【作者】 赵云；

【导师】 曾新吾；

【作者基本信息】 国防科学技术大学 ， 力学， 2010， 博士

【摘要】 调制气流声源是一种经典的频谱可控的大功率强声源,一直以来人们对其内部流场和流动致声过程缺乏细致深入的分析。本文建立了调制气流声源的数值仿真模型,搭建了声源内部流场和近距离声场的实验测试系统。对一种射流式调制气流声源在不同工况和结构参数下的换能过程和声场特性进行了较为系统的研究。本文首次完成了声源装置中超声速射流调制的设计和实现,通过对激励调制信号的特殊设计,获得了较为显著的声压级增益。本文考虑了若干不同形式的声源内气动系统设计,对相关的瞬态流场进行了仿真分析,从中探讨了喉道形状对声能转换过程的影响。本论文的主要研究内容和结论如下:对调制气流声源内部瞬态流场和近距离声场的数值仿真模型进行了研究。为克服传统一维准稳态理论预测声源性能的局限性,基于瞬态雷诺时均方程耦合FW-H声类比的混合气动声学方法,建立了可应用于具体声源装置的气流声源仿真模型。声源内部流场仿真结果与实测结果的比较说明,将混合气动声学方法应用于气流声源性能预测和换能过程研究在理论和实现上是可行的。基于粒子图像测速(PIV)、单点测压和强声场测试方法,建立了调制气流声源流动致声实验系统,获取了不同气室压力、几何结构参数和激励信号等条件下的声源内部速度场、压力场和近距离声场数据。研究了声源装置可调参数对内部流场和外部声场特性的影响。稳态流场实测结果证实了内流场具有流动分离、管口回流和剪切分层等特性。声源频率响应测量结果表明,内气动系统的高速流动对调制部件的振动过程有影响。声源内压力扰动量级随气室压力和激励信号强度的增加而增加,静压频谱结果中能量集中在基频和高次谐波。不同声源参数下,近距离声场结果在量级和频谱特性等方面的变化与扰动压力的变化保持一致。基于所建立的声源仿真模型,进一步研究了气室压力、喉道入口宽度、调制频率和气体工作介质等参数对声源性能和声能转换过程的影响。模拟结果表明,喉道内压力扰动量级随气室压力的增加而逐渐趋于饱和,改变喉道入口宽度对真实的调制面积比和声学区域静压值的变化影响不大,不同调制频率对应的声源内部静压值分布及其变化过程具有显著差异。对于高频调制,声源内部存在明显的非线性波形失真和附加衰减作用,在剪切层中可见旋涡的产生和对流过程。超声速调制有望从原理上突破声压级输出随气室压力的增加而趋于饱和的限制。本文通过实验测试和模拟分析两种途径,研究了超声速射流受限调制的换能过程和增益特性。声场实测结果表明,当气室压力和激励信号强度较高时,超声速调制能够在低频和频率响应峰值附近等调制度较高的频段产生更高的声压级输出。500Hz超声速调制实验中1米处RMS声压级大于150dB,比相同条件下的声速调制结果高8dB左右。为解释声压级增益的频率相关性,模拟分析了设计马赫数3、频率500Hz的超声速调制换能过程。模拟结果验证了超声速调制的增益效果,但发现调制过程中喷管内未完全形成超声速流动,原因是超声速流动形成的速度低于喷口面积变化的速度。为实现对真实超声速射流的调制,提出了延时调制的概念。模拟结果验证了采用所设计的延时调制信号能够较大幅度提高超声速调制的入口流量和压力扰动量级。为研究声源内气动系统具体设计对换能过程的影响,考虑了多种喉道型线形状。在相同工况条件下,比较了内部瞬态流场的演化过程。不同设计获得的平均流动和声能转换过程存在显著差异。喉道设计的更改影响了压力扰动的形成和压力波形的频谱特性。其中,高速主流与压力扰动之间的相互作用、喉道出口两侧的阻抗匹配情况被认为是重要的影响因素。此外,为降低部分喉道设计对应的结果中压力波形的严重失真和压力扰动的空间不均匀性,本文提出了一种外喷式内气动系统设计。模拟结果验证了外喷式设计的作用效果。更多还原

【Abstract】 Air-Modulated Speaker (AMS) is one of the most popular high power and high intensity acoustic sources due to its high sound pressure level (SPL) output and wide frequency responses. Most of the previous works on AMS are concerned with its sound field. However, it is well understood that acoustic characteristics of AMS are closely related to the unsteady flow and energy conversion process inside the source, which are essential to the research of sound generation mechanism and optimal design of the AMS device. In this paper, a numerical simulation model of AMS was developed and an experimental system was established for recording the internal unsteady flow and near field sound pressure. In order to understand the flow and sound characteristics, numerical simulations and experimental tests of the flow and sound fields were carried out for various working conditions. For the first time, supersonic flow modulated speaker (SFMS) was designed and implemented. Considerable SPL gain of SFMS was obtained after introducing a special time-delay modulation function. Meanwhile, the role of vocal tract playing in the sound generation was studied by the comparison of unsteady flow simulation results in different vocal tract designs.The main works are summarized as follows:The numerical model of AMS was established for the internal transient flow and near field nonlinear acoustic field. Without the quasi-steady hypothesis on derivation of model equations, a model of the internal flow was developed by utilizing a finite volume compressible CFD solver and dynamic mesh technique. The intensive sound field was calculated based on a hybrid method with Reynolds Averaged Navier-Stokes (RANS) and FW-H analogy. As a result, the model of AMS can be applied to handle complex geometric structure of the actual device for various working conditions. Agreements in steady flows were obtained between simulation results and flow field measurements. The hybrid method was proved to be feasible for performance prediction of AMS and better understanding the sound generation mechanism.To understand the energy conversion process, several experimental methods in fluid dynamics were used. The steady velocity field of a two dimensional AMS model was captured by the particle image velocimetry (PIV) technique. Both flow field static pressure and near field sound pressure were recorded by a flow-induced sound monitoring system.Experimental results of the internal flow and acoustic field near the source were obtained for various chamber pressures, geometric parameters or driving signals. Velocity field of the PIV results reveals that there are two dominant characteristics in the steady flow. One is the pressure recovery process with flow separation on the outer wall, and the other is the shear layer with vortex formation between main flow and reversed flow. According to the sound pressure data, frequency responses of the source are related with the chamber pressure. It is implied that the voice coil oscillation is coupled with the high speed jet flow. Variation of the static pressure in vocal tract increases with the increment of the chamber pressure and the driving signal amplitude. The fundamental and second harmonic frequency components in the disturbing pressure were obvious in the source generation zone during a monochromatic modulation. Spectral composition of the internal static pressure is coincident with that of the near field sound signal under the same working conditions.Based on the numerical model, the performance and energy conversion process of AMS were compared for different chamber pressures, vocal tract inlet sizes, modulation frequencies and gas types. Simulation results under various working conditions have demonstrated that SPL output becomes saturated at high chamber pressure conditions, which is consistent with the quasi-steady theory result. When the size of vocal tract inlet is changed, disturbing pressure in the acoustic zone and the true modulation area ratio are almost the same. Variations of the static pressure in the transient flow are closely related to the modulation frequency. In a low frequency modulation, amplitude of negative disturbing pressure decreases along the main flow direction. In the case of high frequency modulation, a positive pressure zone was formed. The classical nonlinear effects were also found in this case, such as waveform distortion, shock wave and additional attenuations. When the modulation frequency increased, vortex flow was also enhanced in the shear layer.Numerical and experimental studies were carried out for the SFMS. It was considered as a feasible way to overcome sound saturation at high chamber pressure conditions. Experimental results have shown that, compared with the sonic flow case, SPL output gain of the SFMS is obtained only when the following two requirements are fulfilled. Firstly, a high chamber pressure and a high amplitude driving signal are required. Secondly, a specified frequency band is needed to obtain large displacement for voice coil oscillation, such as a frequency band around the peak value in the frequency response curve. Experimental results show that, SPL gain for SFMS at the position of 1 meter from the source was 8dB at chamber pressure 0.6MPa, driving current 10A and modulation frequency 500Hz. In order to explain the frequency dependence of SPL gain, numerical simulations employing a mach-3 nozzle were carried out for a fixed modulation frequency 500Hz and different chamber pressures. SPL gain of the SFMS is also confirmed by the simulation results. However, they have demonstrated that supersonic jet is not established near the nozzle exit even for chamber pressure 1.2MPa. This is because that time needed for supersonic flow development inside the nozzle is similar to the modulation period. In order to realize the true supersonic flow modulation, a time-delay modulation function was designed to replace the original sine function. Simulation results of the time-delay modulation have validated that the flow speed near the nozzle exit has been greatly added to supersonic. Both of the mass flow rate and SPL output remarkably increase in the time-delay case.The function of vocal tract on the energy conversion in AMS was studied by the comparison of unsteady flow simulation results for different vocal tract designs under the same working condition. There are considerable differences in the average flow and sound generation mechanism among results for different designs. Development of flow disturbance and spectral composition of the transient pressure are also influenced by the vocal tract contour. It is pointed out that two factors related with the contour design affect the energy conversion process. One is the impedance mismatch on the vocal tract exit; the other is the interaction between high speed main flow and disturbance propagation. They are also responsible for the internal transient flow changes. In order to reduce the wave distortion of the flow disturbance inside the vocal tract, a novel external jet design was proposed. The advantage of this new design was validated by the simulation of the energy conversion process in AMS.更多还原