Noise In Aircraft Research Articles

Effects of slat track on the flow and acoustic field of high-lift devices

Most of the published works related to noise generation in high-lift devices only consider clean idealized geometries, whereas real aircraft wings contain important parts called the slat tracks that connect the slat and the main wing. To investigate the effects of slat tracks on the ambient flow and noise, simulations are performed for wings based on the 30P30N airfoil. Two configurations are considered, i.e., the 30P30N wing with and without a slat track. Improved detached-eddy simulation is carried out to compute the unsteady near-field flow. The far-field noise propagation is calculated using the Ffowcs Williams-Hawkings equation. The results show that the installation of the slat track has observable effects on the aerodynamic and aeroacoustic performances of the high-lift devices. The maximum lift coefficient of the high-lift device and the stall angle of attack is reduced. Low-frequency broadband noise radiated from the main wing is significantly increased and the directivity of the overall sound pressure level is changed due to the slat track. The far-field noise from the configuration with the slat track displays noticeable sensitivity to the angle of attack. The slat track has an amplification effect on the high-pressure fluctuation on the main wing, especially at a small angle of attack. This leads to nearly 10 dB in the far-field noise based on the discussed wing section. To reveal the relative noise mechanism, vortical structures near the slat track, large-scale spanwise flows above the main wing, and the vortex dynamics are investigated. A spiral vortex pair is captured as the most important flow structure. It is generated in the spanwise direction by the interaction of slat wakes with the boundary layer on the upper surface of the main wing. The pressure fluctuations on the main wing are highly correlated with this flow structure, which cannot be ignored for the study of noise in a real aircraft.

Embedded large eddy simulation of transitional flow over NACA0012 aerofoil

An accurate computation of near-field unsteady turbulent flow around aerofoil is of outstanding importance for aerofoil trailing edge noise source prediction, which is a representative of main contributor to airframe noise and fan noise in modern commercial aircraft. In this study, an embedded large eddy simulation (ELES) is fully implemented in a separation-induced transitional flow over NACA0012 aerofoil at a moderate Reynolds number. It aims to evaluate the performance of the ELES method in aerodynamics simulation for wall-bounded aerospace flow in terms of accuracy, computational cost and complexity of implementation. Some good practice is presented including the special treatments at RANS-LES interface to provide more realistic turbulence generation in LES inflow. A comprehensive validation of the ELES results is performed by comparing with the experimental data and the wall-resolved large eddy simulation results. It is concluded that the ELES method could provide sufficient accuracy in the transitional flow simulations around aerofoil. It is proved to be a promising alternative to the pure LES for industrial flow applications involving wall boundary layer due to its significant computational efficiency.

Improved Technique for Evaluation of Wall Pressure Fluctuations from Turbulent Boundary Layer

The wall pressure fluctuations produced by a turbulent boundary layer (TBL) are the main excitation of structural vibrations and cabin noise in aircraft. To precisely estimate the structural vibration excited by a TBL, an improved technique is proposed and developed for the prediction of wall pressure fluctuations induced by a high-speed flow. First, an integrated expression is given to describe wall pressure fluctuations in different situations, and a conventional phase array technique is employed to identify the wave numbers of the wall pressure fluctuations and noise. Then, the pressure fluctuations are separated from the noise by a modified subsection approaching method. To improve the resolution of the low-frequency noise, an accelerated focusing method is developed and used in this paper. A variable spacing array design is proposed to improve the calculation accuracy of the broadband pressure fluctuation separation. In addition, a sensitivity analysis is carried out to fix the error range of the noise wave numbers in which the pressure fluctuation and noise calculation converges to the actual value. Finally, a wind-tunnel test is performed to validate the proposed method in this work.

Full-scale experiments on the reduction of propeller-induced aircraft interior noise with active trim panels

Results are presented of full-scale experiments on the active reduction of rotor-induced passenger cabin noise in a Dornier Do728 aircraft. Two active sidewall panels (smart linings) are used to reduce the sound pressure in a control region in front of the linings. The fuselage pressure distribution associated with the first five harmonics of a generic counter-rotating open rotor (CROR) engine is emulated with a loudspeaker array. Each smart lining is equipped with two inertial force actuators. It uses up to eight error microphones and implements an adaptive feedforward controller. The two smart linings are driven in parallel. A maximum SPL reduction of 11.3 dB is achieved in the controlled area. The mean SPL reduction over 18 microphones is 6.8 dB. At the most critical second frequency, a mean SPL reduction of 9.3 dB is achieved by the two smart linings working in parallel. It is observed that the SPLs near the lining (at the window seat positions) are significantly higher than the SPLs near the aisle. This leads to the conclusion that the major part of the sound energy is transmitted through the linings, which is seen as an argument for the suitability of the smart lining concept.

A Statistical Energy Analysis (SEA) model of a fuselage section for the prediction of the internal Sound Pressure Level (SPL) at cruise flight conditions

Comfort plays an increasingly important role in the interior design of airplanes. In general, comfort is defined as ‘freedom from pain, well-being’; in scientific literature, indeed, it is defined as a pleasant state of physiological, psychological and physical harmony between a human being and the environment or a sense of subjective well-being. Cabin noise in passenger aircraft is one of the comfort parameter, which creates straightaway discomfort when exceeding personal thresholds. In general the cabin noise varies by the seat position and changes with flight condition. It is driven by several source types, which are transmitted through different transfer paths into the cabin. In the forward area the noise is mainly dominated by the turbulent boundary layer described by pressure vortexes traveling along the fuselage surface.In this paper evaluation of the Sound Pressure level, for the medium-high frequency range, of an aircraft fuselage section at different stations and locations inside the cabin has been performed numerically by using Statistical Energy Analysis (SEA) method. Different configurations have been considered for the analysis: from the “naked” cabin (only primary structure) up to “fully furnished” (primary structure with interiors and noise control treatments) one. These results are essential to understand which are the main parameters affecting the noise insulation. Furthermore the Power Inputs evaluation has been determined to see the contribution of each considered aeronautic component on the acoustic insulation. Finally, the effect of a viscoelastic damping layer embedded in the glass window has been evaluated.

Innovative passive multifrequency propeller device for noise and vibration reduction in turboprop fuselage

One of the main comfort issue affecting the passenger comfort into a turboprop aircraft fuselage is the propeller tonal noise and the related vibrations. It is well known that propeller rotation during flight generates the main noise sources, depending upon its rotational angular velocity, number of blades, power at shaft generating aircraft thrust and blades geometry. Thanks to the progress behind the control systems of the blades rotations, an innovative highly selective DVA has been conceived. The purpose of the research activity has been improving the performances of the standard passive tonal noise control system used for the BPF tuned noise and vibration attenuation in turboprop aircraft. Due to specific commercial need, the use of bi-tuned frequency can lead at a passive noise reduction at two RPM regimes. Generally, the turboprop aircrafts use only two RPM regimes: 100% at take-off, climb and approach, 86% during cruise, climb and descent. An innovative passive bi-tonal device capable to be tuned at two different frequencies in order to optimize the fuselage noise reduction at two different flight regimes (100% and 86%), has been designed and numerically verified. The functional effectiveness of the bi-frequential tuned device has been analysed by finite elements simulations on a linear beam, representative of the turboprop fuselage frame. The outcomes achieved within this activity encourage the advancement of this research sector, as a support to the needs of the turboprop aeronautical industry. According to the long experience gained by the research group, the proposed multifunctional concept can be a valid technology solution ready to be manufactured as well as validated in flight.

Development of a direct aerodynamic drag measurement system for acoustic liners

The objective of this paper is to establish the validity of a measurement apparatus for determining aerodynamic drag caused by acoustic liners. Conventional acoustic liners for ducted turbofan engine nacelles have been proven to reliably reduce engine noise in commercial aircraft for a narrow frequency range. Traditional acoustic liners consist of a porous facesheet, honeycomb core, and solid backing. As technology has developed, new liner designs, including variable-depth cores, show promise of reducing noise in a broad frequency range. Additionally, the next generation of aircraft design may allow for additional aircraft surface area to be covered by acoustic liners, further reducing aircraft noise perceived on the ground. Before these advanced acoustic technologies can make their way onto commercial aircraft fleets, the aerodynamic drag caused by the liners must be understood. Using the Mach 0.6 Wind Tunnel at the University of Notre Dame, a direct drag measurement system has been developed utilizing a linear force balance. In combination with indirect drag measurements taken at NASA Langley’s Curved Duct Test Rig, measurement confidence has been established. This paper will detail the capabilities of the direct drag measurement system at Notre Dame and explore its intended future use in developing new liner technologies.

Noise and vibration phenomena in aircraft wheel and brake systems.

There is a wide variety of noise and vibration phenomena in aircraft brake systems for which must be accounted in the design process of such systems. These phenomena include such modes as whirl and squeal, the latter of which can be quite different from its counterpart in automotive systems and has likewise received much less attention in the literature than its automotive counterpart. Consequently, an overview of such phenomena with representative results from simulations and experiments will be presented to highlight the differences. Complicating matters is that brake-induced vibration often involves strong coupling with the aircraft structure, thereby necessitating a system level understanding beyond the brake itself. This aspect poses a particular problem to a brake component supplier in how to ensure favorable noise and vibration behavior for the full aircraft system, particularly early in the development cycle. To accomplish this goal, Goodrich has developed a successful methodology combining simulation and testing to assess the noise and vibration behavior at an earlier stage. The methodology implements simulations to guide appropriate brake component tests that are more tractable for the brake supplier yet sufficiently mimic the full system environment. With the combined simulation and testing, a reduction in actual testing can be achieved while improving component noise and vibration performance.

Towards a Quieter Low Pressure Turbine: Design Characteristics and Prediction Needs

This article examines Low Pressure Turbine (LPT) noise in modern aircraft and shows how difficult it is to design a quiet LPT. LPT noise has long been neglected in terms of research and prediction capability due to it being considered of lesser importance. Recent developments in LPT design have led to what is termed a “turbine noise storm,” in which the LPT is designed to be cut-off but in the end there is significant LPT noise and little or no ability to reduce it. These developments include the trend toward lower blade counts and solidities. The situation is further hurt due to the inability to predict LPT noise, particularly for the new type of designs. The inability to reduce the LPT noise once the engine and airplane is designed is due to there being no available light-weight high-temperature acoustic-lining technology that can be added to fix a LPT noise problem without a significant penalty. In an attempt to understand what makes some engines' LPT quiet while others are very loud, modern turbine designs are characterized into several LPT noise “classes” and important aspects of each class are shown. Finally, the need for a semi-empirical prediction is discussed including what essential elements this prediction needs to have to be relevant to modern aircraft preliminary design.

Control of aircraft interior noise using heterogeneous (HG) blankets

This study is concerned with the passive control of vibration and sound radiation of interior cabin noise in aircraft at low frequencies (<500 Hz) using heterogeneous (HG) blankets. HG blankets consist of poroelastic media with small embedded masses, which act similar to a distributed mass-spring-damper system. HG blankets have shown significant potential to reduce low-frequency radiated sound from structures, where traditional poroelastic materials have little effect. A mathematical model of a double panel system with an acoustic cavity and HG blanket was developed using mobility and impedance matrix methods. Theoretical predictions are validated with experimental measurements. Results indicate that proper tuning of the HG blankets can achieve in broadband reductions in sound transmission through the double panel system with less than 10% added mass. Future work includes expanding the model and experiment to multiple panel systems.

Speech intelligibility index calculations in light aircraft cabin during flight

High levels of cabin noise in small general aviation aircraft significantly deteriorate the quality of speech communications and potentially endanger the safety of flight. Several ground and inflight cabin noise measurements on new generation Cessna 172R were made during various phases of flight. The results are analyzed and used for Speech Intelligibility Index (SII) calculations, in order to quantify the influence of cabin noise on speech communications between crew members.

Noise: My 62 years of it!

Imagine getting paid for having fun! Well, in retrospect, it was fun; but there were several tough challenges. Even those are worth remembering. From 1941 to 1982, there were acoustic torpedoes, HVAC acoustics, noise and vibration in auditoriums, aircraft and airport noise, OSHA and industrial noise control, power plants, community noise problems, vbration, railroad and subway vibration control, legal acoustics, noise manuals, and noise courses—and a few other things that don’t fit into those neat categories. Some specific jobs could be named, but that would take away the suspense and the surprise. But 1941 to 1982 is only 41 years. How about the other 20-odd years?

航空機中の電磁ノイズおよび宇宙放射線の測定

放射線計測器および航空機自身からの電磁ノイズをフィールドメータを用いて調べた。前者は後者よりも小さいことが証明された。次に, 成田からバンクーバ間の高度約10kmの民間航空機の宇宙線中の中性子と電離成分による線量率をそれぞれ減速3HeカウンタおよびCsI (T1) シンチレーションカウンタを用いて測定した。測定データは計算コードCARI-2による結果と比較され, 両者はほとんど一致した。

Active noise and vibration control for turboprop aircraft

Interior noise in modern turboprop aircraft is a serious problem as it is directly related to passenger comfort, crew performance, and aircraft weight. For turboprop aircraft the most important noise sources are the propellers generating intense low-frequency noise which is, in general, difficult to reduce by passive measures. Therefore, extensive work has been conducted on active noise control using loudspeakers as secondary sources and microphones as error sensors. In addition, new control techniques have been investigated applying structural actuators for controlling the structural fuselage modes coupling to the critical acoustical modes. The present paper demonstrates the feasibility of using the new structural actuator technique in turboprop aircraft. Multiple piezoceramics have been attached to the structure of the Acoustic Test Cell (ATC) based on the Dornier 328 turboprop aircraft. The ceramics are driven according to a theoretical optimization process based on measured transfer functions. Noise reduction for the first three BPFs has been calculated and measured for different piezo configurations. The achieved results are compared and discussed. It is shown that the new technique of active vibration control is a powerful tool not only to control vibrations but also to significantly reduce the interior noise levels in modern turboprop aircraft.

Model-Based Feedback Control of Acoustic Radiation from Vibrating Structures by Means of Structural Control

In this work, it is investigated how classical techniques of linear feedback control design can be applied to the problem of the reduction of acoustic radiation from vibrating structures for cases where the disturbance is broadband and where no reference is available. Much of the work carried out to date in the field of active noise and vibration control has concentrated on applications where either the disturbance to be cancelled is periodic (propeller noise in aircraft,...) or a reference signal, highly correlated with the disturbance, is available (air conditioning duct noise,...) such that a feedforward control approach can be used. When the disturbance is broadband and where no reference is available, feedforward control cannot be used and feedback control must instead be used. Feedback control theory is well established and a vast amount of analytical tools are available to the feedback control designer. However, due to the inherent delays associated with the propagation of sound waves, feedback control of acoustic fields is prone to being unstable. In this paper, a controller is presented which feeds back a measure of the structural response (vibration) of the system in order to determine the control force that needs to be applied to the vibrating structure in order to reduce the total acoustic energy radiated by the vibrating structure.

Multivariable structural acoustic control with static compensation

The active control of turbulent boundary layer noise in commercial aircraft has gained significant interest in recent years, and, due to the stochastic nature of the disturbance, feedback control strategies are required. Linear quadratic Gaussian control has been proposed in the past [Baumann et al., J. Acoust. Soc. Am. (1991)], but compensators resulting from such control approaches are typically quite involved. The thrust of the current effort is to demonstrate that acceptable performance in structural acoustic control can be achieved with static compensation implemented through a multiple input, multiple output (MIMO) array of colocated transducers. Since static compensation requires a matrix of constant gains, it is easily realized in analog or digital hardware. By augmenting the system dynamics to include radiation filters, the dual Levine–Athans algorithm was implemented to design the appropriate static compensator. Results for various MIMO control cases are presented.

Feedback control of sound radiation from a plate excited by a turbulent boundary layer

In this paper it is shown that the problem of applying active control to a simply supported plate excited by a turbulent boundary layer can be presented in a form which allows the application of optimal control theory. Linear regulator theory and stochastic linear regulator theory are briefly summarized. The approach taken follows that of Baumann et al. who show that the optimal control theory can be applied to structural acoustic problems if the sound power radiated by the structure is estimated from the vibration signal by means of a ‘‘radiation filter.’’ A modal model for the vibration of an elastic structure is presented and the state space realization of the model is given. It is shown that a simple Corcos model of the turbulent boundary layer excitation can be modeled by using a number of ‘‘excitation filters.’’ The state space realization of these filters can be combined with the state space model of a vibrating plate to give a model to which optimal control theory can be applied. In similar fashion the sound power radiated by the plate can be estimated by means of a radiation filter matrix. A detailed account of the calculation of such a radiation filter matrix is given. The state space forms of the excitation filters and radiation filter matrix can be combined with the state space model of the plate to give a system state space realization which can be used in the solution of a linear quadratic regulator problem. Optimal control analysis is then applied to the state equations and optimal reductions in the far-field radiated power are obtained for various arrangements of control forces. These results give insight into the possibilities for obtaining useful reductions in turbulent boundary layer induced noise in aircraft by the application of feedback control.

Noise attenuation panel

A noise attenuation panel used to attenuate noise in aircraft engines includes a cellular core and a facing sheet formed with an array of holes. The holes are laser drilled to provide: (i) hole size variation over the facing sheet; (ii) non-circular hole cross section; (iii) polygonal hole cross section; (iv) hole locations not contiguous with walls of the cellular core; and (v) inclined holes passing through the facing sheet in a direction inclined to the normal to the facing sheet.

Active control of noise and vibration: Overview and current applications

In this introductory presentation the principles and components of active control systems will be briefly overviewed. Current research topics and areas of application will be discussed including duct noise, machinery isolation, wave control in beams, interior noise in aircraft, and structurally radiated sound. The use of adaptive materials in active noise and vibration control will also be touched upon. The presentation will finish with a discussion on the present limitations of active control and future promising directions.

Active control of interior noise in a business aircraft using piezoceramic control actuators

This paper describes results of experiments in which active control of interior noise in a business aircraft is achieved using piezoceramic actuators bonded to the fuselage structure. The experiments were performed in a real aircraft fuselage (Cessna Citation 3) with engine noise simulated by a point force shaker attached to the engine pylon, and multiple arrays of piezoelectric control actuators in conjunction with an LMS adaptive controller. The cost function used was the summation of the pressure squared at four error microphones located at head height in proximity of the two rear-mounted passenger seats. Results are given for on-resonance and off-resonance cases and sound level reductions from 3 to 15 dB were demonstrated. The global nature of the control and influence of the actuator/sensor positioning is discussed.