Airframe Noise Research Articles

Improvement of noise calculation method considering aerodynamic noise condition during aircraft landing

Recently, while aircraft engine noise has become quieter, aerodynamic noise such as airframe noise, gear, and flaps have become relatively louder. In addition, the WHO environmental noise guidelines for the European region require consideration of small noise effects, and it has become necessary to increase the validity of calculations at distant airports, where aerodynamic noise fluctuations are large. Therefore, Nakazawa et al. reported a method to improve the validity of noise prediction for distant airports by correcting the existing noise calculation method to take aerodynamic noise into account. However, the predicted results deviated to a smaller extent than the measured results, indicating that the correction method needs to be improved. This method was improved, and using the results of highly accurate microphone array measurements with 195 microphones, NPD data for correction was prepared from aircraft noise by component using J-FRAIN, taking aerodynamic noise

The mitigation of airframe noise using a Krueger flap as a leading-edge device in a high-lift configuration

High-lift device is a potent airframe noise contributor. The conventional slat, commonly used in a high-lift device, is known as one of the dominant noise sources. Here, we combine an experimental and a numerical approach to investigate the noise generation of a conventional slat and two Krueger flaps as leading-edge devices in a high-lift configuration. To reduce the computational cost, a short span of the entire high-lift device is selected as the focusing region for scale-resolving and acoustic computations using Improved Delayed Detached Eddy Simulation (IDDES) coupled with the Ffowcs-Williams and Hawkings (FWH) analogy. Simulation results show that turbulent cross flows induce slightly lower-level noise spectra at frequencies less than 300 Hz when the two spanwise sides of the FWH permeable integral surface are closed. Wind-tunnel test results show that the reference and optimal Krueger configurations effectively attenuate the dominant tone of the conventional slat by 4 and 6 dB, respectively. Besides, the optimal configuration is very effective in noise reduction in a wide frequency range. However, the reference Krueger configuration increases the noise level by 0 − 4 dB at frequencies exceeding 5, 500 Hz. Overall, the optimal Krueger configuration is a good choice for high-lift device noise mitigation.

Proposing Method for Airframe Noise Prediction Benefitting Indonesian Aircraft Design

Proposing Method for Airframe Noise Prediction Benefitting Indonesian Aircraft Design

Acoustic characterization of the NASA Langley 14- by 22-Foot Subsonic Tunnel using single-microphone analysis techniques

An experimental campaign was carried out to assess recent acoustic modifications to the NASA Langley Research Center 14- by 22-Foot Subsonic Tunnel. This effort was undertaken in preparation for future rotorcraft, Advanced Air Mobility, and airframe noise acoustic tests. The tunnel is a closed circuit and typically operates in an open-jet configuration for acoustic studies. A vertical linear array of microphones and a phased array were placed on traverses outside of the core flow. Pole-mounted acoustic sources with known waveforms were used to identify sources of reflection under static conditions. A similar process was replicated with more compact sources in an aerodynamic fairing for flow speeds up to Mach 0.16. This allowed for acoustic characterizations of the test section core flow and bounding shear layer. Periodic averaging was investigated as a method for isolating acoustic sources and was shown to be capable of isolating periodic acoustic signals even for poor signal-to-noise ratio conditions. Benefits and limitations of single-microphone processing methods are identified. A companion paper utilizes a phased array in an effort to address the identified limitations to more traditional single-microphone data collection.

Development of aircraft noise simulation framework J-FRAIN based on component-wise sound source models

In order tomake detailed and accurate predictions of aircraft noise around airports, we have developed “J-FRAIN”, a new aircraft noise simulation framework that can precisely predict the time histories of noises emitted from each major aircraft noise source during the landing approach phase at ground observation points. This article describes the elements of the developed framework—data acquisition, sound source modeling, propagation modeling, and ground noise prediction—and present some application examples. To develop the framework, we first deployed a 30m-diameter microphone array under the final approach path to an international airport to measure acoustic maps of several civil aircraft types in flight. The sound powers of major aircraft noise-emitting components were then estimated quantitatively by domain integration of the deconvolved acoustic maps at five emission angles in the plane of the glideslope, and the directivities of each noise source were determined. Next, component-wise sound source regression models for engines and airframe noise sources were created based on the physical relationships between engine rotation speed, airspeed, and the deployment angle of high-lift devices, and the coefficients in each model were determined to minimize the root-mean-square error between the measured and predicted sound power levels. The phenomena of atmospheric absorption and ground effect during the propagation of radiated sounds were also incorporated into the framework. Actual flight parameter values were used as inputs to the completed framework, and it was confirmed that the predicted time histories of sound pressure levels on the ground agreed with measured data to within 2 dB if ground effect was properly considered. Finally, as sample applications that use the unique features of the proposed J-FRAIN framework, the article discusses the evaluation of the contributions of each aircraft noise source at noise observation points under the final approach path to the airport and the impact assessment of flight operations on noise.

Pilot Assistance Systems for Energy-Optimized Approaches: Is It Possible to Reduce Fuel Consumption and Noise at the Same Time?

Air traffic has appreciable environmental impacts, especially regarding gaseous emissions and noise. Recent studies have shown that the energy management during approach is a driving factor regarding environmental impact and is especially challenging for pilots. In a previous project, a newly developed pilot assistance system called LNAS (Low Noise Augmentation System) showed the potential of energy-optimized approaches to reduce fuel consumption and noise. Within the SESAR Exploratory Research project DYNCAT, novel functions based on LNAS have been integrated in the flight management system. In this contribution, results from real-time simulations with the enhanced FMS are presented, and mitigation of the environmental impact is analyzed. It was shown that with DYNCAT, the energy management could be improved, resulting in a later configuration and engines mostly in idle. With DYNCAT, procedures were also flown more uniformly and the variability in noise and fuel outcomes was reduced. However, the results revealed a trade-off for optimizing noise and fuel consumption simultaneously, whereby both parameters can be improved along specific optimum curves. A perfect strategy to minimize noise would be to first reduce speed and only secondly height, as high speeds lead to higher levels of airframe noise and sound exposure increases with decreasing distance. In contrast, saving fuel might be achieved by reducing the flight time, as the engines consume fuel even when being in idle.

A Preliminary Evaluation of Morphing Horizontal Tail Design for UAVs

Morphing structures are a relatively new aircraft technology currently being investigated for a variety of applications, from civil to military. Despite the lack of literature maturity and its complexity, morphing wings offer significant aerodynamic benefits over a wide range of flight conditions, enabling reduced aircraft fuel consumption and airframe noise, longer range and higher efficiency. The aim of this study is to investigate the impact of morphing horizontal tail design on aircraft performance and flight mechanics. This study is conducted on a 1:5 scale model of a Preceptor N-3 Pup at its trim condition, of which the longitudinal dynamics is implemented in MATLAB release 2022. Starting from the original horizontal tail airfoil NACA 0012 with the elevator deflected at the trim value, this is modified by using the X-Foil tool to obtain a smooth morphing airfoil trailing edge shape with the same CLα. By comparing both configurations and their influence on the whole aircraft, the resulting improvements are evaluated in terms of stability in the short-period mode, reduction in the parasitic drag coefficient CD0, and increased endurance at various altitudes.

Trailing-edge far-field noise and noise source characterization in high inflow turbulence conditions.

Airframe noise currently is a bottle neck in various applications, e.g., wind energy, maritime applications, and aircraft. Airframe noise is significantly increased by the presence of inflow turbulence. High inflow turbulence influences the boundary layer and wall-pressure fluctuations close to the trailing edge of airfoils. In this research, measurements of boundary layer and wall-pressure fluctuations near the trailing edge of an airfoil are conducted to investigate how the inflow turbulence affects the trailing-edge noise generation mechanism. Far-field noise measurements of additional three airfoils are shown to understand the role of the airfoil geometry in the dominant noise source for the cases of inflow turbulence and to generalize the observed increase in trailing-edge noise. Inflow turbulence leads to an increase in both the wall-pressure spectrum and spanwise correlation length. Trailing-edge noise increases due to the inflow turbulence in the entire frequency range at least 2 dB up to more than 15 dB for all the cases. The contribution of leading- and trailing-edge noise to the total noise varies with the airfoil geometry and inflow velocity, with the trailing-edge noise dominating in a larger frequency range for the thickest airfoil and for lower velocities.

Interactional aerodynamics and acoustics of a rotor with an airframe in hover

The demand for the development of urban air mobility (UAM) powered by electric systems has been steadily rising across private and public sectors. Most UAM flights incorporate a distributed electric propulsion system to enhance aircraft safety and reliability, which entails an increase in the number of rotors or propellers. Consequently, aerodynamics and aeroacoustics are significantly influenced by strong interactions between the rotor and the airframe. In this study, we conducted a computational investigation to examine the impact of rotor–airframe interaction on aerodynamic and aeroacoustic characteristics. This examination considered variations in airframe shape and the distance between the rotor and airframe. The aerodynamic analysis was executed using the lattice-Boltzmann method simulation, in which acoustic predictions were made using the Ffowcs Williams–Hawkings(FW–H) acoustic analogy with a permeable surface. The airframe consists of two geometries: a cylinder and a cone. Tip vortex breakdown and the transition into the turbulent wake state were captured in both airframes, and a fountain flow was affected by the downwash circulation generated under certain proximity of airframe cases. The acoustic prediction results showed that high-intensity noise radiated over the broad surface of the airframe in the conical airframe case. Significant thrust force fluctuations and an increase in noise level were observed at the smallest rotor tip clearance, S/R=−0.1, compared to the isolated rotor. Furthermore, the noise contribution of the rotor and airframe was compared, revealing that the airframe noise level was even higher than the rotor noise at S/R=−0.1.

Study on noise calculation method considering gear and flap conditions during aircraft landing

Looking at recent aircraft noise by component, engine noise has become quieter, while aerodynamic noise such as airframe noise, gear, and flap noise has become relatively louder. Gradually, it has become necessary to take aerodynamic noise into account appropriately. Therefore, using the regression calculation results of aircraft noise by J-FRAIN, we examined to modify performance data and Noise-Power-Distance (NPD)data, which represent component changes consistent with the AERC model, an existing aircraft noise model in Japan. For performance data, gear and flap parameters were added to the already used altitude, speed, and thrust. For NPD data, correction NPD data were prepared for the gear and flap changes using the J-FRAIN calculation results. Using these data, noise calculations were performed considering gear and flap status during aircraft landing. In addition, aircraft landing noise was measured under a flight path of about 30 km around Narita Airport in Japan to confirm the validity of the noise calculation results. Some measured aircraft gear and flap status were obtained and the results of the matched noise calculations were compared with the measured noise.

Increase of trailing-edge noise due to inflow turbulence

Altough the regulatory imposition of low-noise limits, airframe noise currently limits the design space of several applications, such as, wind energy, marine, and aeronautics. The noise of those applications is significantly increased by the presence of inflow turbulence. It has been believed that only the leading-edge component is increased. However, recent results show that the trailing-edge noise is also increased because of the free-stream turbulence penetration in the airfoil boundary layer. This research measures the increase of trailing-edge noise due to inflow turbulence for NACA 63018 and NACA 0008 airfoils. Noise measurements were conducted for velocities ranging from 15 to 40 m/s, with and without inflow turbulence. A rod installed upstream of the airfoils generated an inflow turbulence of ≈ 12%. The wall-pressure fluctuations were measured close to the trailing edge to analyze the near-field phenomenon. The inflow turbulence increases the wall-pressure spectrum level in the entire frequency range and the spanwise correlation length in the low-frequency range. The trailing-edge noise increases due to the inflow turbulence in the entire frequency range at least 2 db up to more than 15 dB for all the cases. The contribution of leading- and trailing-edge noise to the total noise varies with the airfoil geometry, with the trailing-edge noise dominating in a larger frequency range for the thickest airfoil.

Toward Airframe Noise Reduction for Passenger Aircraft

Airframe noise is one of the main noise sources of the latest passenger aircraft during the approach phase. To demonstrate noise reduction technology based on unsteady Computational Fluid Dynamics (CFD) and wind tunnel tests, JAXA conducted flight tests using the JAXA Experimental Aircraft "Hisho" in 2016 and 2017 in cooperation with industries in Japan. The noise data obtained from flyover noise source measurements successfully showed consistent characteristics with the noise reduction designs for the flaps and the main landing gear. The noise measurement data from the flight test 2017 were used to validate the wind tunnel tests and CFD used in the design phase. Subsequently, the research activities proceeded towards the research and development targeting regional jet aircraft, aiming for practical application in passenger aircraft. This paper overviews these research activities toward developing airframe noise reduction technology for passenger aircraft.

The importance of experimental airframe noise research

Numerical methods progress rapidly in terms of capabilities and efficiency. Accordingly, one could conclude that experimental activities will sooner or later disappear. In this context the paper will exemplify the use of numerical methods for the proper design and conduct of wind tunnel experiments. Examples will cover the research topics on high lift system's aero-acoustic and landing gear noise. Finally, the paper will describe the development of selected noise reduction concepts from their first experimental assessment in wind tunnels up to being flight tested.

Expert decision support system to improve the analysis of airframe noise measurements

For the localization and estimation of aeroacoustic sources on complex aircraft models in wind tunnel measurements beamforming and related methods are well known. The task of the aeroacoustic experts is to identify and classify the relevant sources within these beamforming maps. In this paper, we present an expert decision support system to support this task. The system comprises three steps: The first step identifies aeroacoustic sources in sparse beamforming maps and extracts their corresponding spectra without manual definition of regions of interest. The second step calculates the acoustic properties of the sources based on spectral and spatial information. The third step clusters the identified sources based on these acoustic properties. In this paper, data are evaluated based on CLEAN-SC beamforming maps of two airframe half-model wind tunnel measurements. A variety of aeroacoustic features are proposed that capture the characteristics of the source spectra. The result of this expert decision support system is a list of all relevant sources clustered according to their acoustic behavior at different Mach numbers. The aim of the system is to support aeroacoustic specialists in analyzing beamforming results and identifying relevant and spurious sources in-situ.

Assessment of an Open-Source Aircraft Noise Prediction Model Using Approach Phase Measurements

An open-source simulation model for aircraft noise prediction is presented and validated using backpropagated noise measurements for a state-of-the-art engine and aircraft. The validation is focused on approach procedures and was performed using ground-based noise measurements that were taken at 17 recording stations for a total of 18 consecutive flights carried out during the morning of 8 April 2021. The flights were performed using two A321neo aircraft with LEAP-1A engines. It is demonstrated that the presented noise model provides a satisfactory estimation of the source noise for varying approach configurations and flight conditions. Configurations using a greater number of high-lift devices are particularly well predicted in the mid- and high-frequency regions, whereas the lower configuration settings show greater spectral deviations, which are partly attributed to measurement uncertainties caused by the increased aircraft–microphone distance. The model can predict the overall mean total sound intensity level within a 2 dB accuracy for all configurations, while the average predicted level at each microphone differs by less than 3 dB from the measurement average, for all cases except one. Variation in aircraft speed showed to have a strong impact on the predicted total noise, which matches the well-recognized sixth-power Mach number far-field sound intensity scaling law for airframe noise models, while the measurements indicated a less significant dependency. This is mainly due to installation effects and noise reduction measures that are not included in the models. Nevertheless, the variations in the spectra of the predicted and measured noise showed similar patterns.

A novel hybrid method for aerodynamic noise prediction of high-lift devices

Aerodynamic noise of High-Lift Devices (HLDs) is one of the main sources of airframe noise, and has immediate impacts on the airworthiness certification, environmental protection and security of commercial aircraft. In this study, a novel hybrid method is proposed for the aerodynamic noise prediction of HLD. A negative Spalart-Allmaras (S-A) turbulence model based Improved Delayed Detached Eddy Simulation (IDDES) method coupling with AFT-2017b transition model is developed, in order to elaborately simulate the complex flow field around the HLD and thus obtain the information of acoustic sources. A Farassat-Kirchhoff hybrid method is developed to filter the spurious noise sources caused by the vortex motions in solving the Ffowcs Williams-Hawkings (FW-H) equation with permeable integral surfaces, and accurately predict the far-field noise radiation of the HLD. The results of the 30P30N HLD indicate that, the computational Sound Pressure Levels (SPLs) obtained by the Farassat-Kirchhoff hybrid method conform well with the experimental ones in the spectrum for the given observation point, and are more accurate than those obtained by the Farassat 1A method. Based on the hybrid method, the acoustic directivity of the HLD of a commercial aircraft is obtained, and the variation of the SPLs in the spectrum with the deflection angle of the slat is analyzed.

A Numerical Study on the Flap Side-Edge Noise Reduction Using Passive Blowing Air Concept

The flap side-edge is a vital contributor to airframe noise. In this study, we propose a novel flap side-edge noise reduction method based on the concept of active blowing air. A long slot is opened from the flap’s lower surface to the tip surface to induce a secondary jet flow, which is driven by the local pressure difference between the flap’s lower surface and the tip surface. The unsteady flow field around the flap side-edge was computed by the lattice Boltzmann solver PowerFLOW, and the far-field noise was predicted by the FW-H equation. It is demonstrated that the dominant features of the flap side-edge flow are the double vortex structures, and the new passive blowing air reduction method can achieve about 3.3 dB noise reduction. Moreover, the underlying noise reduction mechanism has been analyzed and revealed. It is shown that the secondary jet flow from the long slot on the flap side-edge would dissipate the flap side-edge vortex and displace the flap side-edge vortical structure away from the flap surface, thus resulting in a decrease in the pressure fluctuations on the flap side-edge surface. As a result, the flap side-edge noise was reduced. In contrast to the current active air blowing technique, the newly proposed blowing air technique is passive and quite simple and does not require an extra air source or control system. This novel flap side-edge noise reduction technology provides a new flow control strategy and noise reduction methodology and can be further optimized.

Progress in understanding seismic wind noise

Previous work at NCPA on predicting and measuring wind noise in the ground used a quasistatic calculation employing Shield’s wind noise correlation function. This method predicted the vertical displacements at the surface well, but under predicted the horizontal displacement and predicted a rapid decay of levels with depth that was not observed in the measurements. A new measurement of the wind noise correlation coefficients using flush mount microphones was presented at the Seattle Meeting (2aPa1). In this talk additional measurements and improved methods for evaluating the coefficients are described. An improved calculation of the seismic levels due to wind induced pressures at the ground has been developed based on the wavenumber-frequency analysis developed for studying airframe noise due to turbulence. This calculation correctly predicts the surface vertical component and the near constant seismic level in the first 40cm of the ground. It still under predicts the horizontal component, but the under prediction is smaller than that predicted by the quasistatic calculation. [This research was sponsored by the Department of the Navy, Office of Naval Research under ONR Award No. N00014-18-2489.]

Lessons learned on the use of data surfaces for Ffowcs Williams-Hawkings calculations: Airframe noise applications

The use of integral solutions to the acoustic-analogy-based Ffowcs Williams-Hawkings (FW-H) formulation is the de facto industrial and academic approach for assessing far-field acoustic behavior from airframe noise simulations. The methodology utilizes time-accurate flow variables obtained at arbitrary data surfaces to determine far-field spectra. The solid surface approach, where the time history of pressure is recorded on the surface of the aircraft, is generally preferred. Permeable surfaces, where pressure and velocity are recorded on a user-defined surface enclosing the aircraft, are also used. This paper seeks to share some of the valuable lessons we learned over the past several years on the limitations of the FW-H formulation and best practices for its successful application. We show results for various configurations comprising isolated landing gears, a business jet, a large commercial transport, and a generic low boom supersonic prototype, all at landing conditions. We demonstrate that the solid FW-H approach has significant accuracy limitations for most realistic cases and that the permeable FW-H approach may also fail in specific scenarios, requiring additional costly steps to extract the correct far-field noise spectra. Suggested guidelines for proper FW-H calculations are also given.

Identification of the two-sources at the aircraft slat/fuselage juncture based on phased array

Slat/fuselage juncture noise or slat horn noise has been identified as a major airframe noise component and is commonly assumed to be similar to the flap side-edge noise. However, whether the slat/fuselage juncture noise has similar characteristics to flap side-edge noise has not been confirmed. In this study, the slat/fuselage juncture noise sources in realistic aircraft configurations are investigated based on phased microphone arrays. Two different source mapping methods of L1/2-DAMAS and CLEAN-SC are applied to the airframe noise benchmark test DLR1 for cross validation. It is shown that both methods obtained identical noise characteristics and source strength distributions. The slat/fuselage juncture noise is revealed to be completely different from the flap side-edge noise. The spectrum of the slat/fuselage juncture noise is Helmholtz-dependent, rather than Strouhal-dependent, as the flow Mach number increases. The broad peak of the spectrum shifts towards a higher frequency as the angle of attack increases. Furthermore, both the two source mapping methods all identified two noise sources existing at the aircraft slat/fuselage juncture. The first noise source is the inboard slat side-edge vortex scattering and the second noise source is the horseshoe vortex on the fuselage. It is demonstrated that the side-edge vortex scattering dominates the spectrum, especially in the middle- and high-frequency domains. The horseshoe vortex is a major noise contributor in the low-frequency domain. These findings provide improved insight into the underlying noise generation mechanisms and can help develop prediction methods for slat/fuselage juncture flow noise.