Acoustic characteristics of pulse detonation engine with ellipsoidal reflector

Acoustic characteristics of a pulse detonation engine(PDE) with and without an ellipsoidal reflector are numerically and experimentally investigated. A two-dimensional(2 D) non-splitting unstructured triangular mesh Euler solver based on the space-time conservation element and solution element(CE/SE) method is employed to simulate the flow field of a PDE.The numerical results clearly demonstrate the external flow field of the PDE. The effect of an ellipsoidal reflector on the flow field characteristic near the PDE exit is investigated. The formation process of reflected shock wave and reflected jet shock are reported in detail. An acoustic measurement system is established for the PDE acoustic testing. The experimental results show that the ellipsoidal reflector changes the sound waveform and directivity of PDE sound. The reflected shock wave and reflected jet shock result in two more positive pressure peaks in the sound waveform. The ellipsoidal reflector changes the directivity of PDE sound from 20 to 0. It is found that the peak sound pressure level(PSPL) and overall sound pressure level(OASPL) each obtain an increment when the PDE is installed with a reflector. The maximum relative increase ratio of PSPL and OASPL are obtained at the focus point F2, whose values are 6.1% and 6.84% respectively. The results of the duration of the PDE sound indicate that the reflecting and focusing wave generated by the reflector result in the increment of A duration and B duration before and near focus point F2. Results show that the ellipsoidal reflector has a great influence on the acoustic characteristic of PDE sound. The research is helpful for understanding the influence of an ellipsoidal reflector on the formation and propagation process of PDE sound.     

空腔流动的大涡模拟及气动噪声控制

大涡模拟(LES)和三维的Ffowcs Williams-Hawkings声学比拟方法相结合,研究空腔过流的一种噪声控制措施.空腔的底板/后墙使用多孔壁板,因此流体可以穿透空腔壁面,多孔效果使用Darcy压力-速度关系模拟.声源流场由LES计算,声辐射和远声场由声学比拟获得.结果表明,这种措施有效地减弱了空腔内的压力脉动和远场声辐射,低频脉动Rossiter模数对应的波动幅值被有效抑制,声源中偶极子占优项大幅度减小,从而抑制了声辐射.     

可压缩涡流场中空泡运动规律及声辐射特性研究

基于可压缩流体力学基本理论, 通过边界积分方程, 采用不同表面压力模型, 求解空泡在计及可压缩性的涡流场中的运动规律; 通过表面离散及坐标变换, 采用Kirchhoff动边界积分方程, 将空泡表面视为运动变形边界, 作为直接噪声源, 获得涡流场中空泡运动产生的时域声压分布; 分析了涡流场参数对空泡运动规律及声辐射特性的影响. 研究结果表明: 计及流场可压缩性, 空泡的脉动幅度会随时间减弱, 辐射声压幅值随之减小; 空泡在涡流场中会发生延展、 颈缩、 撕裂, 并在撕裂后子空泡中形成射流; 当流场中的压力减小时, 空泡运动过程中的最大半径与撕裂前的最大长度逐渐增加, 且当流场中压力较小时, 空泡撕裂时形成的子空泡增多; 空泡辐射声压的指向性较弱, 撕裂会使辐射声压产生突变, 形成极大峰值; 随着涡通量的增大或空泡数的减小, 空泡脉动周期及其诱导的辐射声压波动周期随之延长, 辐射声压峰值逐渐滞后并减小. 本文结果旨在为涡流场中空泡运动规律及声辐射特性的相关研究提供参考. 关键词： 可压缩 涡流场 空泡 声辐射     

基于FW-H方程的Hartmann哨辐射声场数值研究*

该文基于声类比法,通过求解FW-H方程,对不同喷口与谐振腔位置关系的Hartmann哨的声场分布进行了分析,计算了不同位置关系的Hartmann哨的声指向性。得到了以下结论:喷口与谐振腔位置关系对Hartmann哨的发声基频几乎不产生影响。对称位置关系的Hartmann哨的声场呈对称趋势,且在垂直于喷流方向上达到最大;非对称位置关系的Hartmann哨不再呈现对称趋势。Hartmann哨的最大声压级随着偏离对称位置的距离的增加有小幅增加,最大声压级出现的方向向着谐振腔方向移动。     

水下超声速气体射流诱导尾空泡实验研究

两相射流与空化问题对采用喷气推进的水下高速运载器而言不可避免.本文通过水洞实验,探究了回转体在水流场中由亚声速及超声速气体射流诱导形成尾空泡的形态特征,发现了四种不同类型的诱导尾空泡,并探讨了相应的形成机理和控制条件.通过高速图像采集及数字处理技术,得到了不同弗劳德数和通气流量系数下诱导尾空泡的瞬时及时间平均形态.通过气体射流数值解及射流耦合空泡闭合理论模型与实验图像的对比分析,得到如下结论:根据形态特征,将观察到的射流诱导尾空泡划分为泡沫状、完整、部分破碎和脉动泡沫状四类,其中诱导产生的部分破碎尾空泡在形态上与超空泡存在明显差异,脉动泡沫状则为诱导空泡所特有;气体射流受到空泡阻挡发生回射后对应的实际通气流量系数是控制空泡形态的关键;诱导空泡类型转变可以通过Paryshev提出的射流空泡耦合模型预测,但必须在考虑射流空间结构和流动损失的前提下;进行上述修正后,诱导尾空泡形态变化规律与理论模型估算得到的实际流量系数相符合.     

利用质量流注入的流激孔腔噪声共振峰抑制

通过数值仿真揭示了开口前缘垂直注入质量流和前壁面平行注入质量流抑制流激孔腔噪声的机制,研究了多参数影响下脉动压力峰值降噪量和总降噪量随质量流注入速度的变化规律。开口前缘垂直注入质量流通过抬升剪切层,避免漩涡冲击开口后缘,抑制流激孔腔噪声脉动压力峰值;在一定范围内质量流注入速度越大,脉动压力峰值降噪量越大,但是低频部分引起的抬升也会越高,导致总降噪量先增大后减小;经优化后的峰值降噪量和总降噪量分别可以达到15 dB和9.5 dB。开口前壁面平行注入质量流则是通过加强开口处剪切层的稳定性,避免发生漩涡脱落,达到抑制流激孔腔噪声的目的;当质量流入口面积大于孔腔开口前壁面积2/3时,不仅可以显著降低流激孔腔噪声脉动压力的峰值,并且可以很好地抑制其它频段噪声的抬升;质量流注入速度为来流速度的0.5倍时,脉动压力峰值降噪量和总降噪量分别可以达到18 dB和15.4 dB。     

Computation of the sound field of a jet from characteristic turbulence parameters measured by crossed beam techniques

The sound field radiating from a jet is strongly dependent upon the turbulence in the jet. To describe the sound sources in a hot jet, a method has been developed, based upon the measurement of infra-red radiation of the jet, and a hybrid processing of the measured signal, which enables the computation of characteristic properties of the turbulence at various points inside the jet (convection speed, integral length scale, life time and intensity of turbulence).From these quantities the acoustic far field can be mapped, as a function of the polar distribution of the spectrum, the turbulence intensity and the total sound pressure field.     

合成双射流对下游声压级影响试验

在Ma=0.4的来流条件下, 利用安装在主翼后缘处的合成双射流激励器对襟翼上的流动进行控制, 在风洞中开展了合成双射流对下游声压级影响的研究. 基于脉动压力测量结果, 结合油流显示试验, 得到了合成双射流对下游不同流动状态区域声压级影响的一些结论. 对于附着流, 在其峰值频率附近激励会明显提高其声压级; 对于受旋涡主导的流动, 恰当的合成双射流控制可以降低声压级, 激励频率较为关键. 在俯仰运动过程中, 对于附着流, 激励提高了声压级, 但不改变其迟滞特性; 对于受旋涡主导的流动, 激励对声压级的影响与攻角有关, 能够减弱其迟滞特性, 但激励强度对迟滞特性的影响较小, 减小声压级的最佳激励与运动历程有关.      

Roles of initial condition and vortex pairing in jet noise

This is a study of the effect of initial condition on sound generated by vortex pairing in a low Mach number, cold air jet (0·15 ⩽ M ⩽ 0·35). Data has been taken, both flow velocity fields and sound pressure far fields, in a quality anechoic facility, with careful documentation of the effect of initial condition on the sound field of jets of two different geometries (i.e., circular and elliptic). Explanations are presented for most of the observed effects by applying Möhring's theory of vortex sound to vortex filament models of coherent structures in the jets. The explanations also draw upon experience with coherent structure dynamics. The sound source of interest here is that associated with the pairing of shear layer vortices. The evolution of these vortices is greatly affected by the initial condition as is their resultant sound field. The elliptic jets with laminar boundary layers show azimuthal directivity, namely, sound pressure levels in the minor axis plane were greater than in the major axis plane. This difference decreases as the nozzle boundary layer undergoes natural transition with increasing jet speed. When the nozzle boundary layer is tripped, making it fully turbulent and removing the shear layer mode of pairing, the elliptic jet sound fields become nearly axisymmetric. What appears to be the most acoustically active phase of vortex pairing has been modeled, and the resulting sound field calculated for the circular jet. Supporting evidence is found in the experimental data for the validity of this model. The model explains the connection between the initial condition and the far field sound of jets. Interestingly, a general result of Möhring's theory is that motions of vortex rings (of any arbitrary shape) can produce only axisymmetric sound fields if the rings remain in a plane. This implies that the observed asymmetric directivity of the laminar elliptic jet sound field must be due to non-planar ring motions of the vortical structures. The primary contribution of this paper is to examine quantitatively the role of vortex pairing in the production of jet noise; the results are used to reemphasize that “pairing noise” cannot be dominant in most practical jet sound fields, contrary to claims by other researchers.     

Experimental verification of the quasi-steady approximation for aerodynamic sound generation by pulsating jets in tubes

Voice production involves sound generation by a confined jet flow through an orifice (the glottis) with a time-varying area. Predictive models of speech production are usually based on the so-called quasi-steady approximation. The flow rate through the time-varying orifice is assumed to be the same as a sequence of steady flows through stationary orifices for wall geometries and flow boundary conditions that instantaneously match those of the dynamic, nonstationary problem. Either the flow rate or the pressure drop can then be used to calculate the radiated sound using conventional acoustic radiation models. The quasi-steady approximation allows complex unsteady flows to be modeled as steady flows, which is more cost effective. It has been verified for pulsating open jet flows. The quasi-steady approximation, however, has not yet been rigorously validated for the full range of flows encountered in voice production. To further investigate the range of validity of the quasi-steady approximation for voice production applications, a dynamic mechanical model of the larynx was designed and built. The model dimensions approximated those of human vocal folds. Airflow was supplied by a pressurized, quiet air storage facility and modulated by a driven rubber orifice. The acoustic pressure of waves radiated upstream and downstream of the orifice was measured, along with the orifice area and other time-averaged flow variables. Calculated and measured radiated acoustic pressures were compared. A good agreement was obtained over a range of operating frequencies, flow rates, and orifice shapes, confirming the validity of the quasi-steady approximation for a class of relevant pulsating jet flows.     

A study on generation of noise by high-speed pulsating jets

This paper describes the noise generation in an exhaust system of a reciprocating engine and focuses on the noise generated by shock/vortex interaction. The pulsating flow through the exhaust pipe consists of the compression and expansion wave, shock wave being generated by the non-linearity of the compression wave at its head. The jet noise is produced when the pulsating flow is discharged from the pipe end into atmosphere. The numerical simulation based on a finite difference method and experiment were made, the result of both of them being compared. First, the flow field in the pipe was obtained to easily discuss the characteristic of the pulsating jet in terms of the pressure history in the pipe. The jet structure was visualized by using schlieren and shadowgraph techniques. Sound pressure measurements at various locations were made in order to survey the directivity of the noise. The comparison between the result of numerical calculation and experiment showed a good agreement. A noise source related to shock/vortex interaction was confirmed by the numerical study clearly.     

On the acoustical relevance of supraglottal flow structures to low-frequency voice production

The supraglottal flow exhibits many complex phenomena such as recirculation, jet instabilities, jet attachment to one vocal fold wall, jet flapping, and transition to turbulence. The acoustical relevance of these flow structures to low-frequency voice production was evaluated by disturbing the supraglottal flow field using a cylinder and observing the consequence on the resulting sound pressure field. Despite a significantly altered supraglottal flow field due to the presence of the cylinder, only small changes in sound pressure amplitude and spectral shape were observed. The implications of the results on our understanding of phonation physics and modeling of phonation are discussed.     

高速受限混合层结构和效应实验研究

受限混合层的流动主要是喷流与自由来流相互剪切形成的混合层受到壁面的限制而形成的一种流动.文章采用后向台阶平板模型研究了高速高压比条件下的受限混合层的典型流场结构以及冷却效率.实验自由来流Mach数为5, 喷流的Mach数为1.28, 喷流总压为0.2~0.7 MPa, 通过调整冷喷气流的总压, 基于纹影流动显.形成喷口附近波系的欠膨胀流动现象的深刻认识, 提取波系特征与流动参数之间的规律.基于流动显示及实验测量结果, 通过分析流场中大尺度结构的空间演化规律, 揭示流动参数对于冷却效率的影响规律及物理内涵.采用快响应压敏漆(FRPSP)技术在高超声速风洞开展热流分布和冷却效率研究, 获得了平板对受限混合层冷却效率的影响.      

Near-field acoustic characteristics of screech jet exhausted from a nozzle with a hard reflecting plate

The standing wave in the near field of the screech jet exhausted from a nozzle with a hard plate works on the jet flow as the forcing wave by the location of a reflecting plate, and then jet flow is considered to be changed. Moreover, the reflector location from the nozzle changes the sound pressure contours of the near field. Intensity maps of the screech tone which indicate the propagation to the jet axial direction or the radial direction of the jet by the presence of the reflector plate have not been explored. In the present paper, acoustic characteristics in the near field of the screech tone with the reflecting plate are studied using an optical wave microphone, which can measure the sound propagating for both vertical and horizontal directions to the jet axis. As a result, the standing wave in the near field of the screech jet with the reflector has two types: One is the standing wave between the hydrodynamic pressure fluctuation propagating jet downstream and the sound pressure propagating upstream, and the other is the standing wave by the difference between the wavelength of the sound wave and the wavelength at the place close to the jet.     

The sound field of a rotating monopole in a plug flow

A theoretical study is performed on the sound field generated by a rotating point monopole in a jet flow, the mixing layer of which is simulated by a velocity discontinuity. Its sound in the far field is compared to the sound field generated by a rotating monopole in a uniform flow in the absence of a velocity discontinuity, which makes it possible to estimate the size of the sound refraction effect.     

发动机喷流对火箭气动特性影响

开展了考虑底部发动机喷流影响的火箭气动特性CFD仿真设计,比较了有/无喷流时火箭附近流场结构、表面压力分布、整体气动力/力矩特性在亚/超声速段的差异,结果显示,发动机喷流对火箭亚声速段的轴向力、法向力和俯仰力矩特性均有较为显著的影响,且有减小尾部空气舵气动控制力矩的影响,而超声速段的影响仅限于轴向力。该仿真结果与飞行试验气动辨识结果较为一致。基于仿真分析结果,可建立一种折中考虑喷流影响的气动特性设计方法,供火箭精细化气动特性设计参考使用。      

On the interaction of a sound pulse with the shear layer of an axisymmetric jet,II: Heated jets

The fluctuating field of a heated jet excited by a sound pulse is simulated numerically. The fluctuations in both the flow field and the far field are studied. The flow field results depend crucially on the stability properties of the heated jet. The altered stability properties due to the heating cause a reduction in the overall fluctuations and a shift of the instability waves into lower frequencies. These changes cause a similar downward shift in the far field spectrum and a reduction in the total far field noise. The results provide a partial explanation in terms of stability theory of experimentally observed properties of the noise of heated jets.     

A theoretical model of the pressure field arising from asymmetric intraglottal flows applied to a two-mass model of the vocal folds

A theoretical flow solution is presented for predicting the pressure distribution along the vocal fold walls arising from asymmetric flow that forms during the closing phases of speech. The resultant wall jet was analyzed using boundary layer methods in a non-inertial reference frame attached to the moving wall. A solution for the near-wall velocity profiles on the flow wall was developed based on a Falkner-Skan similarity solution and it was demonstrated that the pressure distribution along the flow wall is imposed by the velocity in the inviscid core of the wall jet. The method was validated with experimental velocity data from 7.5 times life-size vocal fold models, acquired for varying flow rates and glottal divergence angles. The solution for the asymmetric pressures was incorporated into a widely used two-mass model of vocal fold oscillation with a coupled acoustical model of sound propagation. Asymmetric pressure loading was found to facilitate glottal closure, which yielded only slightly higher values of maximum flow declination rate and radiated sound, and a small decrease in the slope of the spectral tilt. While the impact on symmetrically tensioned vocal folds was small, results indicate the effect becomes more significant for asymmetrically tensioned vocal folds.     

低速开式空腔自激反馈流场结构与流致噪声的风洞试验研究

本文主要针对低速开式空腔流动自激振荡产生噪声问题,在0.55 m×0.4 m航空声学风洞开展了不同低马赫数(0.1/0.15/0.2/0.25)条件下长深比为2的空腔腔内流场结构和噪声特性风洞试验研究。通过利用高频粒子图像测速技术捕捉腔内流场结构,分析了腔内声波传递路径;完成空腔远场噪声和壁面压力测试,分析了噪声自激振荡模态和简正波模态,并对空腔壁面脉动压力和远场噪声进行压/声相关性研究。结果表明:空腔内部除主涡外,在腔口前缘处剪切涡与腔口后缘处碰撞涡明显存在;在875 Hz,1288 Hz,1875 Hz,2050 Hz四个频率附近出现了由声腔共振所致的单频噪声;壁面压力与远场噪声密切相关,在壁面压力主频位置有明显单频噪声出现。      

Flow visualization of vortex structure in a pulsed rectangular jet

Pulsating jet is visualized using hydrogen bubble method to clarify the vortex nature in the near field of the jet. This study focused on the development in space and time of vortex structures evolution in low aspect-ratio rectangular jet with pulsation. Pulsation means large-amplitude, low-frequency excitation which is expected to increase the mixing and spreading of the jet and to accelerate its transition from a rectangular form to an axisymmetric form. It was deemed appropriate to investigate whether jet characteristics of a pulsating, submerged jet flow can be altered by including pulsations. The difference of the vortex deformation process is discussed in relation to pulsating conditions. Consequently, the pulsation leads to the formation of vortices at regular intervals, which are larger than those occurring in a steady jet. The results show that the streamwise interaction, between leading vortex and trailing vortex rolled up at nozzle lips, strengthens with increasing pulsating frequency. The spanwise drift of the vortex becomes strongly apparent at large amplitude and high frequency conditions. The drifting start position does not change regardless of pulsating condition. The convection velocity of vortex increases at lower frequency and larger amplitude.