圆孔声学非线性效应的数值模拟

本文发展了一种离散涡模型,模拟了在高声强声波作用下圆孔处发生的涡脱落过程。进而计算了圆孔中的声质点速度,并分析了孔中速度的畸变情况、最后给出了圆孔的非线性声阻和声抗的理论值。非线性声抗的理论预测是现有的研究圆孔声学非线性效应的准稳态模型没有满意解决的问题,因而所做的有关尝试是本文工作的特点之一。     

大气压直流微等离子体射流研究

介绍了一种结构简单、 制作方便的微米量级大气压等离子体射流。这种微等离子体射流由直流电源驱动,可在多种工作气体(如Ar,He,N₂等)中实现大气压放电,产生高电流密度的辉光放电。为了确定微等离子射流产生的激发物种成分,测量了以Ar和N₂为工作气体的等离子体发射光谱。利用发射光谱相对强度比值法测量了氩气微等离子体射流的电子激发温度。实验显示,其电子激发温度约为3 000 K,这远低于大气压等离子体炬的电子激发温度。利用N₂的二正带发射光谱得到微等离子体的振动温度约为2 500 K;利用其电学参数估算电子密度在1013cm-3量级。利用此微等离子体射流进行了普通打印纸表面处理的应用实验。结果显示,这种微等离子体射流能够明显的提高普通打印纸的亲水性。     

激光声源特性研究及海洋应用

激光声作为水下声源的一种新的激发方法,具有高声强、窄脉冲、宽频谱等优点。基于激光声激发的线性及非线性理论,建立了激光声信号特性的初步模型,研究了激光在水中激发声波的特性,研究表明：在热膨胀激发状态,增大激光光强可以提高激光声的转换效率;光强大于击穿阈值后,激光声的激发机制为非线性的光击穿机制。激光声具有的高距离分辨能力和海洋抗干扰能力,对水下目标探测、空载平台等领域有着较广的应用前景。     

加载波前沿宽度对铝表面微射流的影响

利用光滑粒子流体动力学方法,计算了金属表面沟槽在冲击下的微射流现象,详细讨论了微射流对加载波前沿宽度的依赖性. 计算结果表明：随加载波前沿宽度增加, 射流质量和头部速度都减小,同时低速喷射物所占比例增大. 对结果分析得出,微射流由沟槽斜面粒子在冲击加卸载后获得轴向速度,并在对称轴附近碰撞形成;射流体由沟槽斜面的金属薄层构成;当加载波前沿变宽,形成射流的金属层变小,这是由于沟槽斜面粒子的碰撞速度降低,碰撞角度增大,部分粒子碰撞时不满足射流强度封锁条件,而不再形成射流. 关键词： 微射流 光滑粒子方法 加载波前沿     

SPH方法对金属表面微射流的数值模拟

 光滑粒子流体动力学（Smoothed Particle Hydrodynamics）方法是一种无网格的拉氏计算方法,具有计算格式简单、易于计算大变形问题等优势。用二维SPH程序模拟了冲击加载下金属铝表面的沟槽状缺陷产生微射流的过程,得到了喷射物的总质量、最大速度和质量-速度曲线。还计算了相同深度、不同夹角的沟槽微射流,并与实验结果进行了比较。结果表明：计算得到的喷射物总质量和最大速度与实验结果符合较好,SPH方法对于估算金属表面的微射流提供了一种有效的数值模拟手段。     

高温壁面液体射流冲击瞬态沸腾传热的实验研究

以水作为冷却介质,对高温壁面在射流冲击淬冷时的瞬态换热特性进行了实验研究,获得了介质在不同过冷度、不同射流速度下的完整沸腾曲线。实验结果表明。无论是增大工质过冷度还是提高射流速度,总会使得热壁面的冷却速率加大。在一定的过冷度和射流速度下壁温变化呈现快－慢－快的特点。临界热流密度随平均壁温变化率的增大而增大,二者之间存在线性关系．     

微喷管甲烷非预混射流火焰燃烧特性实验研究

为发展微尺度燃烧器并拓展微尺度燃烧理论,对具有外部伴流空气的甲烷非预混微喷管射流火焰燃烧特性进行了实验研究。微喷管采用内径为710μm、425μm及280μm的不锈钢管,通过实验得到了微喷管非预混射流火焰的火焰形态、高度、最小熄灭流速及吹熄极限,并与常规尺度(管内径2 mm)非预混射流火焰进行了对比。研究表明微喷管射流火焰只有层流火焰一种形态;微喷管射流火焰高度主要取决于燃料流速而不受外部伴流速度影响;微喷管射流火焰的吹熄极限随伴流速度先增加后减小,而微射流火焰的最小熄灭流速受伴流空气速度影响较小,随管径减小微喷管射流火焰的可燃范围急剧减小。     

利用X射线激光进行激光等离子体射流的诊断

射流是激光惯性约束聚变(ICF)、天体物理学等领域中一种普遍存在的非线性现象.在实验室对射流现象进行模拟、诊断等方面的研究对于理解ICF、天体物理中的相关现象具有重要参考价值.采用纳秒激光辐照特殊形状的圆孔靶产生等离子体射流,利用波长为13.9nm的X射线激光作为光源对特定时刻的射流进行阴影成像,获得了清晰的射流阴影图像,与理论模拟结果定性一致. 关键词： 射流 X射线激光 等离子体诊断     

L-R两步欧拉方法在铝材料微喷射中的应用

应用三维弹塑性流体力学Lagrangian-Remapping两步欧拉计算方法对铝材料微喷射现象进行了数值模拟研究。计算了Asay实验中表面刻有相同深度、不同夹角沟槽的金属铝微喷射模型,计算得到的微喷物总质量、最大射流速度和实验结果均符合较好。进一步展开了对相同深度、更大夹角范围沟槽微喷射的数值模拟。分析认为喷射最大速度随沟槽角度的增大呈线性下降趋势。同时给出了喷射系数随沟槽角度的变化的拟合关系曲线,看到由于材料强度及沟槽角度变化后造成的波系关系变化的影响,随着沟槽角度增加,喷射系数曲线呈明显非线性发展。     

微气泡激发的声微流对细胞声孔效应的影响

理论及实验研究了微气泡激发的声微流对声孔效应的影响。实验采用低幅度(0.05~0.3MIPa)连续超声波信号照射MCF-7细胞,PEI:DNA的复合质粒和造影剂气泡的混合溶液,通过扫描电子显微镜测量细胞膜声孔效应。结果表明声孔大小随着激励声压的幅度和照射时间的增加而增大,平均孔径范围为100 nm~1.25μm。基于Marmottant微气泡振动模型的理论计算结果表明微气泡振动所产生的声微流引起的剪切力在低幅度超声引发声致穿孔作用中起着关键作用。      

Jet forking driven by pipe tone

The present work deals with an experimental investigation of flow of air through a square-edged circular orifice at the downstream end of a circular duct. Self-excited acoustic oscillations at the natural duct modes are observed for certain flow velocities when the orifice is sufficiently thick. For a specific Reynolds number based on the orifice diameter and the mean jet velocity (9150 < Re < 9850), the jet forks into two trains, with the alternating vortices falling into the same branch of the forked train. Whereas this phenomenon has been reported earlier to have occurred when the density ratio of the jet is less than 0.72, the present results show that it is possible for a jet having the same density as the ambient atmosphere. The jet forking is coincident with jump in the acoustic frequency from one natural acoustic mode to another with comparable amplitudes of both the modes.     

穿孔板在切向流条件下非线性效应的理论与实验研究

流过声衬表面的切向流和高声压级的声场对于穿孔板共振结构的声学特性有这非常显著的影响。本文从波涡相互作用机制出发,发展了一种旨在研究矩形穿孔板在切向流条件下非线性效应的离散涡模型。同时用双传声器法测量了不同几何参数的穿孔板的声阻抗随偏流速度及声压级的变化情况。将本文的实验与理论结果进行了对比分析,对比的结果表明本文的实验与理论结果符合良好。     

液体材料超声处理过程中声场和流场的分布规律研究

针对合金熔体等液体材料的超声处理过程,选取水作为透明模型材料,采用数值模拟计算和示踪粒子实验方法,研究了20和490 kHz两种频率超声作用下水中的声场和流场分布.结果表明,增大变幅杆半径能够提高水中声压水平,扩大空化效应的发生区域.当超声频率为20 kHz时,水中声压最大值出现在超声变幅杆下端面处,且声压沿传播距离的增大而显著减小.如果超声频率增加至490 kHz,水中的声压级相比于20 kHz时明显提高,且声压沿着超声传播方向呈现出周期性振荡特征.两种频率超声作用下水中的流场呈现相似的分布特征,且平均流速均随着变幅杆半径增大表现出先升高后降低的趋势.变幅杆半径相同时,20 kHz频率超声作用下水中的平均流速高于490 kHz频率超声.采用示踪粒子图像测速技术实时观察和测定了水中的流速分布,发现其与计算结果基本一致.     

圆孔非线性声阻抗三维时域仿真与特性分析

为研究有限振幅声波作用下圆孔的非线性声学特性,提出了基于三维时域计算流体动力学(CFD)仿真的圆孔非线性声阻抗提取方法,通过求解层流方程来模拟声信号在圆孔及上下游的传播,以及采用横向周期性边界条件来考虑高穿孔率时圆孔之间相互作用的影响。研究了不同幅值声波作用下孔径、厚度和穿孔率对声阻抗的影响规律,通过对质点振速幅值、频率和板厚等组成的无量纲参量进行非线性回归分析,得到了圆孔非线性声阻抗的拟合公式,并将其转换为可考虑多频声波影响的时域模型。最后结合声阻抗时域模型和有限差分方法计算了直通穿孔管消声器在小振幅和有限振幅声波作用下的传递损失,通过与实验测量结果的比较,验证了拟合公式的准确性和实用性。     

超声波多次回波反射法测量油料密度技术研究*

该文通过同时测量油料的声阻抗和声速来测量油料的密度。通过对超声波在不锈钢壁内的多次反射回波信号作快速傅里叶变换,取各次回波信号中心频率下的幅值在对数坐标系下作数据拟合,由拟合曲线斜率并结合多次反射理论求得油料的声阻抗;测量由聚苯乙烯壁面反射回来的二次回波信号计算油料的声速。实验中将温度控制在20?C,测量了8种油料的声阻抗和声速,进而求出8种油料的密度。实验结果表明8种油料密度测量结果的最大误差为1.2%。该方法适合油料密度在线、快速、准确测量。     

Experimental examination of vortex-sound generation in an organ pipe: A proposal of jet vortex-layer formation model

Aero-dynamical models of sound generation in an organ pipe driven by a thin jet are investigated through an experimental examination of the vortex-sound theory. An important measurement requirement (acoustic cross-flow as an irrotational potential flow reciprocating sinusoidally) from the vortex-sound theory is carefully realized when the pipe is driven with low blowing pressures of about 60 Pa (jet velocities of about 10 m/s). Particle image velocimetry (PIV) is applied to measure the jet velocity and the acoustic cross-flow velocity over the mouth area at the same phase by quickly switching the jet drive and the loudspeaker-horn drive. The vorticity of the jet flow field and the associated acoustic generation term are evaluated from the measurement data. It is recognized that the model of the “jet vortex-layer formation” is more relevant to the sound production than the vortex-shedding model. The acoustic power is dominantly generated by the flow–acoustic interaction near the edge, where the acoustic cross-flow velocity takes larger magnitudes. The acoustic generation formula on the vortex sound cannot deny the conventional acoustical volume-flow model because of the in-phase relation satisfied between the acoustic pressure at the mouth and the acoustic volume flow into the pipe. The vortex layers formed along both sides of the jet act as the source of an accelerating force (through the “acceleration unbalance”) with periodically alternating direction to oscillate the jet flow and to reinforce the acoustic cross-flow at the pipe mouth.     

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.     

ACOUSTIC IDENTIFICATION OF COHERENT STRUCTURES IN A TURBULENT JET

Fluid-dynamic events associated with noise generation in a subsonic jet are educed by conditioning in-flow velocity and pressure signals on farfield sound measurements. The jet is located in an anechoic chamber, and farfield noise measurements are performed simultaneously with in-flow anemometric and acoustic measurements at a number of distances x from the nozzle (0?x/D?20, with D the jet diameter). The experimental data are then analyzed with a conditional averaging procedure using peaks in the acoustic signal as a trigger. An analysis of the method is developed and supported by numerical simulations. The averaging procedure permits the identification of the average time signatures of in-flow velocity and pressure associated with noise-generating coherent structures in the flow, their position at the emitting instants and their temporal statistics. The physical properties of the events associated with the averaged time signatures are then discussed.     

On the interaction of a sound pulse with the shear layer of an axisymmetric jet

The behavior of a sound pulse from a simulated source in a jet is investigated both experimentally and numerically. Both approaches show that in the low and medium frequencies the far field acoustic power exhibits a marked amplification as the flow velocity increases. Experimentally this changes to an attenuation at the higher frequencies which cannot be computed by the numerical model. This amplification is traced to shear noise terms which trigger the instability waves that are inherent within the flow.