High Speed Jet Noise Research Articles

Subsonic and Supersonic Jet Noise Predictions from Statistical Source Models

Subsonicandsupersonicjetnoiseisdeterminednumerically from statisticalsourcemodels.Thegoalistodevelop prediction methods for high-speed jet noise for application to aeronautical and space transportation systems. In this framework, a combination of a k-≤ turbulence closure with an acoustic analogy provides an interesting way to compute such radiated acoustic e elds. Three acoustic analogies are investigated. First, the classical Lighthill theory in combination with Ribner' s results is applied to calculate jet mixing noise. The second method relies on the Goldstein -Howes convected wave equation, which is used to improve the predicted supersonic jet mixing noise in the upstream direction. It is necessary to properly account for acoustic wave convection, and then, one e nds that the Doppler factor features an exponent of i 3 in the associated power law. A model based on the Ffowcs Williams-MaidanikanalysisthenisdevelopedtoestimatetheMach-wavenoisecomponentthatdominatesforward arc radiation when theconvection Mach number is supersonic. Comparisons between aerodynamicand calculated acoustic results on the one hand, and available measurements on the other hand, are carried out. It is shown that the last two models yield improved supersonic jet mixing noise predictions.

Aeroacoustics

SummaryThe high noise levels of early turbojet aircraft forced attention to be given to the mechanics of the noise generation process and ways were urgently sought for silencing their high speed propulsive flows. The subject of aeroacoustics was born with this problem, the subject being the variety of ways in which sound and flow interact, with the early emphasis being on sound generated aerodynamically. Sound was thought to be a by-product of the flow arid believed to have a negligible back-reaction on it.The search for noise suppression has been extremely successful, so much so, that engine noise is no longer a problem likely to constrain future aircraft operations. Silencing technology has made enormous strides by avoiding as much as possible the very high speed jet flows that were a feature of early engines. But that avenue is not available to supersonic aircraft, which still wait for ways of making their operation acceptably quiet to be identified.Aeroacoustics has brought into being a much deeper insight into the interactions of sound and flow, and some of the early presumptions about the mechanisms of noise creation have had to be rethought. Some of the advances have come from entirely new directions, for example, from anti-sound, a silencing idea based on the destructive cancellation of interfering waves, which has advanced to a state that provides a useful new technology. Some new understanding follows the discovery that flow-acoustic interactions are not always the one-way process previously assumed. Sound can affect flow, especially so when the flow is unstable. It is of course the very unstable flows that make most noise so, taken together, these developments indicate that control might one day be exercised by actively managing the unsteady flows, making them more useful and desirable than before. Indeed examples are now known where control has made possible the avoidance of a powerful performance-limiting instability that previously made that flow regime a prohibited operational zone, a development of aeroacoustics that is probably far more significant than its noise suppression role. That line of thought is expanded in this paper, which concludes with the speculation that active control might now be offering an avenue for approaching die high speed jet noise problem from a completely new direction. Of course it is far too early to know whether that approach can provide a way for removing the take-off noise constraint currently threatening the viability of supersonic transports, but it might; that makes it very important indeed, for it is hard to see any alternative.

Relationship between the instability waves and noise of high-speed jets

The relationship between the instability waves and noise of hot jets at moderate supersonic Mach number is investigated. The relative importance of the Kelvin-Helmholtz instability waves and the supersonic instability waves as sources of noise is considered. The results show that, for the range of Mach number and jet-to-ambient temperature ratio considered, the Kelvin-Helmholtz instability waves have much higher total amplification and higher phase speed.

‘Crackle’: an annoying component of jet noise

The paper describes an investigation of a subjectively distinguishable element of high speed jet noise known as ‘crackle’. ‘Crackle’ cannot be characterized by the normal spectral description of noise. It is shown to be due to intense spasmodic short-duration compressive elements of the wave form. These elements have low energy spread over a wide frequency range. The crackling of a large jet engine is caused by groups of sharp compressions in association with gradual expansions. The groups occur at random and persist for some 10−1s, each group containing about 10 compressions, typically of strength 5 × 10−3 atmos at a distance of 50 m. The skewness of the amplitude probability distribution of the recorded sound quantifies crackle, though the recording process probably changes the skewness level. Skewness values in excess of unity have been measured; noises with skewness less than 0·3 seem to be crackle free. Crackle is uninfluenced by the jet scale, but varies strongly with jet velocity and angular position. The jet temperature does not affect crackle, neither does combustion. Supersonic jets crackle strongly whether or not they are ideally expanded through convergent-divergent nozzles. Crackle is formed (we think) because of local shock formation due to nonlinear wave steepening at the source and not from long-term nonlinear propagation. Such long-term effects are important in flight, where they are additive. Some jet noise suppressors inhibit crackle.

Noise of High Speed Jets with Rough Combustion

An experimental program to demonstrate the effect of unsteady combustion processes in a combustion-augmented jet flow is described. Characteristics of noise for several combustors, having different flame holders and several lengths, operating in a jet temperature range from 1100°C to 1500° and a jet Mach number range from 0.2 to 0.35 (jet exit velocity ranging between 400 and 800 ft/sec) are presented. Data were collected from six B&K 14-in. microphones positioned at an angular separation of 15°in a quadrant of 50-ft radius from the 2-in. diam nozzle exit. Kistler type 601 pressure transducers are mounted in the duct wall of the combustor which is held on a pylon 12 ft above the ground. The experimental data collected and analyzed to date shows a directivity pattern with a nearly circular pattern, which has a peak in the vicinity of 45° to 60° from the jet axis. Comparison of internal pressure fluctuations with the farfield noise shows many simularities in the spectrum, an observation which is verified by the strong cross correlation between the two signals. Combustor and flame holder geometry are found to affect the level and frequency content of the noise. [Supported by NASA grant.]

Developments of a Generalized Theory of Jet Noise

Following a generalized theory of supersonic jet noise, explicit analytical expressions for identifying jet noise mechanisms and numerical calculations of high-speed jet noise characteristics have been obtained. This new theory is uniformly valid in the transonic and supersonic speed ranges, and it includes implicitly refraction and convection of sound through the jet flow. Given results include the transition from U-law to the U-law, differences in directivity for high and low frequencies, sound source distribution, noise spectrum, and the coupled effects of refraction and convection. The implications of these results and their correspondences with results in the existing literature are discussed.

On the Aerodynamic Noise of a Turbulent Jet

Summary A new model is advanced for analyzing the broad-spectrum noise of a turbulent jet. The shear layer bounding the turbulent jet is assumed to play an important role in modifying the quadrupole sound radiation from the interior. To the sound-emit­ ting, small-scale turbulent eddies (with frequencies much higher than those of large-scale eddies), the laminar shear layer has an irregular contour, as if the large-scale turbulent motions were frozen. The linearized analysis is then applied to the laminar shear layer to relate the acoustic oscillations across it. The concept of geometrical acoustics is generalized to represent the passage of an acoustic ray through a laminar shear layer. Acoustic rays may be traced across the shear layer as transmis­ sion and refraction, but they may also be apparently absorbed or generated by the laminar layer. This generation is visual­ ized as the schematic representation, within the framework of geometrical acoustics, of the action of the Reynolds stress in transferring energy from the shearing mean flow to the acoustic waves. Such action of the Reynolds stress can be neglected in ordinary acoustics when the acoustic medium is not moving at speeds comparable to the speed of sound in the medium. How­ ever, this action is of crucial importance in the aerodynamic noise of high-speed turbulent jets where the Reynolds stress is the fun­ damental element of the radiating quadrupoles, according to Lighthill. Those acoustic waves that become stationary with respect to the local mean flow somewhere in the interior of the shear layer are significantly modified by the viscous action through the critical layer. The shear layer therefore serves as a selective amplifier of the acoustic waves passing through it. Kinematically, the shear layer brings about the preferred downstream emission. Dynamically, the shear-layer augmentation signifi­ cantly increases the polar peak noise level. The acoustic power output per unit solid angle for such downstream emissions aug­ mented by the shear layer (including the polar peak) varies as Uj8, as predicted by Lighthill, but without Lighthill's convective corrections. On the other hand, the acoustic power output per unit solid angle nearly normal to the jet, due to the transmitted downstream-propagating waves, varies roughly as £//. Heating the jet gas increases the shear-layer augmentation and may in­ crease the polar peak noise level by several db. The silencing action of the edge notches and edge teeth may also be inter­ preted as due apparently to the result of possible distortion of the shear-layer profiles.