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Aeroacoustics of a High Aspect-Ratio Jet
Abstract
Circulation control wings are a type of pneumatic high-lift device that have been extensively researched as to their aerodynamic benefits. However, there has been little research into the possible airframe noise reduction benefits of a circulation control wing. The key element of noise is the jet noise associated with the jet sheet emitted from the blowing slot. This jet sheet is essentially a high aspect-ratio rectangular jet. Thus, to fully understand the noise of a circulation control wing, the noise of high aspect-ratio rectangular jets must also be understood. A high aspect-ratio nozzle was fabricated to study the general characteristics of high aspect-ratio jets with aspect ratios from 100 to 3000. The jet noise of this nozzle was proportional to the 8" power of the jet velocity. It was also found that the jet noise was proportional to the slot height to the 312 power and slot width to the 1/2 power.
... They found that the sound power scaled with the velocity to the power of 8 following Lighthill [7] and an equivalent length scale hL, where h and L are the rectangular jet height and width respectively. Munro and Ahuja [8] performed flow and noise measurements on a rectangular nozzle with aspect ratios ranging from 100 to 3000. They found that the jet noise data collapsed when scaled by the jet velocity to the power of 8 and an equivalent length scale h 3/2 L 1/2 . ...
... 10 dB (a) Lw normalised using V 8 j hL [6]. Figure 12: Comparison of the jet noise scaling laws found from previous studies applied to the data from current study. Figure 12(b) shows the normalised sound power level using the scaling law found by Munro and Ahuja [8]. The normalised sound power level collapses to within less than 10 dB (compared with the 5 dB spread obtained using the current scaling laws, see fig. ...
Noise measurements have been performed on rectangular jets of aspect ratios ranging from 49 to 987 with the aim of determining the appropriate velocity and length scaling to be used in an empirical noise prediction model. The results have shown that the velocity exponent is a function of the nozzle aspect ratio, decreasing with increasing nozzle aspect ratio. In an effort to establish a general prediction model, the velocity exponent of 7 was chosen as the best compromise to represent all the measured data. The analysis of the noise measurements from high aspect ratio nozzles of varying jet height and width has shown that, for the range of aspect ratios considered, the jet sound power level scales with the nozzle height to the power of 3 and the nozzle width to the power of 1. The derived jet noise scaling has been validated with independent experimental data and shows good agreement.
... To understand and quantify the aeroacoustic characteristics and benefits of the Circulation Control Wing, Munro, Ahuja and Englar [7,8,9,10] have recently conducted several acoustic experiments comparing the noise levels of a conventional high-lift system with that of an advanced CC wing at the same lift setting. The present Computational Fluid Dynamics (CFD) study [11] is intended to complement this work, and numerically investigate the aerodynamic characteristics and benefits associated with the CC airfoil. ...
... According to recent acoustic measurements [7,8], the jet slot height has a strong effect on the noise produced by the CC airfoil. These studies indicate that a larger jet slot will reduce the noise at the same momentum coefficient compared to a smaller slot. ...
Circulation Control technology is a very effective way of achieving high lift forces required by aircraft during take-off and landing. This technology can also directly control the flow field over the wing. Compared to a conventional high-lift system, a Circulation Control Wing (CCW) can generate comparable or higher lift forces during take-off/landing with fewer or no moving parts and much less complexity. In this work, an unsteady three-dimensional Navier-Stokes analysis procedure has been developed and applied to Circulation Control Wing configurations. The effects of 2-D steady jets and 2-D pulsed jets on the aerodynamic performance of CCW airfoils have been investigated. It is found that a steady jet can generate very high lift at zero angle of attack without stall, and that a small amount of blowing can eliminate vortex shedding at the trailing edge, a potential noise source. It is also found that a pulsed jet can achieve the same high lift as a steady jet at lower mass flow rates, especially at a high frequency, and that the Strouhal number has a more dominant effect on the pulsed jet performance than just the frequency or the free-stream velocity.
... When a planar jet is used to reduce the flow-induced noise, it is expected to use the jet with the lowest speed to shelter the noise source. This is because the jet can generate substantial self-noise, i.e. jet noise (Tam 1998;Munro and Ahuja 2003). The self-noise intensity is positive correlated with the jet speed. ...
To avoid the complexity of the edge definition by the half width, a new approach to defining the leeward edge of the planar jet in crossflow is introduced in this paper. Particle Image Velocimetry (PIV) experiments were performed to measure different flow regimes within the single jet and the dual jets configurations in crossflow. Based on the experimental data acquired, a series of velocity profiles were extracted from the flow field. In each profile, a velocity threshold was given to distinguish the regions sheltered and the regions not sheltered by the planar jet. The boundary of these regions was accordingly recognized as the leeward edge. Furthermore, fitting of the edge was carried out using a second order polynomial so as to enable a mathematical expression of the leeward edge. An application of the proposed approach towards the flow induced noise reduction using a planar jet is also discussed in this paper. In addition, the PIV frame assembly algorithm used in this study is reported.
... In summary, ignoring the complexity of the additional noise source, it is safe to say that the bevel primarily adds noise, and does not create 'shielding' of the jet noise. 7 American Institute of Aeronautics and Astronautics ...
... Only limited hydrodynamic computational studies were carried out, e.g. the DNS study of Stanley et al (23) . On the aeroacoustics side, Kouts and Ku (24) and Munro and Ahuja (25) investigated experimentally very high aspect ratio rectangular jets, providing two dimensional directivities and frequency spectra for the emitted sound. ...
A Large Eddy Simulations (LES) technique combined with the acoustic analogy has been used to study the development and basic radiation of an incompressible planar jet with a Reynolds number of 3000. The nozzle was included into the simulations allowing to prescribe various velocity profiles at the nozzle entrance. The results showed that in the case of welldeveloped (parabolic) profiles, the antisymmetrical mode develops and dominates further downstream leading to a sinuous distortion of the potential core. A symmetrical mode, resulting in a jet 'puffing' prevailed in the case of undeveloped (top-hat) profiles. Mean flow characteristics were influenced by the shape of the inflow profile, however, the major differences were observed in the development of the fluctuations. R.m.s. values of the velocity fluctuations were significantly higher in the region beyond the end of the potential core and before the flow reached a self-preserving state for a jet evolving from a parabolic profile. A compact solution of Lighthill's analogy was used to calculate the basic radiation. It is shown that if the spanwise length of the computational exceeds the correlation length, the results obtained were not affected by the spanwise length of the domain. The results show that the majority of sound is generated by the region beyond the end of the potential core. A distinctive peak is observed in the acoustic spectra at Sth ≈ 0.1 for a jet evolving from the parabolic profile.
... Only limited hydrodynamic computational studies were carried out, e.g. the DNS study of Stanley et al (23) . On the aeroacoustics side, Kouts and Ku (24) and Munro and Ahuja (25) investigated experimentally very high aspect ratio rectangular jets, providing two dimensional directivities and frequency spectra for the emitted sound. ...
Low speed circular, elliptic and planar jets are investigated computationally for basic sound generation and hydrodynamics. The jets are assumed to be incompressible and are simulated using the large eddy simulation (LES) approach. The emitted sound is calculated using Lighthill's acoustic analogy. Two formulations are used, Lighthill's stress tensor formulation and Powell's vortex sound formulation. A new boundary correction for Powell's formulation is developed in order to account for the finite size of the computational domain. Low to moderate Reynolds number jets are simulated. Good agreement with known hydrodynamic results is achieved. This includes the nature of the transition process, e.g. enhanced mixing and axis switching in the elliptic jet and in some statistical results. The new boundary correction for Powell's formulation proves to be vital in order to achieve good agreement with Lighthill's formulation. Some success in high frequency prediction at least for the circular and elliptic jets is achieved in terms of getting the expected asymptotic behaviour. Both formulations show that the elliptic jet noise level is mildly lower than the circular jet noise level. Good to very good agreement is achieved in terms of directivities and frequency spectra with known results for the various jets.
... Tam and Zaman 38 compared the similarity spectra with subsonic jet noise data from elliptic, rectangular, tabbed, and six-lobed nozzles. Munro and Ahuja 39 showed good agreement for high aspect ratio rectangular nozzles. The existence of two seemingly universal similarity spectra offers strong experimental support for two-noise source model 1,2 . ...
The two-noise source model for predicting jet noise claims that the radiated jet noise is composed of two distinct sources one associated with the small-scale turbulence and another associated with the large-scale turbulence. The former source is claimed to radiate noise predominantly at larger angles with respect to the downstream jet axis, whereas the large-scale turbulence radiates predominantly at the shallower angles. A key objective of this effort is to experimentally validate this model using correlation and coherence measurements. Upon the successful validation of the two-noise source model for jets exhausting from multiple nozzle geometries driven at Mach numbers ranging from subsonic to supersonic, a three-microphone signal enhancement technique is employed to separate the contribution of the small-scale turbulence from that of the large-scale turbulence in the far-field. This is the first-ever quantitative separation of the contributions of the turbulence scales in far-field jet noise measurements. Furthermore, by suitable selection of far-field microphone positions, the separation of the contribution of any internal or core noise from that of the jet-mixing noise is achieved. Using coherence-based techniques to separate the contributions of the small-scale turbulence, large-scale turbulence, and any internal or core noise from far-field exhaust noise measurements forms the backbone of this effort. In the application of coherence-based multiple-microphone signal processing techniques to separate the contributions of the small-scale turbulence, large-scale turbulence, and any internal or core noise in the far-field, research efforts focus on three techniques (1) the coherent output power spectrum using two microphones, (2) an ordinary coherence method using the three-microphone technique, and (3) the partial-coherence method using five microphones. The assumption of jet noise incoherence between correlating microphone is included in each of these methods. In light of the noise radiation mechanisms described within the framework of the two-noise source model and their spatial characteristics as experimentally determined in the far-field, the assumption of jet noise incoherence is evaluated through a series of experiments designed to study jet noise coherence across a variety of nozzle geometries and jet Mach numbers ranging from subsonic to supersonic. Guidelines for the suitable selection of far-field microphone locations are established. Ph.D. Committee Chair: Ahuja, Krishan K.; Committee Member: Cunefare, Kenneth; Committee Member: Lieuwen, Tim C.; Committee Member: Mendoza, Jeff; Committee Member: Sankar, Lakshmi
... For this case, with nearly same amount of power generated, the larger jet slot height is more attractive than the smaller jet slot height because it consumes less power (approximately 45% less) to produce the jet flow. Moreover, according to the acoustic study of jet noise [110], a larger jet slot produces less noise than a smaller jet slot. ...
With the advantage of modern high speed computers, there has been an increased interest in the use of first-principles based computational approaches for the aerodynamic modeling of horizontal axis wind turbine (HAWT). Since these approaches are based on the laws of conservation (mass, momentum, and energy), they can capture much of the physics in great detail. The ability to accurately predict the airloads and power output can greatly aid the designers in tailoring the aerodynamic and aeroelastic features of the configuration. First-principles based analyses are also valuable for developing active means (e.g., circulation control), and passive means (e.g., Gurney flaps) of reducing unsteady blade loads, mitigating stall, and for efficient capture of wind energy leading to more electrical power generation. In this present study, the aerodynamic performance of a wind turbine rotor equipped with circulation enhancement technology (trailing edge blowing or Gurney flaps) is investigated using a three-dimensional unsteady viscous flow analysis. The National Renewable Energy Laboratory (NREL) Phase VI horizontal axis wind turbine is chosen as the baseline configuration. Prior to its use in exploring these concepts, the flow solver is validated with the experimental data for the baseline case under yawed flow conditions. Results presented include radial distribution of normal and tangential forces, shaft torque, root flap moment, surface pressure distributions at selected radial locations, and power output. Results show that good agreement has been for a range of wind speeds and yaw angles, where the flow is attached. At high wind speeds, however, where the flow is fully separated, it was found that the fundamental assumptions behind this present methodology breaks down for the baseline turbulence model (Spalart-Allmaras model), giving less accurate results. With the implementation of advanced turbulence model, Spalart-Allmaras Detached Eddy Simulation (SA-DES), the accuracy of the results at high wind speeds are improved. Results of circulation enhancement concepts show that, at low wind speed (attached flow) conditions, a Coanda jet at the trailing edge of the rotor blade is effective at increasing circulation resulting in an increase of lift and the chordwise thrust force. This leads to an increased amount of net power generation compared to the baseline configuration for moderate blowing coefficients. The effects of jet slot height and pulsed jet are also investigated in this study. A passive Gurney flap was found to increase the bound circulation and produce increased power in a manner similar to the Coanda jet. At high wind speed where the flow is separated, both the Coanda jet and Gurney flap become ineffective. Results of leading edge blowing indicate that a leading edge blowing jet is found to be beneficial in increasing power generation at high wind speeds. The effect of Gurney flap angle is also studied. Gurney flap angle has significant influence in power generation. Higher power output is obtained at higher flap angles. Ph.D. Committee Chair: Sankar, Lakshmi; Committee Member: Englar, Robert; Committee Member: Holmes, John; Committee Member: Jagoda, Jechiel; Committee Member: Ruffin, Stephen
... To further understand the aeroacoustic characteristics and benefits of the Circulation Control Wing, Munro, Ahuja and Englar [16, 17, 18, 19] have recently conducted several acoustic experiments comparing the noise levels of a conventional high-lift system with that of an advanced CC wing at the same lift setting. The present Computational Fluid Dynamics (CFD) study [20] is intended to be a complement to this work, and to numerically investigate the aerodynamic characteristics and benefits associated with the CC airfoil. ...
The influence mechanism of jet on aerodynamic noise control in the pantograph region at different sinking heights was numerically studied using an Improved Delayed Detached Eddy Simulation (IDDES) model and the Ffowcs Williams-Hawkings (FW-H) equation. Active flow control was achieved by setting jet slots at the leading edge of the cavity to predict the noise generated by the jet itself. The results showed that in the pantograph region with a sinking height of 500 mm, the shear layer was lifted by the jet, which prevented high-speed turbulence caused by flow separation from entering the cavity. Therefore, the model with jet control device reduced the overall far-field noise on the side of the pantograph by 1.6 ∼ 3.9 dB. Through flow field analysis, for the pantograph region where flow separation occurs in the front of the cavity, the jet needs to lift the shear layer enough to cross the front of the pantograph and prevent flow separation, thereby reducing the aerodynamic noise. For the pantograph region without flow separation in the front of the cavity, the jet may introduce additional noise sources, deteriorating the aerodynamic noise in the pantograph region.
The present study deals with the design, development, and fabrication of a nozzle with flowconditioning assembly and using it to study jet flow acoustics. The flow is conditioned by severalmeans before the nozzle exits and the conditioned flow is allowed to exit the nozzle as jet. Thisconditioned jet flow produces aerodynamically generated noise and the same is measured using amicrophone. The microphone is placed perpendicular to the jet axis and the measurements arerecorded for various regimes. The results will be discussed based on the acoustic spectra and thejet decay characteristics.
This paper covers the design, analysis, manufacture and testing of an experimental rig for aeroacoustic measurements on high aspect ratio rectangular jets. The design of the rig is detailed with emphasis given to flow control and acoustic characteristics. Results of the far-field noise spectra and directivity are presented from the initial tests. These show that the jet noise is notably directional about both the major and minor jet axes and that the peak frequency component is weakly dependant on both aspect ratio and Mach number. The 'zone of silence' close to the streamwise axis can also be seen clearly. Flow visualization tests were also conducted that showed how the flow develops close to the nozzle.
The objective of the work described in this paper is to use the three-microphone coherence-based signal processing technique to separate the contributions of the small-scale turbulence and the large-scale turbulence from far-field exhaust noise measurements. Both round and rectangular model-scale jets were tested for both subsonic and supersonic Mach numbers. Detailed cross-correlation and coherence data are gathered in the exhaust far-field region of each jet. Considerable evidence in support of the two-noise source model has resulted. This is the first ever quantitative separation of the contributions of the two turbulence scales in far-field jet noise measurements.
Acoustic and aerodynamic measurements were performed in NLR's Small Anechoic Wind Tunnel on the air curtain concept, which is intended for landing gear noise reduction. The idea of this concept is to apply an upstream blowing slot to deflect the flow around a landing gear (component), thus reducing the local flow speeds and therefore the aerodynamically generated noise. Prior to the wind tunnel tests, a design study was carried out to assess the possible benefit of an air curtain and to define the test set-up. For the subsequent proof-of-concepts tests, two-dimensional half-models were mounted on an endplate which was attached to the lower edge of the wind tunnel nozzle. Blowing was applied through a slot in the endplate, upstream of the model. Microphone array and PIV measurements were performed to characterize the acoustics and aerodynamics of the air curtain. Tests were done for different wind tunnel speeds, blowing speeds, slot geometries and model geometries. For the relatively quiet baseline half-models in the present tests, broadband noise reductions of 3-5 dB were obtained using an air curtain with normal blowing (i.e. perpendicular to the main flow). The noise reductions could be increased to 5-10 dB by oblique blowing (30° upstream) or by applying a small flow deflector directly before the blowing slot. For full models larger noise reductions are anticipated.
The sound generated by a 2-D, 20% thick elliptical circulation control airfoil with single slot blowing is experimentally investigated in the University of Florida Aeroacoustic Flow Facility. Experiments are conducted at a variety of freestream and slot jet velocities for three different slot heights h. The airfoil's lift characteristics compare favorably with data from a previous research study in a conventional closed-wall wind tunnel. A large aperture microphone array reveals low noise contamination in the UFAFF test section. Data from farfield microphones indicate both tonal and broadband noise sources. Vortex shedding from the rounded trailing edge occurs when jet momentum levels are insufficient to prevent flow separation. High frequency tonal noise is also identified due to vortex shedding, where fD/Ujet = 0.21, from the D = 0.28 mm thick slot lip. For large jet velocities, high frequency broadband slot jet noise is identified as the dominant noise source for fh/Ujet > 0.1. Jet velocity amplitude and frequency non-dimensionalization collapse the high-frequency portion of the spectra reasonably well when freestream velocity is low or zero. For curvature and slot noise generated by the external boundary layer flow, local mean velocities at the trailing edge and above the slot are not more effective than freestream and jet velocities when non-dimensionalizing frequency. Finally, freestream amplitude and Strouhal number scaling show a remarkable collapse of spectra for a variety of freestream velocities, jet velocities, and slot heights for a constant momentum coefficient.
An integrated airframe propulsion system enables a functionally silent aircraft of Quiet Aircraft Technology (QAT) to reduce aircraft noise below the noise level in polluted areas. The noise generated by the propulsion systems is reduced by reducing the jet velocities and redesigning the fan and other turbomachinery components. The noise generated by scattering unsteady flow structures in the boundary layers at the trailers edge are attenuated by hiding the trailing edge where the distributed propulsion system is embedded in the airframe. The aircraft propulsion system takeoff noise produced by high-speed exhaust jet and is reduced by decreasing the jet velocity below current levels. The high-bypass-ratio-engines integration problem is solved by distributing the propulsion system by multiple small engines or multifan engine concept. The turbomachinery with low noise flow throttling capability yields the drag which meets the technical challenges to facilitate a quiet engine airbrakes.
Hydrodynamics and sound radiation of a low speed planar jet with Re=3000 have been studied by large eddy simulation combined with Lighthill's acoustic analogy. Jets evolving from both well-developed (parabolic) and undeveloped (top-hat) mean velocity profiles have been simulated. The results showed the following: (i) initial domination of a symmetrical mode for jets evolving from top-hat profiles and prevailing of an antisymmetrical mode resulting in a sinuous distortion of the potential core for jets evolving from parabolic profiles, and (ii) shape of a mean velocity profile has some effect on mean flow characteristics; however, the major differences were observed in the development of the fluctuations. Velocity fluctuations were significantly higher for jets evolving from a parabolic profile in the region beyond the end of the potential core before the flow reached a self-preserving state. To calculate the basic sound radiation, the sources in Lighthill's equation were treated either as compact in all directions or as noncompact in the spanwise direction. The spanwise length of the computational domain was found to have a little effect on the results obtained with compact in ail directions solution provided that spanwise length exceeds the correlation length. Results showed that the majority of sound was generated by the region beyond the end of the potential core.
The rapid air travel growth during the last three decades, has resulted in runway congestion at major airports. The current airports infrastructure will not be able to support the rapid growth trends expected in the next decade. Changes or upgrades in infrastructure alone would not be able to satisfy the growth requirements, and new airplane concepts such as the NASA proposed Super Short Takeo and Landing and Extremely Short Takeo and Landing (ESTOL) are being vigorously pursued. Aircraft noise pollution during Takeoff and Landing is another serious concern and efforts are aimed to reduce the airframe noise produced by Conventional High Lift Devices during Takeoff and Landing. Circulation control technology has the prospect of being a good alternative to resolve both the aforesaid issues. Circulation control airfoils are not only capable of producing very high values of lift (Cl values in excess of 8.0) at zero degree angle of attack, but also eliminate the noise generated by the conventional high lift devices and their associated weight penalty as well as their complex operation and storage. This will ensure not only satisfying the small takeoff and landing distances, but minimal acoustic signature in accordance with FAA requirements. The Circulation Control relies on the tendency of an emanating wall jet to independently control the circulation and lift on an airfoil. Unlike, conventional airfoil where rear stagnation point is located at the sharp trailing edge, circulation control airfoils possess a round trailing edge, therefore the rear stagnation point is free to move. The location of rear stagnation point is controlled by the blown jet momentum. This provides a secondary control in the form of jet momentum with which the lift generated can be controlled rather the only available control of incidence (angle of attack) in case of conventional airfoils. The use of Circulation control despite its promising potential has been limited only to research applications due to the lack of a simple prediction capability. This research effort was focused on the creation of a rapid prediction capability of Circulation Control Aerodynamic Characteristics which could help designers with rapid performance estimates for design space exploration. A morphological matrix was created with the available set of options which could be chosen to create this prediction capability starting with purely analytical physics based modeling to high fidelity CFD codes. Based on the available constraints, and desired accuracy metamodels has been created around the two dimensional circulation control performance results computed using Navier Stokes Equations (Computational Fluid Dynamics). DSS2, a two dimensional RANS code written by Professor Lakshmi Sankar was utilized for circulation control airfoil characteristics. The CFD code was first applied to the NCCR 1510-7607N airfoil to validate the model with available experimental results. It was then applied to compute the results of a fractional factorial design of experiments array. Metamodels were formulated using the neural networks to the results obtained from the Design of Experiments. Additional validation runs were performed to validate the model predictions. Metamodels are not only capable of rapid performance prediction, but also help generate the relation trends of response matrices with control variables and capture the complex interactions between control variables. Quantitative as well as qualitative assessments of results were performed by computation of aerodynamic forces and moments and flow field visualizations. Wing characteristics in three dimensions were obtained by integration over the whole wing using Prandtl's Wing Theory. The baseline Super STOL configuration was then analyzed with the application of circulation control technology. The desired values of lift and drag to achieve the target values of Takeoff and Landing performance were compared with the optimal configurations obtained by the model. The same optimal configurations were then subjected to Super STOL cruise conditions to perform a tradeoff analysis between Takeoff and Cruise Performance. Supercritical airfoils modified for circulation control were also thoroughly analyzed for Takeoff and Cruise performance and may constitute a viable option for Super STOL and STOL Designs. The prediction capability produced by this research effort can be integrated with the current conceptual aircraft modeling and simulation framework. The prediction tool is applicable within the selected ranges of each variable, but methodology and formulation scheme adopted can be applied to any other design space exploration. Ph.D. Committee Chair: Dimitri N. Mavris; Committee Member: Daniel P. Schrage; Committee Member: Lakshmi N. Sankar; Committee Member: Robert C. Michelson; Committee Member: Robert J. Englar
This paper summarizes the results of studies undertaken to investigate revolutionary propulsion-airframe configurations that have the potential to achieve significant noise reductions over present-day commercial transport aircraft. Using a 300 passenger Blended-Wing-Body (BWB) as a baseline, several alternative low-noise propulsion-airframe-aeroacoustic (PAA) technologies and design concepts were investigated both for their potential to reduce the overall BWB noise levels, and for their impact on the weight, performance, and cost of the vehicle. Two evaluation frameworks were implemented for the assessments. The first was a Multi-Attribute Decision Making (MADM) process that used a Pugh Evaluation Matrix coupled with the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS). This process provided a qualitative evaluation of the PAA technologies and design concepts and ranked them based on how well they satisfied chosen design requirements. From the results of the evaluation, it was observed that almost all of the PAA concepts gave the BWB a noise benefit, but degraded its performance. The second evaluation framework involved both deterministic and probabilistic systems analyses that were performed on a down-selected number of BWB propulsion configurations incorporating the PAA technologies and design concepts. These configurations included embedded engines with Boundary Layer Ingesting Inlets, Distributed Exhaust Nozzles installed on podded engines, a High Aspect Ratio Rectangular Nozzle, Distributed Propulsion, and a fixed and retractable aft airframe extension. The systems analyses focused on the BWB performance impacts of each concept using the mission range as a measure of merit. Noise effects were also investigated when enough information was available for a tractable analysis. Some tentative conclusions were drawn from the results. One was that the Boundary Layer Ingesting Inlets provided improvements to the BWB's mission range, by increasing the propulsive efficiency at cruise, and therefore offered a means to offset performance penalties imposed by some of the advanced PAA configurations. It was also found that the podded Distributed Exhaust Nozzle configuration imposed high penalties on the mission range and the need for substantial synergistic performance enhancements from an advanced integration scheme was identified. The High Aspect Ratio Nozzle showed inconclusive noise results and posed significant integration difficulties. Distributed Propulsion, in general, imposed performance penalties but may offer some promise for noise reduction from jet-to-jet shielding effects. Finally, a retractable aft airframe extension provided excellent noise reduction for a modest decrease in range.
The noise from the fine-scale turbulence of high-speed nonaxisymmetric jets is considered. A prediction method is developed by extending the work of Tam and Auriault (Tam, C. K. W., and Auriault, L., "Jet Mixing Noise from Fine-Scale Turbulence," AIAA Journal, Vol. 37, No. 2, 1999, pp. 145-153). A set of improved numerical boundary conditions for use in nonaxisymmetric jet mean flow and turbulence calculation is developed. These new boundary conditions allow the computation to be carried out in a smaller computation domain. It is known that nonaxisymmetric mean flow has a significant impact on the radiated noise spectrum and directivity through refraction. In the Tam and Auriault theory, this effect is accounted for by means of the adjoint Green's function. Here the adjoint Green's function method is extended to nonaxisymmetric mean flows. The adjoint Green's function is first recast into the solution of a sound scattering problem. The sound scattering problem is then solved computationally by computational aeroacoustics methods. Extensive comparisons between calculated and experimentally measured jet noise spectra are presented. They include both rectangular and elliptic jets at supersonic and subsonic Mach numbers. Good agreements are found even for jets with very large aspect ratio.
Lighthill's Acoustic Analogy has been the dominant theory of aeroacoustics especially jet aeroacoustics for almost fifty years. As yet, except for the u8 scaling law, which was derived by dimensional analysis, jet noise prediction based on the Acoustic Analogy approach has not been particularly successful. This paper examines some of the weaknesses in the formulation of the Acoustic Analogy theories. It is concluded that if the analogy is carried out completely, in the sense that the full wave propagation terms are retained in the propagation part of the equations of the analogy, then the theory offers no sensible noise source terms. An example involving an initial-value aeroacoustic problem is presented. This example shows unambiguously that the Lighthill quadrupole noise sources are not the real noise source of the problem.
The effectiveness of jet noise reduction by the use of different nozzle exit geometry is examined. Because there will be thrust loss associated with a nozzle of complex geometry, consideration is confined to practical configurations with reasonably small thrust loss. Only jets with a single stream are considered. The nozzle configurations examined are circular, elliptic, and rectangular. Plug nozzles as well as a suppressor nozzle are included. It is shown that the measured turbulent mixing noise of the jets from these nozzles consists of two independent components. The noise spectrum of each component is found to fit the shape of a seemingly universal similarity spectrum. It is also found that the maximum levels of the fitted noise power spectra of the jets are nearly the same. This finding suggests that nozzle geometry modification may not be an effective method for jet noise suppression.
A theory is initiated, based on the equations of motion of a gas, for the purpose of estimating the sound radiated from a fluid flow, with rigid boundaries, which as a result of instability contains regular fluctuations or turbulence. The sound field is that which would be produced by a static distribution of acoustic quadrupoles whose instantaneous strength per unit volume is rho vivj + pij - a02rho delta ij, where rho is the density, vi the velocity vector, pij the compressive stress tensor, and a0 the velocity of sound outside the flow. This quadrupole strength density may be approximated in many cases as rho 0vivi. The radiation field is deduced by means of retarded potential solutions. In it, the intensity depends crucially on the frequency as well as on the strength of the quadrupoles, and as a result increases in proportion to a high power, near the eighth, of a typical velocity U in the flow. Physically, the mechanism of conversion of energy from kinetic to acoustic is based on fluctuations in the flow of momentum across fixed surfaces, and it is explained in section 2 how this accounts both for the relative inefficiency of the process and for the increase of efficiency with U. It is shown in section 7 how the efficiency is also increased, particularly for the sound emitted forwards, in the case of fluctuations convected at a not negligible Mach number.
The theory of sound generated aerodynamically is extended by taking into account the statistical properties of turbulent airflows, from which the sound radiated (without the help of solid boundaries) is called aerodynamic noise. The theory is developed with special reference to the noise of jets, for which a detailed comparison with experiment is made (section 7 for subsonic jets, section 8 for supersonic ones). The quadrupole distribution of part I (Lighthill 1952) is shown to behave (see section 3) as if it were concentrated into independent point quadrupoles, one in each 'average eddy volume'. The sound field of each of these is distorted, in favour of downstream emission, by the general downstream motion of the eddy, in accordance with the quadrupole convection theory of part I. This explains, for jet noise, the marked preference for downstream emission, and its increase with jet velocity. For jet velocities considerably greater than the atmospheric speed of sound, the 'Mach number of convection' Mc may exceed 1 in parts of the jet, and then the directional maximum for emission from these parts of the jet is at an angle of sec-1 (Mc) to the axis (section 8). Although turbulence without any mean flow has an acoustic power output, which was calculated to a rough approximation from the expressions of part I by Proudman (1952) (see also section 4 below), nevertheless, turbulence of given intensity can generate more sound in the presence of a large mean shear (section 5). This sound has a directional maximum at 45 degrees (or slightly less, due to the quadrupole convection effect) to the shear layer. These results follow from the fact that the most important term in the rate of change of momentum flux is the product of the pressure and the rate of strain (see figure 2). The higher frequency sound from the heavily sheared mixing region close to the orifice of a jet is found to be of this character. But the lower frequency sound from the fully turbulent core of the jet, farther downstream, can be estimated satisfactorily (section 7) from Proudman's results, which are here reinterpreted (section 5) in terms of sound generated from combined fluctuations of pressure and rate of shear in the turbulence. The acoustic efficiency of the jet is of the order of magnitude 10-4 M5, where M is the orifice Mach number. However, the good agreement, as regards total acoustic power output, with the dimensional considerations of part I, is partly fortuitous. The quadrupole convection effect should produce an increase in the dependence of acoustic power on the jet velocity above the predicted U8 law. The experiments show that (largely cancelling this) some other dependence on velocity is present, tending to reduce the intensity, at the stations where the convection effect would be absent, below the U8 law. At these stations (at 90 degrees to the jet) proportionality to about U6\cdot 5 is more common. A suggested explanation of this, compatible with the existing evidence, is that at higher Mach numbers there may be less turbulence (especially for larger values of nd/U, where n is frequency and d diameter), because in the mixing region, where the turbulence builds up, it is losing energy by sound radiation. This would explain also the slow rate of spread of supersonic mixing regions, and, indeed, is not incompatible with existing rough explanations of that phenomenon. A consideration (section 6) of whether the terms other than momentum flux in the quadrupole strength density might become important in heated jets indicates that they should hardly ever be dominant. Accordingly, the physical explanation (part I) of aerodynamic sound generation still stands. It is re-emphasized, however, that whenever there is a fluctuating force between the fluid and a solid boundary, a dipole radiation will result which may be more efficient than the quadrupole radiation, at least at low Mach numbers.
Subsonic jet noise from nonaxisymmetric and tabbed nozzles are investigated experimentally and theoretically. It is shown that the noise spectra of these jets are in good agreement with the similarity spectra found empirically earlier by Tam et al. through a detailed analysis of supersonic jet noise data. Furthermore, the radiated noise fields of the jets under study, including elliptic and large-aspect-ratio rectangular jets, are found to be quite axisymmetric and are practically the same as that of a circular jet with the same exit area. These experimental results strongly suggest that nozzle geometry modification into elliptic or rectangular shapes is not an effective method for jet noise suppression. A lobed nozzle, on the other hand, is found to impact significantly the noise field. Noise from large-scale turbulent structures, radiating principally in the downstream direction, is effectively suppressed. Tabs also impact the noise field, primarily by shifting the spectral peak to a higher frequency. A jetlets model is developed to provide a basic understanding of the noise from tabbed jets. The model predicts that the noise spectrum from a jet with N tabs (N≥2) can be obtained from that of the original jet (no tab) by a simple frequency shift. The shifted frequency is obtained by multiplying the original frequency by N1/2. This result is in fairly good agreement with experimental data.
Measurements of subsonic jet noise made on a model jet rig in the anechoic chamber ofthe National Gas Turbine Establishment are presented. Jet noise spectra for three nozzles of diameters 2·84, 2·4 and 1·52 inches have been obtained. Considerable care has been taken to ensure that unwanted noise from sources such as valves and ducting upstream of the nozzle are insignificant compared with the jet mixing noise.
Attempts have then been made to collapse all the data points obtained on to one curveThis is done first of all by normalizing the 1/3 octave and overall sound pressure levels for the theoretical parameters obtained from Lighthill's theory. Empirical schemes are then presented to collapse the data for all the angles and the frequencies. The prediction schemes for 1/3 octave SPL, PWL and OASPL and OAPWL are considered. The schemes presented predict noise accurately for cold and clean jets.
The theory initiated by Lighthill (1952) for the purpose of estimating
the sound radiated from a turbulent fluid flow is extended to deal with
both the transonic and supersonic ranges of eddy convection speed. The
sound is that which would be produced by a distribution of convected
acoustic quadrupoles whose instantaneous strength per unit volume is
given by a turbulence stress tensor, Tij. At low subsonic
speeds the radiated intensity increases with the eighth power of
velocity although quadrupole convection augments this basic dependence
by a factor |1 - M cos θ|-5, where M is the eddy
convection Mach number and θ the angular position of an
observation point measured from the direction of eddy motion. At
supersonic speeds the augmentation factor becomes singular whenever the
eddy approaches the observation point at sonic velocity, M cos θ =
1. At that condition a quadrupole degenerates into its constituent
simple sources, for each quadrupole element moves with the acoustic wave
front it generates and cancelling contributions from opposing sources,
so essential in determining quadrupole behaviour, cannot combine but are
heard independently. This simple-source radiation is likened to a type
of eddy Mach wave whose strength increases with the cube of a typical
flow velocity. When quadrupoles approach the observer with supersonic
speed sound is heard in reverse time, but is once again of a quadrupole
nature and the general low-speed result is again applicable. The
limiting high-speed form of the convection augmentation factor is |M cos
θ|-5 which combines with the basic eighth power
velocity law to yield the result that radiation intensity increases only
as the cube of velocity at high supersonic speed. The mathematical
theory is developed in some detail and supported by more physical
arguments, and the paper is concluded by a section where some relevant
experimental evidence is discussed.
Measurements of the noise field from a 25 mm diameter subsonic air jet are presented. These results are analysed in some detail by determining both the jet velocity dependence and the directivity of the intensity of the radiation in 1/3-octave bands at particular values of the frequency parameter,
\[
(fD/V_J)(1-M_c\cos\theta).
\]
This procedure should ensure that a particular source in a geometrically similar position in the jet is always observed, whatever the jet velocity, diameter and emission angle.
Measurements of the noise field from three 1.52, 2.4 and 2.84 inch diameter subsonic cold-air jets are presented. The tests were conducted in the anechoic chamber of National Gas Turbine Establishment, Pyestock, on a test rig with a very large contraction ratio (maximum 250:1, minimum 70:1) and with an air supply system which has been shown to produce a jet having a low turbulence level just upstream of the nozzle exit and whose noise is dominated by pure jet mixing noise. Comparison of the results is made with the theory by analysing them in detail by determining both the jet velocity dependence and the directivity of the intensity of the radiation in 1/3 octave bands at particular values of the Doppler corrected Strouhal frequency given by (fD/VJ) (I-Mccosθ).
When a free jet (or open jet) is used as a wind tunnel to simulate the effects of flight on model noise sources, it is necessary to calibrate out the effects of the free jet shear layer on the transmitted sound, since the shear layer is absent in the real flight case. In this paper, a theoretical calibration procedure for this purpose is first summarized; following this, the results of an experimental program, designed to test the validity of the various components of the calibration procedure, are described. The experiments are conducted by using a point sound source located at various axial positions within the free jet potential core. By using broadband excitation and cross-correlation methods, the angle changes associated with ray paths across the shear layer are first established. Measurements are then made simultaneously inside and outside the free jet along the proper ray paths to determine the amplitude changes across the shear layer. It is shown that both the angle and amplitude changes can be predicted accurately by theory. It is also found that internal reflection at the shear layer is significant only for large ray angles in the forward quadrant where total internal reflection occurs. Finally, the effects of sound absorption and scattering by the shear layer turbulence are also examined experimentally.
Thesis (doctoral)--Technical University of Denmark, 1983. Summary in Danish. Bibliography: p. 113-117.
The acoustical radiation characteristics of a high aspect ratio, cold subsonic, model slot jet have been investigated experimentally. Acoustical measurements were made in both a free-field environment and a reverberation field environment. Mean velocity distribution of the slot jet was determined to provide information on the mixing characteristics of the flow. Free-field results obtained reflected a characteristic asymmetrical directivity pattern of the slot jet noise. The total radiated sound power yielded a typical U to the 7-th power dependence on mean jet velocity. The experimental findings are compared with data from similar studies and also with radiation characteristics of circular cold subsonic jets.
It is argued that because of the lack of intrinsic length and time scales in the core part of the jet flow, the radiated noise spectrum of a high-speed jet should exhibit similarity. A careful analysis of all the axisymmetric supersonic jet noise spectra in the data-bank of the Jet Noise Laboratory of the NASA Langley Research Center has been carried out. Two similarity spectra, one for the noise from the large turbulence structures/instability waves of the jet flow, the other for the noise from the fine-scale turbulence, are identified. The two similarity spectra appear to be universal spectra for axisymmetric jets. They fit all the measured data including those from subsonic jets. Experimental evidence are presented showing that regardless of whether a jet is supersonic or subsonic the noise characteristics and generation mechanisms are the same. There is large turbulence structures/instability waves noise from subsonic jets. This noise component can be seen prominently inside the cone of silence of the fine-scale turbulence noise near the jet axis. For imperfectly expanded supersonic jets, a shock cell structure is formed inside the jet plume. Measured spectra are provided to demonstrate that the presence of a shock cell structure has little effect on the radiated turbulent mixing noise. The shape of the noise spectrum as well as the noise intensity remain practically the same as those of a fully expanded jet. However, for jets undergoing strong screeching, there is broadband noise amplification for both turbulent mixing noise components. It is discovered through a pilot study of the noise spectrum of rectangular and elliptic supersonic jets that the turbulent mixing noise of these jets is also made up of the same two noise components found in axisymmetric jets. The spectrum of each individual noise component also fits the corresponding similarity spectrum of axisymmetric jets.
The acoustic source regions of a subsonic cold jet were traced as far as the periphery of the jet by means of the pressure field on a plane rigid surface located in the vicinity of the jet. The centers of these regions were determined to be from 4 to 18 diani downstream of the exit. Additional measurements were made of flow variables within the jet. From these measurements, the downstream kinetic energy dissipation rate was calculated. Finally, the acoustic radiation characteristics of several simple nozzle configurations were measured. Results indicate that, although considerable alternating of directivity and spectral content of the noise result from varying nozzle geometry, only limited reductions in total sound power resulted. © 1971, American Institute of Aeronautics and Astronautics, Inc., All rights reserved.
An experimental program on a model of the F-18 airplane has been conducted to determine the performance of nonaxisymmetric nozzles relative to the aircraft's baseline axisymmetric nozzle at Mach numbers from 0.60 to 2.20. The performance of a two-dimensional convergent-divergent nozzle, a single expansion ramp nozzle (ADEN) and a wedge nozzle were compared to the baseline axisymmetric nozzles. The nonaxisymmetric nozzles (except ADEN) were designed for vectoring and reversing. The axisymmetric nozzle did not have these capabilities. The comparisons presented here are for the nozzles in their full forward thrust mode and for the aircraft at zero angle of attack. The results demonstrate that nonaxisymmetric nozzles can be installed on a close-spaced twin engine fighter with equal or higher performance than the axisymmetric nozzle over the range of Mach numbers tested.
A modeling technique for predicting the axial and transverse velocity characteristics of rectangular nozzle plumes is developed. In this technique, modeling of the plume cross section is initiated at the nozzle exit plane. The technique is demonstrated for the plume issuing from a rectangular nozzle having an aspect ratio of 6.0 and discharging into quiescent air. Application of the present procedures to a nozzle discharging into a moving airstream (flight effect) are then demonstrated. The effects of plume shear layer structure modification on the velocity flowfield are discussed and modeling procedures are illustrated by example.
North Atlantic Treaty Organization Advisory Group for Aeronautical Researchand Development
- M J Lighthill