Maxime Koenig

Farfield filtering and source imaging for the study of jet noise

We present an analysis of the sound field radiated by a high Mach number subsonic jet. The spatial and temporal structures of the sound field are filtered and studied, respectively, by means of Proper Orthogonal Decomposition (POD) and wavelet transforms. The first POD mode is shown to give a near-perfect representation of the fluctuation energy radiation at low angles (in the range 30 ◦ ≤ θ ≤ 50 ◦ ), larger numbers of modes being necessary to completely reproduce the radiation characteristics at higher angles. The wavelet analysis shows, in agreement with previous studies, that the temporal structure of the sound field is characterised by localised high-amplitude events. We implement two threshold intermittency metrics which we use to filter the pressure signals based on the scalogram topology. By varying these metrics we characterise the intermittency of the pressure signals as a function of emission angle. We again find that the sound field can be divided into two families: the fluctuations radiated at low angles (30 ◦ ≤ θ ≤ 50 ◦ ) are characterised by higher levels of global intermittency (an intermittency metric defined with respect to the overall fluctuation energy) than the fluctuations radiated in the angular range θ ≥ 60 ◦ . However, when Farge’s Local Intermittency Measure (defined with respect to the local fluctuation energy at each scale) is used to analyse the data, the fluctuations at all angles show identical behaviour. Results also show that the spectral shapes associated with the high-amplitude events, at all emission angles, are less broadband than those of the unfiltered field, suggesting that the most important source dynamics are not as broadband as the Fourier spectrum would have one believe. Using both the POD and wavelet-filtered signals we decompose the acoustic field into two components: a component which we loosely attribute to coherent structures (CS) and a residuum (R). We compare the CS and R components with the LSS and FSS proposed by Tam et al. 1 We find that neither of these filtering criteria produce a natural division of the acoustic field into two components which match the LSS and FSS shapes. We also show, in the appendix, that the three-microphone approach proposed by Nance & Ahuja 2 to split the acoustic field into two such pieces is very sensitive to the three microphones which are chosen to perform the operation. Finally, we implement a source imaging algorithm, using the CS part of the farfield signature, for both the POD and wavelet-based filtering, in order to establish if our socalled CS signal ensemble can be associated with wavepacket-like sources. Results show that the CS component of our filtering can be associated with a wavepacket-like source mechanism.

Intermittency of the azimuthal components of the sound radiated by subsonic jets

We apply a filtering procedure, based on a continuous wavelet transform, to acoustic pressure and to the velocity field of an experimental Mach 0.6 jet in order to extract the intermittent bursts in the signals. With an intermittency measure based on this filter, it is possible to quantify the significance of these events in the total acoustic intensity. The acoustic pressure is measured by a ring of six azimuthal microphones, allowing decomposition of the sound field into azimuthal Fourier modes. The wavelet filtering is applied to each azimuthal mode, and the results show that the intermittent bursts occur mostly for the axisymmetric mode and for low polar angles. When high energy thresholds are used for the filtering, so as to retain only the most energetic bursts, more than 80 percent of the intermittent radiation is axisymmetric.

Extremum-Seeking Optimisation for Fluidic Jet-Noise Control

In this work we use an extremum-seeking algorithm to optimise a fluidic jet-noise controller. The device, which we call a fluidevron, has been shown to produce reductions in jet noise which are comparable with those achieved using conventional microjets, but the underlying flow-physics have been shown to be very different (see Laurendeau et al. for details). A negative effect produced by the control comprises a high-frequency noise increase. The extremum-seeking algorithm is used to optimise the control either for maximum low-frequency noise-reduction, or for maximum overall noise-reduction. This is achieved through a specification of the frequency-range over which noise reduction is sought. The extremum-seeking is shown to perform well, producing flows with integrated low-frequency gains of the order of 2.5dB when tuned for maximum low-frequency benefit, and flows where the high frequency penalty is virtually eliminated when tuned for maximum spectral range. The extremum-seeking is then implemented using metrics computed from farfield microphones at different polar stations; the control effect is thus found to be omnidirectional. This shows that there is no directional bias in the response of the source mechanisms to the actuation. Finally, the relationship between noise reduction and flow-rate is studied and found to be non-linear: the source mechanisms are most receptive at low flow-rate, a saturation point being reached after which the actuation no longer has any control authority over the source dynamics.

Jet noise reduction by fluidic injection from a rotating plug

We explore a novel jet noise reduction device involving the steady injection of fluid from two diametrically-opposed ports on a rotating centerbody. For the rotation speeds currently possible, noise reductions are observed over a lowfrequency range, up to the rotation frequency. Preliminary results suggest that the noise reduction mechanism may be due to the most unstable flow modes (axisymmetric mode m = 0) being deprived of fluctuation energy due to an excitation of less unstable modes (azimuthal mode m = 2 driven at St = 0.23). INTRODUCTION Jet noise control is most often effected by means of either geometric nozzle modifications (Loheac et al. (2004), Samimy et al. (1993), Zaman et al. (2003)), steady or unsteady fluidic injection (Laurendeau et al. (2008), Castelain et al. (2008), Arakeri et al. (2003), Maury et al. (2011)) and plasma discharge (Samimy et al. (2007)). In all of these cases perturbations are introduced in the vicinity of the nozzle lip, and in the unsteady cases by means of a localised pulsation or injection modulation. In this study we propose an alternative actuation, fluidic perturbations being introduced in the central region of the jet by means of steady injection from a rotating plug; thus we have unsteadiness, but no pulsation. The device is tested on a round jet with Mach number, M = 0.3, Reynolds number, Re = 3.105, and low frequency noise reductions are observed up to the rotation frequency. Analysis of the acoustic and flow measurements (performed by means of stereoscopic, time-resolved PIV) show that for the acoustically beneficial actuation the fluctuation energy of the axisymmetric mode of the flow–which is known to be acoustically important, dominating the sound field at low emission angles–is reduced. This appears to be due to its being deprived of energy on account of the excitation of mode m = 2, which is less unstable than the axisymmetric mode at the acoustically-effective excitation frequency; this observation is based on a linear stability analysis (Michalke (1971)) of the mean-velocity profile just downstream of the centerbody. EXPERIMENTAL SETUP The experiments were performed in the anechoic jet noise facility, “Bruit et Vent” (Noise and Wind), of the CEAT, Poitiers. A Mach 0.3 cold jet is studied in this paper (this being due to limitations in the rotation speeds of the actuator). The jet diameter is equal to 0.05 m. A 0.03 m diameter plug (centerbody) is mounted in the centre of the jet (see figure 1), and is driven in rotation by an electric motor. Air is injected into the plug via a hole in the crankshaft.1 The air is ejected at 240 m/s into the main jet by two 0.0013 m diameter, diametrically opposed, control ports. The control-jet flow rate is less than 0.5% that of the main jet. Microphone measurements were made at 30 diameters from the jet, at downstream angles of 20◦ and 30◦. A moving average was used to smooth the spectra, and peaks associated with sound radiated by the electric motor have been removed by a notch filter in the results presented here. The jet flow velocity was measured by a LaVision timeresolved Stereo-PIV system using a camera with 1024x1024 pixel resolution. The light source was a 10 mJ Quantronix Darwin duo laser (light sheet thickness 2 mm) and the flow was seeded with oil smoke. Three components of velocity were measured in r− θ planes at a range of axial stations by two SA1 Photron cameras. The sampling frequency was 2.7kHz. 10000 PIV image pairs were recorded, this being sufficient for convergence of first and second order statistics. Data-processing consisted of a five-pass correlation routine with 64x64 pixel correlation for the first pass, 16x16 pixel for 1The crankshaft looks like a piece of bucatini. 1 Figure 1: Rotating plug actuator in “Bruit et Vent”. the final pass and with a 50% correlation overlap at each pass. The spatial resolution was one velocity vector every 0.75 mm for the small window size (near the jet exit) and one velocity vector every 1.5 mm for the large window size (around the end of the potential core). Flow fields are analysed for the baseline jet and for jets perturbed by fluidic injection from plugs rotating at St = 0.06 (150Hz), St = 0.12 (300Hz) and St = 0.23 (600Hz). The exit Mach number of the main jet is M = 0.3. At the time of this conference, from a total of 17 axial measurement planes, data from three (x/D = 2.5, x/D = 3 and x/D = 6) is available and will be presented in what follows. ACOUSTIC RESULTS Figure 2 shows results for three different rotation frequencies at a fixed injection flow-rate. We see that, aside from the high-frequency self-noise of the actuator (St > 1.5)2, no difference is observed between the uncontrolled and controlled flows at St0.06 (green dashed line): the actuator does not here produce any change in the flow as far as its lowfrequency sound producing dynamics are concerned. The first response of the jet source dynamics to actuation occurs at St0.12: the jet is now louder (red dashed line); the change from green line to red line in figure 2 occurs abruptly at a rotation frequency of St0.12, indicating a sudden bifurcation of the jet from its baseline equilibrium state to a new louder equilibrium state. The blue dotted line (rotation frequency of St0.23) shows a case where a benefit has been produced at low frequency, and figure 2 (Bottom) shows the evolution between red dashed line and blue dotted line in figure 2 (Top): once the “new” equilibrium state has been provoked, the response of the jet to actuation frequency is smooth, noise reduction being achieved over a progressively broader frequency range as rotation frequency is increased. 2The high-frequency noise increase is believed to be associated with scattering, by the plug, of turbulence associated with the fluidic injection: we have established that this component of the noise has a lower velocity scaling than main jet noise, which means that at higher Mach numbers the high frequency penalty is less severe; preliminary measurements at higher Mach number (Mach 0.6) confirm this. 20 25 30 35 40 45 50 0.1 1 300 60

Installation noise of a turbofan jet engine under an airfoil

Excess noise induced by installation effects are numerically investigated in this work. A realistic turbofan jet engine placed under a NACA0012 profile is considered. Experimental data, regarding the turbulent flow and its acoustics, are indeed available. A RANS simulation is used as input data in an acoustic statistical model to predict mixing noise generated by an isolated jet. This model however needs to be revisited to include installation effects. In order to take account of the presence of the wing, the linearised Euler equations are solved in the time domain for the propagation step.

Experimental and Numerical Study of Jet Noise Reduction of HBPR Engine by Microjet Injection

Research of jet noise reduction by adaptive microjets injection has been conducted under a collaborative research project between Onera, Snecma, IHI, and JAXA. The first paper of this collaborative research was about the microjets noise reduction on a low bypass ratio engine nozzle (Tanaka et al., AIAA 2012-2300). Here we present the experimental and numerical results of a microjet noise reduction on a high bypass ratio (HBPR) double stream nozzle model with and without pylon, tested at the Martel test facility in France. Snecma conducted a microjets noise reduction test, and JAXA took charge of Large Eddy Simulations (LES) to show the current noise prediction capability. The purposes of this research were the development of a noise suppression technology for double stream jet nozzle using microjets, and the acquisition of noise prediction technology for jet noise with low noise devices.