冲击射流中反馈环机理的再研究

冲击射流的噪声抑制对于研究短程起飞和垂直起降飞行器(STOVL)是极其重要的. 为了研究冲击射流噪声尤其是冲击单音与涡结构尺度之间的关系以及反馈声波的上传方向,采用小波分析技术和``声类比'方法来分析冲击单音的传播方向. 研究中用到的冲击射流的速度场由PIV技术给出,冲击单音的频率通过噪声场的测量获得. 利用双正交小波变换来提取冲击射流速度场中含有的波动信息,结合冲击单音的频率特性对噪声场进行研究.研究结果表明大尺度结构是冲击单音的``拟声源'. 此外,还可以看出大尺度涡结构产生的反馈声波一部分向喷嘴出口处传播,形成反馈环;另一部分反馈声波向四周传播.      

小口径轴对称收缩喷嘴射流冲击大平板噪声指向特性研究

实验研究了3毫米口径轴对称收缩喷嘴在各种压比下射流垂直冲击和倾斜冲击坚固大平板产生的噪声的指向特性。发现噪声在过平板法线和喷嘴轴线的平面内呈近似四瓣分布，当喷嘴与平板距离减小时，指向壁射流下游的瓣得到增强，反之，指向喷嘴上游的瓣得到增强。喷嘴压比增加时，指向壁射流下游的瓣得到增强，反之，指向喷嘴上游的瓣得到增强。根据自由射流噪声的基本指向特性、射流冲击噪声基本指向特性、声波在平板处发生镜面反射和声波能量叠加的设定，建立了一个冲击射流总体噪声指向特性的模型，成功解释了实验结果，并揭示了形成冲击射流总体噪声指向特性的内在机理。     

小口径轴对称收缩喷嘴射流冲击大平板噪声特性的实验研究

为研究小口径喷嘴冲击射流的噪声特性，测定了3mm口径的轴对称收缩喷嘴在各种压比情况下产生的亚音速和超音速射流冲击坚固大平板产生的噪声。发现噪声在空间呈近似四瓣分布，当喷嘴与平板距离减小时，噪声指向壁射流下游的瓣到增强，反之，噪声指向喷嘴上游的瓣得到增强。噪声随喷嘴距平板距离的增加呈增强的趋势，在距平板一定距离内有锯齿现象。噪声随喷嘴压比的增加而增强，相应于各种工况，存在一不同的压比值，此压比之前，噪声随压比的增大而迅速提高，但有起伏现象，在此压比之后，噪声平缓地随压比的增大而增强。     

展向喷嘴湍流横向射流大涡模拟及其统计分析

采用大涡模拟方法数值模拟了展向椭圆喷嘴的湍流横向射流,对其大尺度结构的时空演化和湍流脉动速度场的时间序列分析、频谱分析、PDF分析以及时、空截面上的统计平均特性进行分析.结果表明,在射流出口附近的下游核心区中速度脉动剧烈,显现出明显的湍流特征.除了三维涡环脱落、扭曲、变形、摆动所对应频率之外,还存在很宽的湍流基频,它与在喷嘴出口附近产生的三维涡环的时空演化过程密切相关.由于展向椭圆喷嘴的湍流横向射流中的三维涡环快速脱落和强相互作用导致射流尾迹中的强湍流脉动,展向椭圆喷嘴湍流横向射流的PDF空间演化特征结构复杂.在射流核心区的湍流偏应力变化平缓,其统计平均值分布接近左右对称.展向椭圆喷嘴的湍流横向射流脉动速度场具有极为复杂的统计行为,与流向椭圆喷嘴相比具有更好的掺混能力.     

燃气射流冲击传热特性的数值模拟

针对射流传热问题,利用基于RNGk-ε湍流模型的数值方法模拟了射流垂直冲击平板的流动过程,并与实验数据比较,验证了模型的可行性。在此基础上,以火箭喷管入口参数为入口条件,建立了超音速燃气射流垂直冲击平板和冲击浸没平板的计算模型,分析了不同冲击条件下努塞尔数分布规律和温度分布规律, 论述了超音速射流传热的特性及影响传热特性的因素。得到了冲击距离为(14~18)D的努塞尔数取值范围,并表明冲击距离和射流温度是影响传热效率的关键因素;冲击距离增加,传热效率降低,冲击平板表面的射流温度越高,传热效率越高。     

气枪喷嘴高速射流的除水效率研究

为揭示喷嘴除水的机理并进而对气枪喷嘴进行改进和优化设计,本文提出了利用图像分析处理对小尺度气枪喷嘴高速冲击乘风破浪的除水效率的研究方法。该方法将有效除水面积作为衡量喷嘴除水效率的标准,从面实现了对喷嘴整体除水效率的定量测量,并利用该方法对影响气枪喷嘴除水效率的各种因素（一次侧压力,喷嘴到平板的距离和射流攻角）进行了研究,并将实验结果与用热线风速仪及总压探头测量的结果进行了比较,得到冲击射流在平板水平速度分量是蚊蝇 嘴除尘除水效率的决定性因素等结论。     

核磁共振陀螺多层磁屏蔽系统优化设计

核磁共振陀螺是目前世界上体积最小的导航级陀螺。由于核磁共振陀螺通过探测原子核的宏观磁化在静磁场中的进动频率来测量载体的角速度,为获得高精度与大动态范围,需要确保静磁场的稳定性,防止外部磁场的干扰,所以必须对核磁共振陀螺进行磁屏蔽。从核磁共振陀螺磁屏蔽原理出发,通过数学计算和计算机仿真,分析和研究了多层磁屏蔽罩结构参数对磁屏蔽系数的影响,并对核磁共振陀螺磁屏罩进行了优化设计。设计的多层磁屏蔽罩磁屏蔽系数达到了106,满足核磁共振陀螺的使用需求。该工作为核磁共振陀螺仪的整体设计和制造提供了一定的理论依据和参考价值。     

基于模态分解的轴对称超声速射流啸声产生位置数值分析

不完全膨胀超声速射流的势核中会产生准周期的激波栅格结构, 其与剪切层内拟序结构的相互作用会产生激波噪声. 啸声是主要向上游方向传播的、具有离散频率的高强度激波噪声, 其产生是受一种非线性的声反馈环机制驱动. 精确定位啸声的声源位置是定量理解啸声反馈环机制和发展准确的啸声预测模型的一个关键所在. 为了分析近场啸声, 本文采用高精度数值方法直接求解轴对称可压缩Navier-Stokes方程, 数值模拟了完全膨胀射流马赫数为1.10和1.15的圆形声速喷管欠膨胀超声速冷射流, 得到了A₁和A₂两种轴对称模态啸声. 通过傅里叶模态分解、本征模态分解和动态模态分解, 分析了射流时序压力场和速度场, 研究了啸声关联拟序流动结构的空间演化, 精确定位了轴对称模态啸声的声源位置. 研究表明: 啸声关联拟序流动结构存在饱和态区域, 啸声声波是在其饱和态区域产生并向外传播; 在本文所涉及的射流马赫数范围内, A₁和A₂两种轴对称模态啸声的有效声源位置分别是在第4和第3个激波栅格结构的尾缘.      

熔喷双槽形喷嘴气体射流流场初探

在熔喷非织造布加工中,气体射流作为工作介质使聚合物熔体实现拉伸,气体射流流场的研究对熔喷气流拉抻数学模型研究非常重要。熔喷双槽形喷嘴形成的流场可以看作两股平面射流的合成。从单个点涡的性质出发,研究了涡偶的性质和涡偶代替射流的可行性。研究表明,在喷丝孔轴线附近,涡偶和射流的速度分布趋势相同,且有比较相近的速度分布,从而说明以涡偶代替射流是可行的。在此基础上,用两个涡偶分别代替两股射流,然后进行合成,推导出两股射流合成后速度分布的理论公式,该公式的计算结果与实验结果吻合较好。将该公式引入熔喷气流拉伸数学模型,预测出的纤维直径与采用经验公式时的预测结果几乎完全相同。结果表明,应用涡偶代替射流推导出的气流速度分布公式能够较好地描述熔喷双槽形喷嘴气体射流流场,可以用于完善熔喷气流拉伸数学模型。     

出口条件对圆形湍流射流远场区自保持性的影响

报道出口条件对圆形湍流射流自保持性影响的实验研究结果. 对来自渐缩和长管两种不同结构喷嘴的射流,在相同雷诺数条件下,沿轴线进行了速度测量; 研究的统计量包括平均速度、湍流强度、高阶矩、能谱和积分尺度. 实验结果表明,渐缩喷嘴射流比长管射流发展得更快、更容易达到自保持状态. 通过对比发现,在两射流的速度(温度)场中,平均速度(温度)、湍流强度、偏斜因子和平坦度因子都存在明显的异同. 同时发现两射流的积分尺度随轴向距离的增加都成线性增长,且在渐缩喷嘴射流中增长得更快. 通过对比两射流的边界层厚度、径向与轴向湍流强度的比值、湍动能能谱图并结合前人的研究结果,对两射流湍流场所表现出的不同的统计学行为给出了合理的解释.      

Active control of supersonic impingement tones using steady and pulsed microjets

In recent years, it has been demonstrated that direct microjet injection into the shear layer of the main jet disrupts the feedback loop inherent in high speed impinging jet flows, thereby significantly reducing the adverse effects. The amount of noise reduced by microjet actuation is known to be dependent on nozzle operating conditions. In this paper, two active control strategies using microjets are suggested to maintain a uniform, reliable, and optimal reduction of these tones over the entire range of operating conditions. In the first method, a quasi-closed loop control strategy is proposed using steady microjet injection and the proper orthogonal decomposition (POD) algorithm. The most energetic spatial mode of the unsteady pressure along the nozzle diameter is captured using the POD, which in turn is used to determine the distribution of microjet intensity along the nozzle exit. Preliminary experimental results from a STOVL supersonic jet facility at Mach 1.5 show that the quasi-closed loop control strategy, in some cases, provides an additional 8–10 dB reduction compared to axisymmetric injection at the desired operating conditions. The second method consists of a pulsed microjet injection, motivated by the need to further improve the noise suppression. It was observed that the pulsed microjet was able to bring about the same noise reduction as steady injection using approximately 40% of the corresponding mass flow rate of the steady microjet case. Moreover, as the duty cycle increased, the performance of pulsed injection was further enhanced and was observed to completely eliminate the impinging tones at all operating conditions.     

Numerical simulation of an impinging jet on a flat plate

Two-dimensional normal impinging jet flowfields, with or without an upper plate, were analysed by employing an implicit bidiagonal numerical method developed by Lavante and Thompkins Jr. The Jones–Launder K–? two-equation turbulent model was employed to study the turbulent effects of the impinging jet flowfield. The upper plate surface pressure, the ground plane pressure and other physical parameters of the momentum flowfield were calculated at various jet exit height and jet inlet Reynolds numbers. These results were compared with those of Beam and Warming's numerical method, Hsiao and Chuang, and others, along with experimental data. The potential core length of the impinging jet without an upper plate is longer than that of the free jet because of the effects of the ground plane, while the potential core length of the impinging jet with an upper plate is shorter than that of the free jet because of the effects of the upper plate. This phenomenon in the present analysis provides a fundamental numerical study of an impinging jet and a basis for further analysis of impinging jet flowfields on a variable angle plate.     

Effect of impinging plate geometry on the self-excitation of subsonic impinging jets

In the generation of discrete tones by subsonic impinging jets, there exists a difference of opinion as how the feedback is achieved, i.e., the path of the feedback acoustic waves is whether inside the jet or outside the jet? The only available model (Tam and Ahuja model) for the prediction of an average subsonic jet impingement tone frequency assumes that the upstream part of the feedback loop is closed by an upstream propagating neutral wave of the jet. But, there is no information about the plate geometry in the model. The present study aims at understanding the effect of the plate geometry (size and co-axial hole in the plate) on the self-excitation process of subsonic impinging jets and the path of the acoustic feedback to the nozzle exit. The present results show that there is no effect of plate diameter on the frequency of the self-excitation. A new type of tones is generated for plates with co-axial hole (hole diameter is equal to nozzle exit diameter) for Mach numbers 0.9 and 0.95, in addition to the axisymmetric and helical mode tones observed for plates without co-axial hole. The stability results show that the Strouhal number of the least dispersive upstream propagating neutral waves match with the average Strouhal number of the new tones observed in the present experiments. The present study extends the validity of the model of Tam and Ahuja to a plate with co-axial hole (annular plate) and by doing so, we indirectly confirmed that the major acoustic feedback path to the nozzle exit is inside the jet.     

Characteristics of unsteady type IV shock/shock interaction

Characteristics of the unsteady type IV shock/shock interaction of hypersonic blunt body flows are investigated by solving the Navier–Stokes equations with high-order numerical methods. The intrinsic relations of flow structures to shear, compression, and heating processes are studied and the physical mechanisms of the unsteady flow evolution are revealed. It is found that the instantaneous surface-heating peak is caused by the fluid in the “hot spot” generated by an oscillating and deforming jet bow shock (JBS) just ahead of the body surface. The features of local shock/boundary layer interaction and vortex/boundary layer interaction are clarified. Based on the analysis of flow evolution, it is identified that the upstream-propagating compression waves are associated with the interaction of the JBS and the shear layers formed by a supersonic impinging jet, and then the interaction of the freestream bow shocks and the compression waves results in entropy and vortical waves propagating to the body surface. Further, the feedback mechanism of the inherent unsteadiness of the flow field is revealed to be related to the impinging jet. A feedback model is proposed to reliably predict the dominant frequency of flow evolution. The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to this complex flow.     

Drag reduction and heat transfer in surfactant solutions with excess counterion

We present the results of a study of turbulent drag reduction in a small circulating loop using surfactant solutions with excess counterion. In addition, these solutions were used in measurements of heat transfer, both in pipe flow and in an impinging jet. Both frictional drag and heat transfer were reduced in the pipe flow experiments. Measurements of heat transfer in the impinging jet revealed a dependence on the molar concentration ratio of the counterion. When the counterion was added at a molar concentration 30 times higher than that of the surfactant, the resulting surfactant solution did not reduce the rate of heat transfer in the impinging jet. By using this surfactant system in an impinging jet, we show both a reduction in pipe friction and normal heat transfer potential in a circulating heat exchange system. In order to investigate this difference in heat transfer between pipe flows and impinging jet flows, a comparison was made of the wall shear stress between these two flow regimes. The estimated wall shear stress was of the same order in both flows, and thus was not considered to be the primary cause of the difference in heat transfer. It is instead suggested that the micellar structure of the surfactant is influenced by a compressive deformation of the impinging flow in a manner that is different from the shear deformation observed in pipe flow.     

The planar jet-plate oscillator

The aeroacoustic noise generated by a high speed, planar gas jet impinging on a flat plate is investigated experimentally. The jet used in this study is typical of those commonly found in industrial applications such as in various coating control and heat transfer processes. Normal jet impingement on the plate is found to generate strong acoustic tones over a wide range of impingement distances and jet velocities. The characteristics of these tones, as a function of the jet velocity and impingement distance, are quantified. Phase and amplitude measurements of the pressure fluctuations on the impingement plate indicate that the acoustic tones are generated by an antisymmetric instability mode of the jet oscillation. The effect of plate inclination in both the transverse and span-wise directions, with respect to the incident jet, is also studied. The jet-plate tone is found to be much more sensitive to changes in the span-wise plate inclination than to changes in the transverse inclination, but in both cases, a complete suppression of the tone is found to be possible.