Vincent Phong

Quiet Nozzle Concepts for Three-Stream Jets

We present a study of three-stream nozzle concepts with potential to reduce takeoff noise of future commercial supersonic aircraft. The concepts were evaluated at realistic cycle conditions in a subscale acoustic facility. Computations solving the Reynolds-Averaged Navier-Stokes equations provided insight into the changes in the flow field that can impact noise generation. The investigation encompassed long- and short-cowl nozzles in coaxial and asymmetric arrangements where the third stream was concentrated in the downward azimuthal direction. In coaxial configurations, addition of the third stream makes a modest impact on the noise emission, with a small benefit at high frequencies in the aft arc. This benefit is more evident in short-cowl nozzles. Asymmetric arrangements involved offsetting the tertiary duct and/or application of an internal wedge-shaped deflector. The asymmetry produces significant noise reduction in the direction of the thickened tertiary flow, and is more effective at cycle conditions with high specific thrust. Reduction of the skewness of the far-field pressure fluctuations suggests suppression of Mach wave radiation by the asymmetric tertiary flow.

The Very Near Pressure Field of Single- and Multi-Stream Jets

We present experimental data towards the development of a low-order model for the jet noise source for predictions of isolated and installed noise. In the proposed scheme, the source is prescribed on a radiator surface defining the boundary between the inner rotational jet flow and the outer linear pressure field. The source consists of wavepacket-type partial fields whose noise emission can be computed using well-established linear propagation methods. Experiments on singleand dualstream jets are used to define the radiator surface and determine relevant quantities on it, including the convective velocity, axial correlation scale, and azimuthal coherence. It is found that the rotational/irrotational boundary is characterized by negative skewness of the pressure field, leading to a convenient criterion for defining the radiator surface. Space-time correlations of the pressure fluctuation on the radiator surface yield the distribution of convective velocity Uc of the partial fields. In coaxial jets, the outer flow effectively silences the eddies of the primary shear layer in the vicinity of the nozzle exit. This is manifested by a large drop of Uc in the initial region of the jet. Offsetting the nozzles prolongs the low-Uc region on the side of the thickened secondary flow. The resulting reduction in radiation efficiency is deemed the primary reason for the noise benefit of eccentric jets. Circumferential coherence measurements indicate that the partial fields are very localized azimuthally.

Noise Reduction of a Turbofan Bleed Valve

A combined experimental and computational research effort investigated the noise sources of a pneumatic bleed valve used in turbofan engines and developed engineering solutions for attenuating those sources. The experimental effort employed 1/4-scale, rapid-prototyped valve designs which enabled the exploration of a large parameter space. Microphone array systems surveyed the sound and its sources, and Pitot surveys measured the mean velocity downstream of the valve. The numerical code solved the Reynolds-Averaged Navier Stokes (RANS) in both steady and unsteady modes. The research addressed the flow and acoustics of the valve without and with a muffler. The noise of the isolated valve consists of conventional jet noise with strong excess sound originating from the valve exit. The excess sound was traced to vortex shedding by support struts. Streamlining of the struts, aided by computational parametric studies, eliminated this noise component. The noise of the complete valve, with muffler attached, was attenuated by two principal means: using the streamlined struts and adding a honeycomb flow straightener to the muffler cavity. Proper installation of the honeycomb is critical for maximizing noise reduction. The resulting best design achieved a reduction of approximately 8 decibels in perceived noise level.

Investigation of Isolated and Installed Three-Stream Jets from Offset Nozzles

An experimental aeroacoustic investigation of three-stream nozzle concepts with potential to reduce takeoff noise from future supersonic aircraft is presented. We use guidance from previous three-stream nozzle experiments to explore asymmetric designs where both secondary and tertiary streams are concentrated in the downward direction. The impact of increasing the plug size on noise is examined. Enlarging the plug provides moderate noise reduction for axisymmetric and asymmetric nozzle configurations. Nozzle configurations that combine asymmetry in both secondary and tertiary streams provide a distinct noise benefit in the sideline azimuthal direction. Considering the sound pressure level at a fullscale frequency around 300 Hz, the combined effects of the enlarged plug and dual asymmetry yield reductions of 15 dB and 6 dB in the downward and sideline azimuthal directions, respectively, and at angles close to the angle of peak emission. Installation effects with an aft deck indicate minimal impacts on radiated noise, except in the case of a long deck at a scrubbing position. In that case, the ability of the asymmetric nozzles to reduce noise is disrupted. Marginal shielding at high frequency is noted at forward observer angles.

High frequency acoustic transmission loss of perforated plates at normal incidence

A study has been conducted on the transmission loss of perforated plates at normal incidence. The investigation includes a theoretical analysis of the problem with validation through experimentation. The experiments comprised microphone measurements of transmission loss for 11 perforated plates with variable thickness, hole size, and porosity. The theoretical model is based on planar wave propagation through a single contraction/expansion chamber with modifications to account for hole interaction effects. The resulting formula for transmission loss yields superior predictions over past theories for the range of properties investigated. Deviations between experimental measurements and theoretical predictions of transmission loss are less than about 1.5 dB for dimensionless hole diameter d/λ < 0.5. The accuracy of the model does not show a strong dependence on plate thickness-to-diameter ratio or porosity.

Normal incidence acoustic insertion loss of perforated plates with bias flow.

The transmission of sound at normal incidence through perforated plates with bias flow is investigated experimentally and theoretically over a large parameter space. A specially designed experimental apparatus enabled the measurement of insertion loss with bias flow Mach number up to 0.25. A theoretical model for insertion loss was constructed based on inviscid, one-dimensional wave propagation with mean flow through a single contraction/expansion chamber. The mass end correction of the contraction is modified for hole interaction effects and mean flow. Hydrodynamic losses are modeled using a vena contracta coefficient dependent on both perforation geometry and Reynolds number. Losses in acoustic energy that occur in the mixing region downstream of the perforations are modeled as fluctuations in entropy. The proposed model was validated experimentally over a range of plate thickness, porosity, and hole size. The experimental results indicate an increase in insertion loss with increasing frequency, followed by saturation and decline as resonant conditions are established in the perforations. The insertion loss at low frequency increases with increasing Mach number through the perforation. The proposed model captures these trends and its predictions are shown to be more accurate than those of past models.

Normal Incidence Acoustic Transmission Loss of Perforated Plates Subject to Bias Flow

A study has been conducted on the transmission of sound at normal incidence through perforated plates with bias flow. A theoretical model is proposed which characterizes the acoustic response of the plate through the insertion loss. The proposed model is based on inviscid, one-dimensional wave propagation and mean flow through a single contraction/expansion chamber and includes hole interaction effects. Entropy fluctuations are used to model losses in acoustic energy that occur in the mixing region downstream of the perforations. Experimental measurements of insertion loss are conducted in a specially designed facility and are used to assess the validity of the model over a range of plate thickness, porosity, and hole size. Further, the model is compared to past theoretical treatments. The experimental results indicate an increase in insertion loss with increasing frequency, followed by saturation and decline as resonant conditions are established in the perforations. There is an increase in insertion loss as the Mach number through the perforation increases from 0 to about 0.25, and the resonance frequency decreases with increasing Mach number. The proposed model captures these trends and its predictions are superior to those of past models. Experimental measurements of insertion loss at higher hole Mach numbers are challenging as the turbulent fluctuations in the mixing region can overwhelm the acoustic signal transmitted through the perforation.

Acoustic Transmission Loss of Perforated Plates

A study has been conducted on the acoustic response of perforated plates at normal incidence. The investigation includes a theoretical analysis of the problem, with validation through experimentation. The acoustic response was quantified through the transmission loss and absorption coefficient of the perforate. The theoretical analysis is based on planar wave propagation through a single contraction/expansion chamber, with modifications to account for hole interaction effects. The resulting formula for transmission loss yields superior predictions over past theories. The theoretical model is validated through rigorous parametric experimentation. Eleven perforated plates with different thickness, hole size, and porosity were tested. Deviations between experiment measurements and theoretical predictions of transmission loss are shown to be less than about one decibel for dimensionless hole diameter d/λ < 0.5. The accuracy of the model does not show a strong dependency on plate thickness or porosity.

Perceived Noise Assessment of Offset Three-Stream Nozzles for Low Noise Supersonic Aircraft

We assess the potential of three-stream nozzle concepts to reduce the takeoff perceived noise level from future supersonic aircraft. The study encompasses the effects of nonaxisymmetric exhaust configurations as well as the effect of an enlarged plug. Asymmetry in the plume was created via eccentric tertiary and secondary ducts, in combination with a wedge-shaped deflector in the tertiary duct; and by reshaping the primary plug from circular to elliptical cross-section. The eccentricities were directed to increase the thickness of the lower-speed tertiary and secondary flows underneath the fast primary stream, thereby reducing noise in the general downward direction. The plug ellipticity was designed to flatten the primary stream along the major axis of the ellipse, thereby improving its coverage by the lower-speed streams with the goal of improving sideline noise reduction. The enlarged plug design was motivated by sonic-boom signature considerations. Acoustic measurements using helium-air mixture jets from small-scale, rapid-prototyped nozzles generated sound pressure level spectra at a number of polar and azimuthal angles for each configuration. These were converted to estimates of flyover perceived noise level and effective perceived noise level (EPNL) assuming a typical takeoff profile. The effect of the enlarged plug is to reduce the takeoff (downward) and sideline EPNLs, each by about 1.7 dB. Configurations with eccentricity in the tertiary flow only, all other components being axisymmetric, reduced the downward EPNL by as much as 5.1 dB but did not reduce the sideline EPNL. Adding eccentricity to the secondary ducts results in sideline noise reduction of about 2 EPNdB while providing downward reduction of around 5.6 EPNdB. The effect of the elliptical plug is to add another 1.0 dB to the sideline reduction. The best configuration involves the combination of eccentricity in the secondary and tertiary ducts with ellipticity of the primary plug; it yields noise reductions of 5.8 EPNdB and 2.9 EPNdB in downward and sideline directions, respectively. Including the effect of the enlarged plug, a cumulative (takeoff plus sideline) noise reduction of 12 EPNdB is estimated.