Helicopter Rotor Noise Research Articles

Numerical study on the noise propagation characteristics of rotor in non-uniform downwash flowfield Based on Linearized Euler Equations

To study the influence of non-uniform flowfield on the propagation characteristics of helicopter rotor noise, a Hybrid Computational Aeroacoustics (HCAA) method is developed. The acoustic source region is simulated by Computational Fluid Dynamics (CFD) technique with the Unsteady Reynolds Averaged Navier-Stokes equations (URANS) as the governing equations. Acoustic near-field is simulated by Computational Aeroacoustics (CAA) technique with the Linearized Euler Equations (LEE) as the governing equations, and the numerical discretization of the LEE is accomplished by Runge-Kutta Discontinuous Galerkin (RKDG) method. A novel acoustic source extraction method based on pressure and pressure gradient is proposed to accomplish the one-way CFD-CAA weak coupling. The HCAA method is validated through comparisons with noise experimental data of the UH-1H model rotor and the BO-105 model rotor. Based on the proposed HCAA method, the convection and refraction effects of rotor noise under different collective pitch angles are analyzed. The results show that the distortion effect of the rotor noise is most affected by the non-uniformly distributed downwash velocity field, resulting in an increment of acoustic energy below the rotor plane. The effect of non-uniformly distributed downwash velocity on noise propagation increases with the increase of the collective pitch angle. For the UH-1H model rotor, the maximum change of the sound pressure level is 0.8 dB (about 10% change of the effective sound pressure).

Integrated methodology for the prediction of helicopter rotor noise at mission level

This paper presents an integrated approach for the aeroacoustic assessment of four-dimensional rotorcraft operations. A comprehensive rotorcraft code is utilized to model aircraft flight dynamics across complete missions. A free-wake aero-elastic rotor model is employed to predict high-resolution unsteady airloads, including blade-vortex interactions, at each mission element. A rotor aeroacoustics code is developed to calculate source noise and far-field ground acoustic impact. Time-domain acoustic formulations are used to evaluate near-field noise generation across designated acoustic spherical surfaces surrounding the helicopter main rotor. A numerical procedure is developed for the derivation of acoustic spheres on-the-fly, coupled with trajectory-adaptive ground observer grids. The individual analytical models are incorporated into a mission analysis numerical procedure. The applicability of the integrated method on “real-world” rotorcraft operations is demonstrated for two generic, four-dimensional missions, without the need of pre-stored noise data. The proposed approach provides insight into helicopter noise prediction at mission level, elaborating on the coupling of aeroelastic rotor response with rotorcraft flight dynamics and aeroacoustics.

Prediction of helicopter rotor noise in hover using FW-H analogy

The integral formulation of Ffowcs Williams and Hawkings (FW-H) analogy developed by Farassat is implemented to understand the sound generation and propagation of a rotating slender body like the helicopter rotor blade in hover. Using the linear acoustic theory, the thickness and loading noise terms are implemented in a flexible, new, in-house aero-acoustic tool. The pressure distribution on the blade surface is obtained from CFD simulation which sufficiently captures the correct flow characteristics of the rotor blade. Although, there is an improved estimation of the acoustic pressure signal in comparison to existing numerical noise signal results, there is still an under prediction of the peak amplitude compared to experimental values. The code is validated against the experiment for the UH-1H model helicopter rotor equipped with two rectangular blades having NACA0012 section.

Helicopter rotor noise prediction using a convected FW-H equation in the frequency domain

In this paper, high-speed impulsive noise radiated from helicopter rotors in hover and forward flight is computed by using a convected FW-H equation in the frequency domain with a rotating permeable data surface. Compared to the time-domain formulations, the frequency-domain formula avoids retarded time computation and flow data interpolation with respect to the retarded time. In addition, the complicated emission surface construction is obviated, while a supersonic rotating data surface is chosen to account for quadrupole sources outside the sonic cylinder. The selection of a rotating data surface close to the rotor blades improves noise source resolution and thus ensures noise prediction accuracy at a low computational cost, compared to a stationary data surface approach. Besides, the use of a rotating data surface for hovering helicopter rotors enables noise source identification by a steady flow simulation. Application of the methodology to hovering and forward-flight helicopter rotors has shown good agreement with experimental data, thus indicating that the approach is an accurate and efficient computational tool for noise prediction of helicopter rotors operating at high tip Mach number. Since the formula is derived in a moving medium frame, the effect of forward-flight speed on the radiated noise can be explicitly taken into account.

Prediction of helicopter rotor noise in hover

Two mathematical models are used in this work to estimate the acoustics of a hovering main rotor. The first model is based on the Ffowcs Williams-Howkings equations using the formulation of Farassat. An analytical approach is followed for this model, to determine the thickness and load noise contributions of the rotor blade in hover. The second approach allows using URANS and RANS CFD solutions and based on numerical solution of the Ffowcs Williams-Howkings equations. The employed test cases correspond to a model rotor available at the KNRTU- KAI aerodynamics laboratory. The laboratory is equipped with a system of acoustic measurements, and comparisons between predictions and measurements are to be attempted as part of this work.

The Prediction and Analysis of Helicopter Rotor Rotation Noise in Hover

A prediction program for analysis of helicopter rotor rotation noise was developed based on Farassat1A formulation. The acoustic prediction program was validated by the comparison with the WOPWOP program results in the literature.The loads input for the acoustic calculation are obtained from CFD calculation. The directional characteristic analysis was conducted by this program. Some calculations were conducted for evaluating the influence of rotor parameters including rotor speed , blade pitch ,and blade dihedral for the helicopter rotor rotation noise in hover. The results show that some proper changes of rotor parameters can bring benefit in thickness and loading noise of helicopter rotor. Several conductive conclusions were summarized for the helicopter rotor noise reduction.

A Refinement of Asymptotic Predictions and Full Numerical Solution of Helicopter Rotor Noise in the Far Field

The asymptotic analysis of Parry and Crighton [1] for propeller noise in the far field, which is based on Hanson's formulation [2] of the FW-H equation, is refined to second order by Laplace's method [3] for evaluating integrals, accounting for second order contributions near the blade tip for loading and thickness noise. The full numerical solution of Hanson's integrals for both thickness and loading noise is also presented. In particular, the theory is applied to a four-bladed helicopter rotor with tip Mach numbers ranging between 0.5 and 0.7. The aerodynamic loading in this case is obtained using a 3D compressible code based on finite volume method with intensified grid density near the blade tip. The far field angular SPL noise distributions of a helicopter rotor in hover show that the present second order asymptotic formula is in better agreement with full numerical computations than that of the first order formula, especially for thickness noise.

Helicopter Rotor Noise in the Merged Tip-Vortex and Blade Interaction Condition

BVI (blade vortex interaction) phenomena cause unsteady loading and noise in the helicopter rotor. It is difficult to accurately predict BVI aerodynamics and noise, because rotor wake and tip-vortices are numerically dissipated. In the present study, CFD and a time-marching free-wake coupling analysis were used to describe inflow and outflow without wake dissipation. Rotor noise was also computed using the Farassat formula, derived from the Ffowcs-Williams Hawkings equation. To validate aerodynamics and the noise prediction solver, a two-blade rotor was simulated and the results were compared with experimental data. To investigate the vortex interaction of a multi-blade rotor, a four-blade rotor was also simulated. In a slow descent flight condition, tip vortices are merged at the advancing side. The merged tip-vortex and blade interaction were simulated and are discussed in this paper.

Numerical investigation of nonlinear propagation distortion effects in helicopter rotor noise

The effect of nonlinear propagation distortion on helicopter rotor noise is presented based on measured data for low-speed descent and numerical calculations that predict the noise level away from the helicopter with and without nonlinear effects. It is shown that for some frequency bands the difference between linear and nonlinear calculations can be as high as 7 dB. Blade vortex interaction (BVI) noise, the dominant noise contributor during descent, is mainly examined. It is shown that advancing side BVI noise is affected by nonlinear distortion, while retreating side BVI noise is not. Based on signal characteristics at source, two quantities are derived. The first quantity (termed polarity) is based on the pressure gradient of the source signal and can be used to determine whether a BVI signal will evolve as an advancing or a retreating side signal. The second quantity (termed weighted rise time) is a measure of the impulsiveness of the BVI signal and can be used to determine at which frequency nonlinear effects start to appear. Finally, polarity and weighted rise time are shown to be applicable in cases of BVI noise generated from different blade tips, as well as in cases of non-BVI noise.

Recent developments in helicopter rotor noise prediction

Prediction of discrete frequency noise for rotorcraft in steady flight has reached a high level of sophistication and understanding. The primary challenge in such rotor noise predictions is the accurate determination of the loading on and flow field around the blades. Although current rotor noise prediction tools have been demonstrated for steady flight conditions, the utility of rotorcraft comes from their unique ability to hover and maneuver. Such maneuvers often occur near the ground in close proximity to people. This paper describes an initial study to characterize maneuver noise. A maneuver noise prediction system has been developed, which consists of a flight simulation code, free‐vortex wake code, and a maneuver noise prediction code, loosely coupled together. One of the key aspects of this system is the ability to investigate the additional noise caused during the transition from one flight state to another. Several maneuvers including turns, accelerations, and pop‐up/popdown maneuvers were considered. Significant increases in the low‐frequency noise can occur, depending on how aggressive the maneuver is performed. The status of advanced research on predicting acoustic scattering of the rotor noise by the aircraft and the first‐principles prediction of rotor broadband noise will also be presented.

회전익 표면의 정상하중에 의한 소음의 방사특성

회전하는 물체의 표면에 걸리는 정상하중에 의해 발생되는 소음을 이론적으로 해석하여 그 방사특성을 분석하였다. 해석의 편의성을 위해, 물체의 표면에 걸리는 힘은 표면 전체에 고루 분포된 분산하중이 아닌 한 점에 집중된 점하중으로 표현하였고, 소음계산은 엄밀한 Lowson의 하중소음식을 사용하였다. Lowson 식의 각 항들을 수학적으로 보다 자세하게 구분하여 각 항들이 가지는 물리적 특성에 대해 논의하였다. 회전각속도를 변화시켜가며 단일 점하중에 의해 발생되는 소음의 방사특성을 근거리 및 원거리에서 조사하였고 그에 대한 이론적 근거를 제시하였다. 또한 거리에 따른 음압의 크기를 조사하여 근거리와 원거리에서의 음압감소비율 및 회전각가속도의 역할에 대해 논의하였다. 마지막으로, 점하중의 개수가 증가될 때 소음이 오히려 감소하는 현상을 설명함과 더불어 소음저감의 방안과 그 한계점 및 향후 연구주제에 대하여 논의하였다. Loading noise generated by steady aerodynamic force exerted on the rotating body surface is theoretically analyzed and its radiation characteristics is examined as a fundamental research of helicopter rotor noise. For simplicity, the force exerted on each blade is not distributed but concentrated at one point and the noise is evaluated by using Lowson' exact formula with a discussion of the physical meaning of each term in the formula. For a single point force rotating with various angular frequencies, we investigated the radiation characteristics and theoretically explained the physical behavior at near and far-field. By investigating the amplitude of acoustic pressure with various distances, we observed the different decreasing ratio at near- and far-field with the discussion of the effect of acceleration of angular frequency. Finally, the phenomenon that the noise level is reduced everywhere as the number of blade increases is explained with the suggestion of a noise reduction idea, the limitations of this study, and the future research topics.

A moving hum filter to suppress rotor noise in high-resolution airborne magnetic data

A unique filtering approach is developed to eliminate helicopter rotor noise. It is designed to suppress harmonic noise from a rotor that varies slightly in amplitude, phase, and frequency and that contaminates aeromagnetic data. The filter provides a powerful harmonic noise-suppression tool for data acquired with modern large-dynamic-range recording systems. This three-step approach — polynomial fitting, bandpass filtering, and rotor-noise synthesis — significantly reduces rotor noise without altering the spectra of signals of interest. Two steps before hum filtering — polynomial fitting and bandpass filtering — are critical to accurately model the weak rotor noise. During rotor-noise synthesis, amplitude, phase, and frequency are determined. Data are processed segment by segment so that there is no limit on the length of data. The segment length changes dynamically along a line based on modeling results. Modeling the rotor noise is stable and efficient. Real-world data examples demonstrate that this method can suppress rotor noise by more than 95% when implemented in an aeromagnetic data-processing flow.

Low Order Modeling for Transonic Helicopter Noise

The permeable surface form of the Ffowcs Williams-Hawkings equation provides an efficient basis for predicting helicopter rotor noise when the flow over the blades is transonic. It requires knowledge of the temporal variation of fictitious acoustic sources over the permeable control surface. Calculation of these using unsteady computational fluid dynamics (CFD) solvers is feasible for straightforward helicopter motion, but becomes prohibitively time consuming for more complicated maneuvers. Low-order modeling provides an efficient alternative. Because at transonic speeds significant noise originates at the shock surfaces, low-order modeling of the two-dimensional shock dynamics was first performed to provide an insight. Using data from CFD simulations, system identification, a modeling technique suitable for dynamically linear systems, was performed. The position of a well-formed shock was found to respond to variations in blade speed and blade pitch angle with approximate first-order lag relationships; these findings were consistent with harmonic tests. Low-order modeling of the acoustic sources was then performed, although for simplicity attention was restricted to two-dimensional motion and an acoustically compact control surface. Models were obtained, again using system identification, and were successfully used in noise prediction. Analytical consideration of the sources revealed that their dynamics arose mainly due to the shock motion. These findings serve to demonstrate the potential of low-order modeling in predicting transonic helicopter noise.

Aeroacoustic integral methods, formulation and efficient numerical implementation

The paper makes a contribution to the clearer understanding of the physical meaning and domain of applicability of the Ffowcs Williams–Hawkings (FW-H) and Kirchhoff aeroacoustic integral methods. The analytical relationship between the two approaches, and the physical implications involved in the use of the Kirchhoff approach are discussed. In particular, where the Kirchhoff surface cuts into a domain where non-negligible volume sources of Lighthill's Acoustic Analogy are present (domain crossed by a shock, non-uniform flow or vortices for instance), the Kirchhoff integral must be complemented by additional surface integrals to recover the FW-H surface integral. The paper goes on to describe a fast, robust integration technique for computing FW-H or Kirchhoff surface integrals. This integration technique starts from emission time, and treats each integration surface element as non-compact. It obviates the need to build retarded acoustic surfaces and avoids some of the drawbacks of current methods, such as the Doppler singularity when using supersonically rotating grids. The paper then recalls formulas for calculations in the aircraft reference frame, and the computational efficiency of the proposed integration method is demonstrated by its application to the prediction of helicopter rotor noise directivity contours.

European helicopter rotor noise research—Highlights of recent projects

Significant advances in understanding, modeling, and controlling helicopter rotor noise have been made in the course of several major European rotor aeroacoustic research projects over the past years. The availability of the German–Dutch Wind Tunnel (DNW), in particular, has enabled research on relatively large main rotor models (up to typically 40% full scale). Here, the simultaneous application and use of blade-mounted absolute pressure sensors to study the very aeroacoustic source mechanisms, of advanced acoustic-data acquisition systems for the radiated noise, of non-intrusive flow visualization as well as flow and blade deflection measurement techniques to investigate the rotor wake and the blade dynamic response has provided much insight into the physics of rotor aeroacoustics, particularly into the rotor blade-vortex interaction (BVI) impulsive noise phenomenon. In efforts towards controlling BVI impulsive noise, in some research projects, active blade-root control techniques have been investigated to favorably affect the BVI geometry and thus minimize the impulsive noise radiation. Much of the experimental data acquired served as benchmark information to enhance the mathematical modeling of the rotor wake, the rotor airloads, and the noise radiation. In this lecture, several major European helicopter aeroacoustic research projects are discussed.

Recent developments in helicopter rotor noise prediction at NASA

Significant progress has been made recently toward the efficient and robust prediction of helicopter high-speed impulsive noise. The far-field approximation to the Ffowcs Williams–Hawkings (FW–H) quadrupole has been augmented with a rigorous mathematical development. Two new quadrupole formulations—a retarded-time formulation known as Q1A and an emission-surface formulation known as Q2—have been developed and implemented for subsonic and supersonic source motion, respectively. A key numerical advantage of the far-field approximation is that volume integration of the source region surrounding the rotor blades is only performed once in a preprocessing stage. The resulting quadrupole noise computation is reduced to surface integration over the rotor disk in the vicinity of the blades. The solution-adaptive marching-cubes integration algorithm used for the emission-surface formulation will be described. Another recent advance in helicopter noise prediction is the use of the FW–H equation on a permeable integration surface surrounding the source region. This application of the FW–H equation has been shown to be both analytically and numerically superior to the Kirchhoff approach. Furthermore, numerical evidence is presented to demonstrate that the Kirchhoff method is inappropriate when a wake convects through the surface.

Rotor noise due to atmospheric turbulence ingestion. I - Fluid mechanics

In the present analytical procedure for the prediction of helicopter rotor noise generation due to the ingestion of atmospheric turbulence, different models for turbulence fluid mechanics and the ingestion process are combined. The mean flow and turbulence statistics associated with the atmospheric boundary layer are modeled with attention to the effects of atmospheric stability length, windspeed, and altitude. The turbulence field can be modeled as isotropic, locally stationary, and homogeneous. For large mean flow contraction ratios, accurate predictions of turbulence vorticity components at the rotor face requires the incorporation of the differential drift of fluid particles on adjacent streamlines.

The Uses and Abuses of the Acoustic Analogy in Helicopter Rotor Noise Prediction

This paper is theoretical in nature and addresses applications of the acoustic analogy in helicopter rotor noise prediction. It is argued that in many instances the acoustic analogy has not been used with care in rotor noise studies. By this it is meant that approximate or inappropriate formulations have been used. By considering various mechanisms of noise generation, such abuses are identified and the remedy is suggested. The mechanisms discussed are thickness, loading, quadrupole, and blade-vortex interaction noise. The quadrupole term of the Ffowcs Williams-Hawkings equation is written in a new form which separates the contributions of regions of high gradients such as shock surfaces. It is shown by order of magnitude studies that such regions are capable of producing noise with the same directivity as the thickness noise. The inclusion of this part of quadrupole sources in current acoustic codes is quite practical. Some of the difficulties with the use of loading noise formulations of the first author in predictions of blade-vortex interaction noise are discussed. It appears that there is a need for development of new theoretical results based on the acoustic analogy in this area. Because of the impulsive character of the blade surface pressure, a time scale of integration different from that used in loading and thickness computations must be used in a computer code for prediction of blade-vortex interaction noise.

Effects of repitition rate and impulsiveness on annoyance due to simulated helicopter rotor noise

Annoyance judgments of 82 different simulated helicopter impulsive rotor noise stimuli were obtained in an anechoic chamber to examine effects of repetition rate and impulsiveness. The stimuli were generated by computer from predicted Fourier harmonic structure with amplitude and phase components corrected for effects of the audio reproduction system. Impulse repetition rates covered a range from 10 to 115 Hz; crest factors covered a range from 3.2 to 19.3 dB. Each stimulus was judged at three sound pressure levels by 48 subjects. Judgments were converted from a continuous numerical annoyance scale to a decibel like scale by comparing them to judgments of annoyance of a 115-Hz tone presented over a wide range of sound pressure levels. Increases in annoyance with increases in repetition rate were found which were not predicted by common loudness or annoyance metrics and which were independent of noise level. The ability to predict effects of impulsiveness varied between the noise metrics, and was found to be dependent on noise level. The ability to predict the effects of impulsiveness was not generally improved by any of several proposed “impulsiveness corrections.” Instead, the effects of impulsiveness were found to be systematically related to the frequency content of the stimuli. A modified frequency weighting was developed which offers improved annoyance prediction.

A vector-potential approach to radiation from force-like sources

This paper presents an extension of the previously reported analysis of sound radiated by a moving point force, in which the acoustic pressure is represented as the divergence of a vector potential function. A single relationship between the particle velocity and the potential is obtained from the continuity equation. This relationship is employed in the construction of the pressure field above a surface with finite acoustic impedance. Applications to predictions of helicopter rotor noise and to flow noise are discussed.