. Particle image velocimetry measurements provide the mean and fluctuating velocity fields in the near-wake. The control jet deflects the mean flux lines towards the wall, illustrating that the separation is delayed. This effect appears more and more powerful as the pulsed jet velocity increases. Phase averaging of the PIV fields shows that periodic structures are generated by the control, and how these structures modify the aerodynamic forces by entraining the external flow towards the wall. Finally, a few comparisons are made with laminar boundary layers and some general mechanisms are presented for the lift increase.

Synthetic jet control for flows in a diffuser: vectoring, spreading and mixing enhancement

This experimental study examines the efficiency of wall pulsing control that is applied to a confined flow going through a two-dimensional diffuser continued by a duct. One- and dual-side pulsing generated by synthetic jet actuators are tested as control means. Particle image velocimetry is used to investigate instantaneous velocity fields and obtain quantitative information. One-side pulsing provokes vectoring of the primary flow in the same direction as the periodic excitation and increases its lateral expansion. Dual-side pulsing neutralizes vectoring but is more efficient for spreading. In both cases, the inner structure of the primary flow was found to be strongly modified, with the generation of highly periodic large-scale structures which kept the flow attached to the diffuser walls. The fluctuating velocity maps point out that the high-fluctuation zone greatly spreads out under control. The flow is considerably widened and the mixing is enhanced.

Active enhancement of the absorbent properties of a porous material

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.

Modal control of beam flexural vibration

An active control system was developed to control the flexural vibrations of a beam with a modal filtering with only one secondary actuator. Segmented piezoelectric actuators and sensors were used for driving and sensing the bending beam vibrations. The primary actuator was fed by a broadband random disturbance signal in order to excite the first five modes of the structure. However, only the second to fifth modes were controlled. The control algorithm was implemented on a DSP board and the input and output signals were filtered using high order low pass filters. These filters, implemented on the DSP board avoid the degrading effect on the control performances of the higher order modes and which are not controlled. The modal filtering was achieved by computing. To this end, it is based on a previous identification procedure. This latter models, in one step, the dynamics of the structure and also the transfer function of the electronic circuits of the controller. The identified filtered modes were then used to compute the gain matrix using a LQR technique (linear quadratic regulator). Simulations of the active control were carried out and practical implementation of the control algorithms was performed. Experimental and simulation results were then compared and discussed.

Effect of microjets on a high-subsonic jet. Parametric study of far-field noise reduction.

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.

Piezoelectric actuator for pulsating jets

Recent researches in aeronautics showed that fluidic actuator systems could offer possibilities for drag reduction and lift improvement. To this end many actuator types were designed. This paper deals with the design, fabrication and test of piezoelectric actuator in order to generate pulsated jets normal to a surface and control air flow separation. It is based on the flexural displacement of a rectangular metal plate clamped on one of its large edge. Piezoelectric patches cemented on the plate were used for driving into vibration the actuator. Experimental measurements show that pulsed flow velocities are adjustable from 1.5m/s to 35m/s through a 100x1mm2 slit andwithin a 100 to 400 Hz frequency range. Prototype provides the jet performances classically required for active control flow.

Acoustics of turbulent flows: a report on Euromech 142

The European Mechanics Colloquium, Euromech 142, ww held at the Ecole Centrale de Lyon from 23 to 26 September 1981 and WM attended by 70 participants, from 9 countries, active in the fields of (i) sound production by turbulent flows and (ii) the effects of flow and turbulence on the propagation of acoustic waves. For topic (i), attention was mostly paid to shear-layer and jet instabilities and to the flow-surface interaction of flexible boundaries, vibrating blades and rigid thick airfoils. Applications were concerned in particular with propeller noise and airframe self-noise of large aircraft. Impinging shear layers were also considered for single and multiple cavities in which self-sustained oscillations can occur. Another subject discussed at the meeting was the noise from inhomogeneities, with applications to flames. For topic (ii), theoretical formulations were presented for the far field of moving multipole sources in the presence of flow and for sound propagation in ducts of variable cross section, with flow, for frequencies around the cut-off. The effect of turbulence was investigated in terms of the space–time coherence of the transmitted pressure fields. Broadband active sound control in the presence of flow was also considered, with emphasis on the progress made possible by use of digital filters. Finally, new experimental techniques, such as acoustic intensity measurements, were presented and large anechoic wind tunnels and other acoustic facilities were described.

Active-control headset protector using piezoceramic material actuator

This paper describes the achievement of active control headset protector using piezoceramic actuators leading to a noise attenuation of about 20 dB within a 1 kHz frequency span located at around 1 to 2 kHz. To this end, several types of piezoceramic transducers or actuators have been designed and tested. They are based on flexural modes of bimorphs constituted by a thin piezoelectric ceramic disk cemented on a metallic plate. The main problems encountered are the spurious frequency regenerations which mask the noise reduction in the expected frequency range. Thus only a few of them meet the above specifications and can be used for reducing the noise inside the headset protector.

The use of time algorithms for the realization of an active sound attenuation

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.<<ETX>>