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