非均匀桨直升机旋翼厚度噪声分析

分析了桨叶间距非均匀调制对直升机旋翼厚度噪声的影响.桨叶间距按照正弦调制和余弦调制变化时,根据FW-H方程的Formulation 1A公式,计算了直升机悬停状态下的旋翼厚度噪声.桨尖速度为亚音速时的计算结果显示,与桨叶间距均匀的直升机旋翼厚度噪声相比,非均匀桨可以改变厚度噪声的频谱特征,而且在基本保持厚度噪声总声压级不变情况下,降低厚度噪声桨叶通过频率及谐频的线谱幅值.本文的分析和结果有助于认识非均匀旋翼厚度噪声的频谱变化规律.     

共轴双旋翼悬停状态气动噪声特性分析

结合CFD(Computational fluid dynamics)方法和FW-H(Ffowcs Williams-Hawkings)方程,建立了一套适合于悬停状态下共轴刚性双旋翼气动噪声特性计算方法。为了准确模拟共轴旋翼流场的涡干扰现象和非定常特性,基于运动嵌套网格技术与双时间推进方法,采用积分形式的可压雷诺平均Navier-Stokes(RANS)方程作为双旋翼非定常流场求解控制方程,湍流模型选用Baldwin-Lomax模型。通过Farassat 1A公式计算双旋翼气动噪声特性,每个声源微面的位置和载荷信息直接从桨叶表面网格中获取。然后,对水平面内和竖直面内观测点处共轴双旋翼厚度噪声、载荷噪声和总噪声的声压时间历程和频谱特性做了细致对比。模拟结果表明:上旋翼和下旋翼反向旋转的特点对声压时间历程影响显著,不同方向观察点的声压波形峰值对应的相位不同;共轴旋翼流场中存在的文丘里效应、桨-涡干扰现象以及下洗流的作用使得桨叶气动载荷呈现明显的非定常特征,导致共轴双旋翼的载荷噪声辐射强度较大;在低频段,总噪声受厚度噪声主导,而在高频段则受载荷噪声主导。      

旋翼桨尖几何形状对旋翼气动噪声影响的定量计算分析

采用CFD与计算声学相结合的Euler Kirchhoff方法对UH-1H直升机旋翼跨声速悬停流动的远场噪声进行定量计算,并将计算结果与实验值进行比较,验证了Euler Kirchhoff方法的正确性.然后以UH-1H直升机旋翼为基准旋翼,定量计算了对桨尖旋翼翼型厚度、旋翼尖削度、后掠角进行变化后的旋翼悬停流场的远场噪声,分析了旋翼桨尖几何形状对噪声的影响.得出结论:降低桨尖翼型厚度、浆尖尖削、浆尖后掠均可以降低噪声峰值.     

开式轴流风扇气动噪声预测

本文采用LES/FW-H的匹配方法,研究了开式轴流风扇内部旋涡流动特征及其与叶片表面干涉引起的气动噪声之间的联系,同时进行了远场噪声预测,探讨了叶轮不同表面辐射噪声时的频谱分布特征.研究结果表明,开式轴流风扇吸力面附近形成的叶尖涡和前缘分离涡在吸力面叶片表面相应位置形成大压力波动,形成主要噪声源;叶片吸力面的辐射噪声可以通过改善吸力面附近的旋涡流动来降低;低速轴流叶轮由叶轮壁面辐射的噪声以宽频成分为主.     

单叶片桨噪声相移叠加法预报多叶片船舶螺旋桨非空化低频噪声

单个桨叶噪声预报螺旋桨非空化噪声可显著降低计算耗时,本文联合URANS和声类比方程对该方法进行验证,并对螺旋桨噪声的时域特征进行分析。首先计算均匀流中E779A螺旋桨的声辐射,揭示了桨叶上声压分布及测点声压信号的典型周期特征,采用单个叶片噪声相移叠加(简称"单叶片方法")预报螺旋桨噪声的结果,与对螺旋桨所有叶片积分的计算结果吻合良好,验证了均匀流中单叶片方法的可行性。将该方法应用于潜艇伴流场中无侧斜和大侧斜螺旋桨噪声辐射计算,预报结果与所有叶片积分的结果吻合较好,验证了非均匀流场中单叶片方法的可行性,说明单叶片相移叠加法预报螺旋桨普遍进流条件下的辐射噪声是可行的。研究结论也可为对转桨、泵喷等噪声预报提供参考。      

弧形锯齿进口导流罩对轴流风扇性能的影响

本文应用LES/FW-H方法,研究了在导流罩入口采用弧形锯齿结构的半开式轴流风扇的气动声学性能。研究结果表明,采用弧形锯齿进口的周向非等宽导流罩可减小叶尖涡影响区域,降低叶尖涡强度。导流罩进口结构的变化对叶尖涡涡心轨迹有一定影响,弧形锯齿进口导流罩使得叶尖涡轨迹更远离轴心,随着弧形锯齿数目的增加,其叶尖涡轨迹越远离轴心。半开式轴流风扇的总噪声和导流罩的声辐射特性密切相关,采用弧形锯齿进口导流罩可降低风扇气动噪声,其中出口轴线方向可降低3 dB以上,且在一定范围内随着弧形锯齿数目的增加降噪程度越明显。     

风力机近尾迹叶尖区域气动噪声变化规律的试验研究

保证额定来流风速10 m/s不变,在不同尖速比条件下,利用声阵列法对S系列翼型风力机的叶尖区域噪声进行了测试.结果表明:风轮在旋转过程中,风力机叶尖辐射噪声频谱在200 Hz以下主要是由叶片旋转基频及其谐波构成的离散噪声叠加在宽频噪声上而组成的,而在200 Hz左右至600 Hz则是由叶片旋转产生的叶尖涡脱落频率以及风洞动力风扇的二次谐波构成的.随着尖速比的增加叶尖涡的脱落频率在增人,旋转基频声压级在叶片相对半径为0.57处的辐射噪声最大,且随着向叶尖移动,噪声逐渐减小。距离风轮旋转平面的轴向方向x=10~60 cm之间辐射噪声衰减较快,在x=60~100 cm之间衰减较慢.发现了叶尖涡的运动轨迹,叶尖涡是逐渐的向外迁移.     

基于多信道无线传输的旋翼载荷测试技术研究

直升机旋翼系统载荷和强度飞行试验是对真实大气环境中旋翼系统应力载荷谱的研究,它提供的真实数据是理论计算所不能提供的。因此旋翼系统载荷试飞是直升机设计定型试飞中极其重要的项目。针对直升机旋翼系统载荷测试技术需求,采用模块化、冗余度和高集成的设计理念,通过多信道无线传输设计等技术,将采集的动态载荷数据调制、发射与解调,实现了多通道、高带宽和精同步的旋翼系统载荷数据采集与监控。该技术对于直升机旋翼系统载荷试飞中遇到的类似问题具有一定的借鉴意义。     

三维FW-H方程与CAA数值模拟匹配技术研究

本文研究了三维FW-H方程与CAA数值模拟匹配技术。首先验证了适于亚音任意运动声源的Farassat时域公式和适于亚音匀速直线运动声源的Lockard频域公式,采用均匀流中单极子源声辐射问题对两类公式进行了校核。进一步,采用FW-H／CAA匹配技术对风扇／压气机前传声进行了预测。近声场基于轴对称三维CAA方法获得,远声场则基于近声场数据采用三维可穿透FW-H方程时域公式进行预测,并校核算例着重分析了不同积分面对远场声指向性的影响。本文研究证实了FW-H／CAA数值模拟匹配技术的可行性和解决工程实际问题的潜力。     

风向对甲板直升机旋翼流场结构的影响

采用数值模拟方法对船体与直升机旋翼的复合流场进行求解,研究了不同风向时复合流场的流线形态与湍动能分布、旋翼桨叶表面静压分布与旋翼升阻特性.数值模拟的正确性由缩比船模风洞试验验证.研究结果表明:旋翼流场与船体流场间存在相互干扰;侧风会增大复合流场的湍流范围,加剧旋翼周围流场的湍流强度和旋翼桨叶的挥舞振动;右舷15°风向条件不利于直升机的甲板悬停.     

Helicopter noise in hover: Computational modelling and experimental validation

The aeroacoustic characteristics of a helicopter rotor are calculated by a new method, to assess its applicability in assessing rotor performance in hovering. Direct solution of the Euler equations in a noninertial coordinate system is used to calculate the near-field flow around the spinning rotor. The far-field noise field is calculated by the Ffowcs Williams–Hawkings (FW–H) method using permeable control surfaces that include the blade. For a multiblade rotor, the signal obtained is duplicated and shifted in phase for each successive blade. By that means, the spectral characteristics of the far-field noise may be obtained. To determine the integral aerodynamic characteristics of the rotor, software is written to calculate the thrust and torque characteristics from the near-field flow solution. The results of numerical simulation are compared with experimental acoustic and aerodynamic data for a large-scale model of a helicopter main rotor in an open test facility. Two- and four-blade configurations of the rotor are considered, in different hover conditions. The proposed method satisfactorily predicts the aerodynamic characteristics of the blades in such conditions and gives good estimates for the first harmonics of the noise. That permits the practical use of the proposed method, not only for hovering but also for forward flight.     

Frequency-domain method for discrete frequency noise prediction of rotors in arbitrary steady motion

A novel frequency-domain formulation for the prediction of the tonal noise emitted by rotors in arbitrary steady motion is presented. It is derived from Farassat's ‘Formulation 1A’, that is a time-domain boundary integral representation for the solution of the Ffowcs-Williams and Hawkings equation, and represents noise as harmonic response to body kinematics and aerodynamic loads via frequency-response-function matrices. The proposed frequency-domain solver is applicable to rotor configurations for which sound pressure levels of discrete tones are much higher than those of broadband noise. The numerical investigation concerns the analysis of noise produced by an advancing helicopter rotor in blade–vortex interaction conditions, as well as the examination of pressure disturbances radiated by the interaction of a marine propeller with a non-uniform inflow.     

Aeroelastic stability of complete rotors with application to a teetering rotor in forward flight

The derivation of a set of non-linear coupled flap-lag-torsion equations of motion for moderately large deflections of an elastic, two-bladed teetering helicopter rotor in forward flight is concisely outlined. The following degrees of freedom are included in the mathematical model: rigid body flapping, rigid body lead-lag, elastic bending in flap and lead-lag, blade root torsion, and shaft torsion. Quasi-steady aerodynamic loads are considered and the effects of reversed flow are included. The aeroelastic stability of the complete rotor is investigated by using a linearized system of equations of motion. The equilibrium position about which the equations are linearized is obtained by considering the trim state of the helicopter, in true or simulated forward flight conditions. The sensitivity of the aeroelastic stability boundaries to interblade structural and mechanical coupling is illustrated by comparing the complete rotor stability boundaries with those obtained from a single blade analysis for a number of hover and forward flight cases.     

The acoustic spectrum of axial flow machines

Rotor spectra from a variety of axial flow machines have been compared and acoustic trends examined. Types of rotor considered in the investigation included aircraft propellers, helicopter rotors, jet engine compressor and by-pass fans and a range of domestic, automotive and industrial cooling fans. Rotor sizes varied from 15 in to 60 ft in diameter and tip speeds from 200 ft/s to transonic speeds.The study showed that the acoustic output from these various rotor devices can be described by a common characteristic spectrum. In particular, the minimum broad band noise generated by ducted and free field rotors corresponds to laminar and turbulent boundary layer shedding, and discrete excess noise is generated by various degrees of impulsive blade loading. Finally the study showed that aerodynamic details can be interpreted from the acoustic spectra, and as a result, new insight into the mechanisms of rotor noise has been formed.     

直升机交互式桨叶振动调整仿真研究

为解决目前机务维修培训中存在的物理样机难以满足需求的问题,以实际机务维修工作中旋翼振动调整方法为参考,提出在虚拟维修训练系统的基础上实现受训人员对虚拟样机旋翼桨叶进行振动调整仿真操作,以期达到训练目的。根据非沉浸式虚拟维修训练系统的特点设计交互仿真操作逻辑,确定仿真操作步骤,分析座舱振动可视化表示方法,专用虚拟样机的构建方法以及虚拟仪器的实现方式和数据组织结构形式,并将该仿真实验以模块化形式添加到虚拟维修训练系统中,满足拆装条件下对虚拟样机的交互操作需求。最后以欧直松鼠直升机虚拟样机旋翼振动调整方法为例验证该操作仿真逻辑的可用性。     

Noise of a model helicopter rotor due to ingestion of isotropic turbulence

A theoretical and experimental investigation of the noise of a model helicopter rotor due to the ingestion of grid-generated, isotropic turbulence is described. Simulated forward flight and vertical ascent tests were performed with a 0·76 m diameter, articulated model rotor in the United Technologies Research Center Acoustic Research Tunnel. Far field noise spectra and directivity were measured in addition to inflow turbulence intensities, length scales and spectra. Measured inflow turbulence statistics and rotor operating parameters were employed in a theoretical procedure to predict turbulence ingestion noise spectra and directivity. This theoretical formulation represented an absolute level prediction method in that empirical or adjustable constants were not employed. From this study it is concluded that incident turbulence represents a potentially important source of rotor narrowband random (quasi-tonal) and broadband noise. With due regard to the absolute level nature of the prediction method, agreement between theory and experiment is reasonable: quasi-tonal noise at low frequency tends to be overpredicted whereas mid-frequency tonal noise and high frequency broadband noise are well predicted. This theory, therefore, provides a means to predict the contribution of turbulence ingestion noise to overall rotor noise spectra and directivity in cases where reasonable estimates of incident turbulence statistics can be made. Ingestion of main rotor wake turbulence by tail rotors in flight and ingestion of atmospheric turbulence by main and tail rotors in hover would be expected to be the most important sources of turbulence ingestion noise in full scale applications.     

OPTIMUM DESIGN OF A HELICOPTER ROTOR FOR LOW VIBRATION USING AEROELASTIC ANALYSIS AND RESPONSE SURFACE METHODS

An aeroelastic analysis based on finite elements in space and time is used to model the helicopter rotor in forward flight. The rotor blade is represented as an elastic cantilever beam undergoing flap and lag bending, elastic torsion and axial deformations. The objective of the improved design is to reduce vibratory loads at the rotor hub that are the main source of helicopter vibration. Constraints are imposed on aeroelastic stability, and move limits are imposed on the blade elastic stiffness design variables. Using the aeroelastic analysis, response surface approximations are constructed for the objective function (vibratory hub loads). It is found that second order polynomial response surfaces constructed using the central composite design of the theory of design of experiments adequately represents the aeroelastic model in the vicinity of the baseline design. Optimization results show a reduction in the objective function of about 30 per cent. A key accomplishment of this paper is the decoupling of the analysis problem and the optimization problems using response surface methods, which should encourage the use of optimization methods by the helicopter industry.     

A moving medium formulation for prediction of propeller noise at incidence

This paper presents a time domain formulation for the sound field radiated by moving bodies in a uniform steady flow with arbitrary orientation. The aim is to provide a formulation for prediction of noise from body so that effects of crossflow on a propeller can be modeled in the time domain. An established theory of noise generation by a moving source is combined with the moving medium Green's function for derivation of the formulation. A formula with Doppler factor is developed because it is more easily interpreted and is more helpful in examining the physic of systems. Based on the technique presented, the source of asymmetry of the sound field can be explained in terms of physics of a moving source. It is shown that the derived formulation can be interpreted as an extension of formulation 1 and 1A of Farassat based on the Ffowcs Williams and Hawkings (FW–H) equation for moving medium problems. Computational results for a stationary monopole and dipole point source in moving medium, a rotating point force in crossflow, a model of helicopter blade at incidence and a propeller case with subsonic tips at incidence verify the formulation.     

Nondestructive evaluation of helicopter rotor blades using guided Lamb modes

This paper presents an application for turning and direct modes in a complex composite laminate structure. The propagation and interaction of turning modes and fundamental Lamb modes are investigated in the skin, spar and web sections of a helicopter rotor blade. Finite element models were used to understand the various mode conversions at geometric discontinuities such as web-spar joints. Experimental investigation was carried out with the help of air coupled ultrasonic transducers. The turning and direct modes were confirmed with the help of particle displacements and velocities. Experimental B-Scans were performed on damaged and undamaged samples for qualitative and quantitative assessment of the structure. A strong correlation between the numerical and experimental results was observed and reported.