Far-field Noise Characteristics Research Articles

Slat noise measurements in open-jet, hard-wall and hybrid wind tunnel test sections

This paper assesses the comparability of aeroacoustic slat noise measurement in an open-jet, a hard-wall, and a hybrid test section configuration. The 30P30N common research model was tested in the Aeroacoustic Wind Tunnel of the University of Twente at a chord-based Reynolds number up to 1⋅106 and free-stream Mach number 0.15. Pressure distribution measurements were used to determine the aerodynamic similar condition of the flow in the slat cove. These conditions are subsequently validated by time-averaged 2D PIV measurements. The experiments show that general aspects of the flow in the slat cove are similar when the Cp distribution around the leading-edge slat element in each test section configuration is comparable. Overall, the shear layer path and reattachment point are in close agreement, showing that the aeroacoustic noise production mechanisms are comparable. Microphone phased array measurements were subsequently conducted to compare the far-field noise characteristics originating from the slat in each test section configuration. A framework is proposed to correct the acoustic measurements of each test section configuration to a standard free-field condition. Transmission loss corrections are required for the microphone measurements in the hard-wall and hybrid test section configurations, whereas a coherence loss model was validated and applied to correct the microphone measurements in the open-jet. The measured noise spectra in each test section configuration show differences up to approximately 5 dB. In general the differences were found to be smaller. The largest differences are seen for small angles of attack with noise levels in the open-jet test section showing considerably higher values than the hard-wall test section and hybrid test section results. In addition, the behavior of the vortex shedding hump in the high frequency range is different in the open-jet measurements, which could be related to the larger noise level differences at small angles of attack.

Study on prediction in far-field aerodynamic noise of long-marshalling high-speed train.

It is still difficult to conduct numerical calculation of the aerodynamic noise of full-scale, long-marshalling, high-speed trains. Based on the Lighthill acoustic analogy theory, the aerodynamic sound source of the high-speed train is equivalent to countless micro-vibrating sound sources. An acoustic radiation model of the dipole sound source of high-speed trains is established, and a method to predict the aerodynamic noise in the far field of long-marshalling high-speed trains is proposed. By this method, combined with numerical simulation technology, the flow field, noise source, and far-field noise characteristics of high-speed trains with different marshalling numbers are studied. The improved delayed detached eddy simulation method is used for flow field calculation, to obtain aerodynamic noise source information regarding the surface of high-speed trains. The numerical calculation method is verified by wind tunnel testing. The results show that the flow field and noise source characteristics of high-speed trains with different marshalling numbers are similar. The greater the length of the train body, the longer the trailing distance of the train wake, and the stronger of a surface noise source the tail car becomes. The spatial distribution characteristics of aerodynamic noise in the far field of high-speed trains do not change significantly with the length of the train body, but the magnitude of the sound pressure level will increase with the increase in length of the train body. The middle car body parts of high-speed trains with different marshalling numbers have similar noise distributions and sound pressure levels. Based on the noise calculation results of the 3-marshalling high-speed train, the far-field noise of the 5-marshalling and 8-marshalling train models is predicted and found to be in good agreement with the far-field noise of the actual train model. The differences in average sound pressure level are 1.01 dBA and 1.74 dBA, respectively.

Effect of a Chevron Nozzle on Noise Radiation from a Compressible Jet

The effect of a chevron on a radiated (acoustic) component of near-field pressure fluctuations in a high-speed compressible jet is experimentally investigated for a nozzle exit Mach number of 0.8. The main focus in the present experimental study is to examine the effect of a chevron on noise sources, their location in the jet flowfield, and the noise radiation pattern in the far-field through simultaneous measurement of near-field and far-field pressure fluctuations using a cross-correlation technique. The formation of lobes in the mixing layer close to the chevron nozzle exit alters the near-field and far-field noise characteristics as compared to the base nozzle. The overall sound pressure level closer to the chevron nozzle exit increases, whereas downstream of the potential core, it reduces in comparison with the base nozzle. Furthermore, the position of the near-field/far-field cross-correlation coefficient peak suggests that the interaction of chevrons with the jet flow moves the dominant noise source upstream and closer to the nozzle exit. The chevron nozzle disintegrates large-scale flow structures, thereby reducing the noise levels radiating at a lower polar angle in the far-field. The near-field/far-field cross-correlations suggest that an acoustic component of the near-field pressure fluctuations, propagating from the jet to the higher polar angles in the far-field, is generated by fine-scale turbulent flow structures in the region close to the nozzle exit, and the chevron nozzle is found to strongly support this mechanism. An increase in the noise levels closer to the nozzle exit in the near-field and at a higher polar angle in the far-field are due to high-frequency noise sources generated by the chevron nozzle.

Experimental study on aerodynamic noise characteristics of high-lift configuration with variable gap leading-edge slat

The aircraft high-lift devices are important components to ensure the safety of take-off and landing, and it is also the main source of airframe noise in the process of take-off and landing, especially the leading-edge slat. The slat that reduces gap by rotating with the fixed axis is proposed in this paper, and experimental study of far-field aerodynamic noise characteristics of high-lift configuration with the slat is also carried out. The experimental results show that discrete tones in the low-middle frequency range disappear and the broadband frequency amplitude also slightly decline, resulting in a significant decrease in the overall sound pressure level of the high-lift configuration with slat gap of 0, compared with the configuration with conventional leading-edge slat. However, in the process of rotation from conventional slat to seamless slat, more prominent discrete tone in low-middle frequency range or humps in mid-high frequency range appear at angles of attack below 6°, which give rise to the overall increase in the sound pressure level. At angles of attack above 6°, these aero-acoustic phenomena are not obvious. Hence, the overall sound pressure levels of high-lift configuration gradually decrease with the rotation process in larger angles of attack.

Numerical study on the aerodynamic noise characteristics of CRH2 high-speed trains

The aerodynamic noise of high-speed trains not only causes interior noise pollution and reduces the comfort of passengers, but also seriously affects the normal life of residents. With the increase of running speed of trains, aerodynamic noises will be more than wheel-rail noises and become the main noise source of high-speed trains. This paper established a computational model for the aerodynamic noise of a CRH2 high-speed train with 3-train formation including 3 train bodies and 6 bogies, adopted the detached eddy simulation (DES) to conduct numerical simulation for the flow field around the high-speed train, applied Ffowcs Williams-Hawkings acoustic model to conduct unsteady computation for the aerodynamic noise of high-speed trains, and analyzed the far-field aerodynamic noise characteristics of high-speed trains. Studied results showed: The main energy of the complete train was mainly within the range of 613 Hz-2500 Hz when the high-speed train ran at the speed of 350 km/h. In the whole frequency domain, it was a broadband noise. Regarding the longitudinal observation point which was 25 m away from the center line of track and 6m away from the nose tip of head train, the sound pressure level of total noises reached the maximum value 88.9 dBA. The maximum sound pressure level of the noise observation point which was 7.5 m away from the center line of track was around the first bogie of head train. Various components made different contributions to the aerodynamic noise of the complete train, and the order was head train, mid train, bogie system (6 bogies) and tail train. The first bogie of head train made the greatest contribution to bogie system and was the main aerodynamic noise source of the complete train.

Numerical simulation of the slipstream and aeroacoustic field around a high-speed train

The flow field and sound propagation around a three-coach 1/8th scale high-speed passenger train were obtained using a detached-eddy simulation and the Ffowcs-Williams and Hawkings acoustic analogy. The Reynolds number of flow based on the train height and speed was 2,000,000. The numerical results of the flow and aeroacoustic fields were validated using wind tunnel experiments and full-scale data, respectively. Features of overall sound pressure level, sound pressure level and A-weighted sound pressure level of typical measuring points are discussed. The sound propagated by a high-speed train is shown as a broadband noise spectrum including tonal component, where high sound pressure levels are concentrated on the low-frequency range from 10 Hz to 300 Hz. The inter-carriage gap is found to cause distinct tonal noise in contrast to the other parts of the train that cause a broadband noise. The negative log law has been used to study the influence of distance from the centre of track on the sound pressure level, where a good fit is shown at low-frequency ranges. The peak values of A-weighted sound pressure level from both full-scale experiment and simulation results occur at approximately 1 kHz, where simulation results show almost the same range as the experiment. The surface of each component of the train as well as the whole train are chosen as the integral surface for the Ffowcs-Williams and Hawkings computation of the far-field noise characteristics. It was found that the sound source generated by a high-speed train is mainly dipole, and the largest noise was obtained from the leading bogie. The results of this paper provide, for the first time, a better understanding of the aeroacoustic field around a three-coach train model, and the paper has the potential to assist engineers to design high-speed trains with aeroacoustic noise reduction in a better manner.

Slat Noise Suppression with Mass Injection

The efficiency and mechanism of slat noise suppression with upstream mass injection was investigated through validated numerical simulations. Delayed detached-eddy simulation was used to describe the noise sources and Ffowcs Williams–Hawkings integration was carried out to obtain the far-field noise characteristics. Outward mass injection from the suction side of the slat near the cusp uplifted the free shear layer, and the corresponding impingement of free shear layer with slat pressure surface was alleviated. The tonal component of the far-field noise was diminished, and the suppression of the broadband noise was also observed. The inward injection of mass at the pressure side of the slat near the cusp facilitated the three-dimensional flow and the breakup of large-scale spanwise vortices, weakening the strength of the free shear layer, especially at the vicinity of the reattachment point. However, the introduction of strong turbulence at the downstream of the injection slot offset the benefits gained from the suppression of unsteadiness at the reattachment area. Although the low-frequency sound pressure level was lowered, two new strong peaks arose at higher frequencies, which contributed to a higher overall sound pressure level at the upstream direction below the airfoil.

Investigating Correlations Between Reynolds-Averaged Flowfields and Noise for Forced Mixed Jets

Jet engine manufacturers have known for some time that mixer design has a strong influence on jet noise, but the correlation between the two is unclear. The Reynolds-averaged Navier Stokes (RANS) computed steady-state flowfields of several mixers for which the far-field noise characteristics are known are analyzed. Three mixers with different lobe penetration heights are examined. The WIND computational fluid dynamics (CFD) code is used and compared with results from recent experiments conducted at the NASA John H. Glenn Research Center at Lewis Field. The Menter shear stress transport turbulence model and computational meshes of 1.9 million grid points are used for the comparisons. The turbulent kinetic energy profiles predicted by CFD are in good qualitative agreement with experimental data. Inferences are also made as to what correlations can be drawn between the RANS computed flowfield and the experimentally determined far-field noise and how this information may be used as an aid in mixer design. Nomenclature

Aeroacoustic Characterization, Noise Reduction, and Dimensional Scaling Effects of High Subsonic Jets

A comprehensive far-field acoustics database was generated for high subsonic, turbulent jets along with mean total pressure and temperature surveys in the flowfield. The effects of core jet temperature and of external coflow on the far-field noise and mean flowfield characteristics are investigated. The results show peak jet noise generation in aft angles (110 < θ < 150 deg). The mean velocity measurements indicate an average potential core length in the range of 3-8 jet diameters D j , depending on the core jet flow temperatures and speed of external tunnel coflow, after which self-similar velocity decay is evidenced. Mixing devices (tabs) were used to enhance near-field jet mixing and were explored as a means for far-field noise reduction. These devices decreased the peak jet noise but were accompanied by increases in high-frequency noise. The effect of scaling from model-scale to engine-scale conditions shifts spectral peaks to lower frequencies, thereby driving high-frequency mixing noise increases (due to tab-generated flows) into the peak annoyance range.

Aeroacoustic properties of a supersonic diamond-shaped jet

The potential for using a novel diamond-shaped nozzle, which may allow superior mixing and noise characteristics of supersonic jets without significant thrust losses, is explored. Flow visualization and pressure measurements indicate the presence of distinct structures in the shear layers, not normally observed in shear layers of ideally expanded axisymmetric and rectangular jets. The characteristics of these features suggest that they are a manifestation of significant streamwise vorticity in the shear layers. Despite the distinct nature of the flowfield structure of the diamond jet, the far-field noise characteristics are quite similar to those of the corresponding axisymmetric jet, except moderate mixing-noise reduction in the aft quadrant. Furthermore, the global shear-layer growth rates are also very similar to those of the axisymmetric and two-dimensional counterparts. These observations lead one to believe that the presence of streamwise vorticity may not play a significant role on the overall mixing and noise generation of a supersonic jet.

A review of advanced turboprop transport aircraft

The application of advanced technologies shows the potential for significant improvement in the fuel efficiency and operating costs of future transport aircraft envisioned for operation in the 1990s time period. One of the more promising advanced technologies is embodied in an advanced turboprop concept originated by Hamilton Standard and NASA and known as the propfan. The propfan concept features a highly loaded multibladed, variable pitch propeller geared to a high pressure ratio gas turbine engine. The blades have high sweepback and advanced airfoil sections to achieve 80 percent propulsive efficiency at M=0.80 cruise speed. Aircraft system studies have shown improvements in fuel efficiency of 15–20 percent for propfan advanced transport aircraft as compared to equivalent turbofan transports. Beginning with the Lockheed C-130 and Electra turboprop aircraft, this paper presents an overview of the evolution of propfan aircraft design concepts and system studies. These system studies include possible civil and military transport applications and data on the performance, community and far-field noise characteristics and operating costs of propfan aircraft design concepts. NASA Aircraft Energy Efficiency (ACEE) program propfan projects with industry are reviewed with respect to system studies of propfan aircraft and recommended flight development programs.

Effect of initial condition on subsonic jet noise

The initial boundary-layer state can significantly affect the radiated noise from an axisymmetric jet. Jets with initially laminar boundary layers are found to emit more noise. Thus, 'cleaner' far-field noise characteristics are achieved in tripped jets. Data suggest that the additional noise in the initially laminar case partly originates from the first stage of pairing of the coherent shear-layer vortices.

Jet Noise Modification by the "Whistler Nozzle".

The farfield noise characteristics of a subsonic whistler nozzle jet are measured as a function of Mach number (0.25, 0.37, and, 0.51), emission angle, and excitation mode. It is shown that a whistler nozzle has greater total and broadband acoustic power than an excited contraction nozzle; and that the intensity of far-field noise is a function of emission angle, Mach number, and whistler excitation stage. The whistler nozzle excitation produces broadband noise amplification with constant spectral shape; the broadband noise amplification (without associated whistler tones and harmonics) increases omnidirectionally with emission angle at all Mach numbers; and the broadband amplification factor decreases as Mach number and emission angle increase. Finally the whistler nozzle is described as a very efficient but inexpensive siren with applications in not only jet excitation but also acoustics.

Noise and Surface Pressure Response of an Airfoil to Incident Turbulence

A theoretical and experimental investigation of the nosie and unsteady surface pressure characteristics of an isolated airfoil in a uniform mean velocity, homogeneous, nearly isotropic turbulence field was conducted. Experiments were performed with a 23-cm chord, two-dimensional, NACA 0012 airfoil in the UTRC Acoustic Research Tunnel over a Mach number range of 0.1 to 0.5. Far-field noise spectra and directivity as well as surface pressure spectra and cross-spectra were obtained. Incident turbulence statistics were documented. Theory applied to predict far-field noise and surface pressure characteristics from measured inflow turbulence statistics showed good agreement with measurement over the dominant frequency range for all Mach numbers investigated. The theoretical formulation represents a first-principles solution providing absolute level prediction without recourse to empirical or adjustable constants. It takes into account compressibility as well as source noncompactness effects. Correlation measurements demonstrated that all chordwise portions of the airfoil radiated directly to the far-field, but that the leading edge was the dominant noise producing region. b c c0 d / g kx,ky,kz

Cross‐correlation methods for studying near‐ and farfield noise characteristics of flow–surface interactions

Systematic methods based on cross‐correlation techniques are presented for experimental studies of near‐ and farfield noise characteristics in airflow–surface interaction problems. Analyses show that, in near‐ and farfields, the noise characteristics due to the surface contribution of fluctuating pressures and velocities and due to the volume contribution of the turbulence in the flow can be determined separately. Both farfield noise intensities and nearfield acoustic energy fluxes can be expressed in terms of appropriate cross correlations. These correlations can be obtained by making microphone measurements in the farfield, in the nearfield, and on the surface. Examples of the applications to the noise field generated by impinging jets, by surface blowing jets, and by turbulent flow over trailing edges are presented. Advantages of the present method over conventional methods are also discussed.

Cross-correlation methods for studying near- and farfield noise characteristics of several flow-surface interaction problems

Experimental methods based on well-known cross-correlation techniques are proposed for studying the near- and farfield noise characteristics of several flow-surface interaction problems. By using three kinds of microphone measurements—one flush-mounted on the surface, one located in the nearfield, and one placed in the farfield-farfield noise intensities and nearfield acoustic energy fluxes can be obtained. In both near- and farfields, the noise characteristics due to the flow and the surface contributions can be determined separately. Specific applications to the noise generated by impinging jets, by surface blowing jets, and by turbulent flow over trailing edges are discussed. These methods may prove useful for understanding detailed noise characteristics of STOL aircraft employing the externally blown flap, the augmented wing, or the upper surface blowing, which configurations are now under exploratory development.