Vertical distribution of traffic noise—A preliminary study

This paper examines the effect of flight on the sound radiated by a high frequency source embedded in a constant area jet pipe in the presence of flow. Ray acoustics theory and classical results for sound transmission at an interface of relative motion are used. The diffraction of sound at the nozzle lips, the inhomogeneity and irregularity of the interface and the possibility of instability waves being triggered by the incident sound are neglected. Some of the waves characterized by wave-fronts pointing upstream are shown to be convected downstream by the flow and to illuminate the forward arc after refraction at the jet interface. The amount of energy emitted by the source, which is trapped inside the flow, depends only on internal jet pipe conditions. However, the portion of the forward arc which is illuminated by this energy, is a function of flight speed. The radiation into the ambient atmosphere at rest of a basically omnidirectional source peaks at the edge of the downstream zone of silence and falls off rapidly when the observation angle is increased. The flight to static comparison reveals an interesting forward arc amplification due to flight but this occurs in a range of angles where the radiation is basically rather feeble.     

Flow-resonant sound interaction in a duct containing a plate,part I: Semi-circular leading edge

The interaction between flow and flow-induced acoustic resonances near rigid plates with semi-circular leading edges located in a hard-walled duct is described. These plates generate acoustic resonances over flow velocity ranges depending on thickness, chord and trailing edge geometry, together with rigidity, internal dimensions, length of the working section and shape of the terminations of the working section. A potential flow model for the plate with a smooth leading edge is developed, and the acoustic power generated by vortices growing and shedding from the trailing edge is calculated. The rate of growth of the vortices is determined by an instantaneous Kutta condition applied over part of the cycle. This technique simulates the influence of the sound field on vortex growth.     

On the generation of side-edge flap noise

A theory is proposed for estimating the noise generated at the side edges of part span trailing edge flaps in terms of pressure fluctuations measured just in-board of the side edge on the upper surface of the flap. Asymptotic formulae are developed in the opposite extremes of Lorentz contracted acoustic wavelength large/small compared with the chord of the flap. Interpolation between these limiting results enables the field shape and its dependence on subsonic forward flight speed to be predicted over the whole frequency range. It is shown that the mean width of the side edge gap between the flap and the undeflected portion of the airfoil has a significant influence on the intensity of the radiated sound. The results indicate that the noise generated at a single side edge of a full scale part span flap can exceed that produced along the whole of the trailing edge of the flap by 3 dB or more.     

Aerodynamic and energy-efficiency-based assessment of plasma actuator's position on a NACA0015 airfoil

In recent years, there has been an increase in the number of research papers on plasma and its use in active flow control applications. The main objective of this study is to assess the plasma actuator's position on a NACA0015 airfoil in terms of aerodynamic forces. In addition, optimization of the plasma actuator's position and its configuration are studied in order to identify the optimum configuration for improvement in lift coefficient. The experiments are conducted in an open-suction-type wind tunnel at Reynolds numbers of 48,000, 75,000, and 100,000. The plasma actuators are mounted on various positions (x/C) starting from the leading edge to trailing edge of the airfoil. The experimental results on aerodynamic force measurement are presented to illustrate the increasing lift effect of the generated plasma. It is also shown that the plasma actuators used as an active flow control device appears to shift the stall angle of the airfoil. The results of the experimental study suggest that the energy efficiency of airborne systems can be improved with the use of plasma actuators due to its increasing lift coefficient effect. This result becomes a vital finding considering that the same flight can be achieved with less fuel and less amount of environmental pollution for the same distance of journey. It is also worth mentioning that increasing lift effect would mean taking off from a shorter runway or allowing the airborne vehicle with the ability to fly with additional payload.     

基于物理机制模型的民机机体噪声预测

为实现民机总体方案快速评估与优化迭代设计,文章对民机增升装置前缘缝翼及后缘襟翼分别建立了基于民机噪声物理机制的预测模型,在此基础上搭建了机体噪声预测体系,开发了相应的预测工具UNICRAFT.为评估预测工具UNICRAFT的计算精度和效率,文章分别针对翼吊式布局,前缘缝翼/Fowler式襟翼形式,以及尾吊式布局,前缘缝翼\双缝子翼加后退式襟翼形式的增升装置进行了计算校核验证,通过与声学风洞试验结果对比分析,验证了本文发展的预测工具及预测体系的有效性,能够实现飞机级噪声水平的高效预测.      

NOISE GENERATED BY A COANDA WALL JET CIRCULATION CONTROL DEVICE

An analysis is made of the production of sound by a hydrofoil with a Coanda wall jet circulation control (CC-) device. Three principal sources are identified in the vicinity of the trailing edge of the hydrofoil. The radiation at very low frequencies is dominated by “curvature noise” generated by the interaction of boundary layer turbulence with the rounded trailing edge of the CC-hydrofoil; this is similar in character and magnitude to the low-frequency component of the conventional trailing edge noise produced by a hydrofoil of the same chord, but with a sharp trailing edge. Higher frequency sound is produced principally at the Coanda jet slot. “Passive slot noise” is caused by the “scattering” by the slot lip of nearfield pressure fluctuations in the turbulent boundary layer of the exterior mean flow past the slot. This is of comparable intensity to high frequency, sharp-edged trailing edge noise. However, the acoustic spectrum is greatly extended to much higher frequencies if the Coanda jet is turbulent; the sound produced by the interaction of this turbulence with the lip tends to dominate the spectrum at frequencies f (Hz) greater than about U_j/h, where h is the slot width and U_jthe Coanda jet speed. Sample numerical results are presented for a typical underwater application that indicate that at this and higher frequencies the slot noise can be 20 dB or more greater than conventional trailing edge noise, although the differences become smaller as the thickness of the slot lip increases.     

Relationship between the velocity of hypersonic vehicles and the transmission efficiency of radio waves

Analyzing the flow field distributions around a near-space blunt-cone vehicle at different flight speeds, we observe that, when the flight speed exceeds a certain threshold, the relationship between the flow field density and the vehicle speed presents a characteristic opposite to that at the flight condition of the speed lower than the threshold. On this basis, the Runge–Kutta method is applied to solve the ray equations in non-uniform plasma, in which the electron density at any computing point is obtained by the Lagrange interpolation based on the discrete data of the flow field. Then the EM wave attenuation in the sheath is determined by the subsection integral method combining with the electromagnetic theory. The numerical results also show that when the flight velocity is lower than the threshold, the energy transfer efficiency of the EM waves decreases with the increase of the vehicle velocity. However, when the flight speed is greater than the threshold, the energy transfer efficiency increases as the vehicle speed increases. This conclusion will be helpful to further understand the EM wave propagation characteristics for hypersonic vehicles, and to provide a reference for the channel selection of antennas.     

机翼后缘噪声预测研究

机翼后缘噪声是飞机重要的机体噪声源之一。本文基于CFD(Computational Fluid Dynamic)数值模拟和Ffcows Williams-Hall理论,研究应用了一种预测干净机翼后缘气动噪声的方法。采用Menter’s SSTκ-ω湍流模型对翼型和机翼进行N-S方程数值模拟得到后缘附近的湍流特征速度和特征长度,再利用Serhat Hosder的预估方法计算后缘噪声强度级。本文首先计算了NACA0012翼型在7种不同状态的后缘噪声,计算结果与实验值比较,符合很好,从而证明了本文采用的方法的可行性和正确性;然后研究了两个亚音速翼型(NACA 0009,NACA 0012),两个超临界翼型(SC(2)- 0710,SC(2)-0714),EET机翼的不同参数对后缘噪声强度级的影响,得出了对降低后缘噪声有参考意义的结论。     

出气边形状对叶栅中凝结流动影响机理的数值研究

湿蒸汽两相凝结流中凝结过程对流场参数的变化敏感。本文对一个叶型五种出气边形状对应的凝结流动进行了比较,以初步揭示几何参数影响凝结流动的机理。结果显示,出气边形状的改变影响凝结流中水滴状态、熵增、湿度、出口气流角的分布。在本文计算条件下,当出气边为平行于切向的平面时,流动中熵增最小,叶栅出口处湿度也最小,其原因在于这种出气边形状造成的流场参数改变使叶栅中蒸汽凝结过程受到抑制,叶栅出口蒸汽仍然维持了较高的非平衡程度。     

A frequency domain numerical method for airfoil broadband self-noise prediction

This paper describes a numerical approach, based in the frequency domain, for predicting the broadband self-noise radiation due to an airfoil situated in a smooth mean flow. Noise is generated by the interaction between the boundary layer turbulence on the airfoil surface and the airfoil trailing edge. Thin airfoil theory is used to deduce the unsteady blade loading. In this paper, the important difference with much of the previous work dealing with trailing edge noise is that the integration of the surface sources for computation of the radiated sound field is evaluated on the actual airfoil surface rather than in the mean-chord plane. The assumption of flat plate geometry in the calculation of radiation is therefore avoided. Moreover, the solution is valid in both near and far fields and reduces to the analytic solution due to Amiet when the airfoil collapses to a flat plate with large span, and the measurement point is taken to the far field.Predictions of the airfoil broadband self-noise radiation presented here are shown to be in reasonable agreement with the predictions obtained using the Brooks approach, which are based on a comprehensive database of experimental data. Also investigated in this paper is the effect on the broadband noise prediction of relaxing the ‘frozen-gust’ assumption, whereby the turbulence at each frequency comprises a continuous spectrum of streamwise wavenumber components. It is shown that making the frozen gust assumption yields an under-prediction of the noise spectrum by approximately 2dB compared with that obtained when this assumption is relaxed, with the largest occurring at high frequencies.This paper concludes with a comparison of the broadband noise directivity for a flat-plat, a NACA 0012 and a NACA 0024 airfoil at non-zero angle of attack. Differences of up to 20 dB are predicted, with the largest difference occurring at a radiation angle of zero degrees relative to the airfoil mean centre line.     

Formation of a reflected wave under propagation of a pulse with high peak intensity through a Kerr medium

Propagation through a Kerr medium of short pulses depending only on the longitudinal coordinate is investigated. High values of the peak intensity are considered for which the nonlinear part of the relative permittivity is on the order of unity. When a short pulse propagates through such a medium, the leading edge of the pulse is extended, while the trailing edge runs into the slowly propagating central part of the pulse; shock waves are generated at the trailing edge, giving rise to high spatial frequencies and backward reflected radiation. The duration of the pulse increases due to the high-frequency jet that forms at the trailing edge, and the peak intensity decreases. The spectrum of the backward reflected radiation is investigated as a function of the peak intensity of the pulse and the characteristics of the time dispersion of the medium.     

An experimental study of airfoil instability tonal noise with trailing edge serrations

This paper presents an experimental study of the effect of trailing edge serrations on airfoil instability noise. Detailed aeroacoustic measurements are presented of the noise radiated by an NACA-0012 airfoil with trailing edge serrations in a low to moderate speed flow under acoustical free field conditions. The existence of a separated boundary layer near the trailing edge of the airfoil at an angle of attack of 4.2 degree has been experimentally identified by a surface mounted hot-film arrays technique. Hot-wire results have shown that the saw-tooth surface can trigger a bypass transition and prevent the boundary layer from becoming separated. Without the separated boundary layer to act as an amplifier for the incoming Tollmien–Schlichting waves, the intensity and spectral characteristic of the radiated tonal noise can be affected depending upon the serration geometry. Particle Imaging Velocimetry (PIV) measurements of the airfoil wakes for a straight and serrated trailing edge are also reported in this paper. These measurements show that localized normal-component velocity fluctuations that are present in a small region of the wake from the laminar airfoil become weakened once serrations are introduced. Owing to the above unique characteristics of the serrated trailing edges, we are able to further investigate the mechanisms of airfoil instability tonal noise with special emphasis on the assessment of the wake and non-wake based aeroacoustic feedback models. It has been shown that the instability tonal noise generated at an angle of attack below approximately one degree could involve several complex mechanisms. On the other hand, the non-wake based aeroacoustic feedback mechanism alone is sufficient to predict all discrete tone frequencies accurately when the airfoil is at a moderate angle of attack.     

高空离轨发动机流场红外辐射特性研究

高超声速飞行器大攻角机动时,其离轨发动机产生的喷流与高速稀薄的大气来流产生强烈干扰,流场情况复杂,流场红外辐射也是天基红外系统探测的标志性事件.本文针对高超声速飞行器发动机喷流与稀薄来流的相互干扰情况,采用数值求解Navier-Stokes方程模拟干扰流场,采用逐线积分法得到气体红外辐射特性,结合反向蒙特卡洛方法计算得...     

Radiative characteristics of nitrogen upon excitation by volume discharge initiated by runaway electron beam

The amplitude, time, spectral, and energetic characteristics of a volume (diffuse) discharge at an elevated pressure (up to 5 atm) in a gap with an inhomogeneous electric field without an additional preionization source are studied. The concentration and temperature of electrons in the discharge plasma are mea-sured by spectral methods. The effective lifetime of the C ³Π _u state of the nitrogen molecule at the trailing edge of the radiation pulse is shown to be determined by radiative and collisional quenchings. For transitions of the second positive system of nitrogen, a plasma discharge radiation power into the complete solid angle is obtained to be ～120 kW, with a specific radiation power of up to ～50 kW/cm³.     

Sound generated in the vicinity of the trailing edge of an upper surface blown flap

Far field noise data indicated that for practical upper surface blown flap configurations, the noise radiated below the flap is dominated by the noise generated in the vicinity of the trailing edge. The sound field caused by turbulent mixing in the trailing edge wake is investigated experimentally and theoretically. Hot wire measurements were made downstream of the trailing edge to determine the gross turbulent mixing characteristics of the flow. This information is used as input to a theoretical analysis of the sound field. Favorable agreement is found between predicted and measured far field noise directivity at various frequencies and noise power spectra in various directions.     

High frequency sound emission from moving point multipole sources embedded in arbitrary transversely sheared mean flows

Formulas are derived for the high frequency sound emission from moving point multipole sources embedded in an arbitrary uni-directional transversely sheared mean flow. The results are used to study the sound generated by non-axisymmetric turbulent jets. The effect of the asymmetry in both the mean flow and the source distribution is accounted for by a “circumferential directivity factor”, which is easily calculated from the solution of a second order ordinary differential equation in the general case and from an explicit formula when the mean flow is symmetric but the source location is not. This factor is used to assess the potential of employing asymmetric velocity profiles that redirect the sound upward to reduce the noise radiation below the flight path of a jet aircraft.     

Vortex shedding noise of low tip speed,axial flow fans

Noise and performance tests were conducted on three low tip speed, half-stage, axial flow fans to determine the nature of the vortex shedding noise mechanism. Each fan was 356 mm in diameter and had eight equally spaced, variable pitch blades. The noise measurements were made in a free field environment and the fan back pressure and speed were varied during the tests. An acenaphthene coating on the blades was used to determine the regions of laminar and turbulent flow.Vortex shedding can be a significant source of noise when the fan is operated in a lightly loaded condition. Essentially it is due to instabilities in the laminar boundary layer on the suction side of the blade where these instabilities are in the form of Tollmien-Schlichting (T-S) waves. These instabilities interact with the trailing edge of the blade and generate acoustic waves which radiate from the trailing edge and form a feedback loop with the source of the instabilities. Vortex shedding noise can contribute as much as 5 dB in overall noise level and up to 22 dB at higher frequencies (8–14 kHz).Serrations located at the leading edge, at the mid-chord, or near the trailing edge on the suction side were found to reduce the vortex shedding noise significantly. The mid-chord location was found to be the most satisfactory because, as well as eliminating the noise, the serrations provided a 3% improvement in peak efficiency. This improvement occurred because separation of the laminar boundary layer was prevented on the suction side. On the other hand, serrations placed at the other two locations tended to degrade fan performance.     

基于等离子体环量控制的翼型气动特性

为了研究等离子体环量控制对翼型的影响特性,采用基于唯象学的等离子体气动激励数学模型和二维雷诺平均N-S方程,选取NCCR 1510-7067N环量控制翼型,数值模拟后缘半径对升力和效费比的影响规律,并进行优化。设计最佳后缘半径模型进行低速风洞实验,获得迎角-4~12,速度6,10,15 m/s下的压力分布和升力特性。研究表明:后缘半径过大或过小都不利于Coanda效应的产生,确定最佳后缘半径与弦长的比值为0.048,效费比97.69。低雷诺数下,随着迎角的增加,出现了层流长泡分离和短泡分离,等离子体射流不仅改善了尾部流场,还通过环量增加抑制层流分离,提高了升力。     

Cosmic rays and dosimetry at aviation altitudes

Recent concerns regarding the effects of the cosmic radiation field at aircraft altitudes on aircrew have resulted in a renewed interest in detailed measurements of the neutral and charged particle components in the atmosphere. CR-39 nuclear track detectors have been employed on a number of subsonic and supersonic aircraft to measure charge spectra and LET spectra at aircraft altitudes. These detectors are ideal for long term exposures required for these studies and their passive nature makes them suitable for an environment where interference with flight instrumentation could be a problem. We report here on measurements and analysis of short range tracks which were produced by high LET particles generated mainly by neutron interactions at aviation altitudes. In order to test the overall validity of the technique measurements were also carried out at the CERN-CEC field which simulates the radiation field at aviation altitudes and good agreement was found with dose values obtained using mainly heavy ion calibration.