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Study on the Breakdown of Compressible Jet and the Mechanism of its Aeroacoustic Emission

可压缩射流击穿及其气动声发射机理研究

基本信息

批准号：
10450373
负责人：
NAKAMURA Yoshiaki
金额：
$ 7.94万
依托单位：
Nagoya University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B).
财政年份：
1998
资助国家：
日本
起止时间：
1998 至 2000
项目状态：
已结题

项目摘要

In this study both numerical simulation and experiments regarding high-speed jets were performed. In the experiments mini-jets were used as an actuator to enhance jet mixing, the results of which are summarized as follows : (1) The flapping mode of actuation was most effective in mixing, where the axial length of the potential core was reduced by 35%, compared with that of the baseline. (2) As the frequency of actuation becomes higher, the jet breaks down more upstream. (3) As under-expanded jets are intrinsically stable, it does not change so easily by external disturbances, compared with the case of optimum jet. (4) As for noise, only steady actuation can decrease the noise level. On the other hand, regarding numerical simulations, the following results were obtained. (1) In axisymmetric calculation of jet, periodical roll-up of the jet shear layer was observed. These vortices merge with each other, and show the motion of acceleration, which leads to sound emission. (2) In 3D simulation, some instability produced upstream cannot maintain the state of being axisymmetric in the downstream, and three-dimensional unstable modes appear. As a result, the degree of jet breakdown is weakened, which reduces Mach wave-like sound emissions. (3) The velocity distribution along the jet axis shows good agreement between 3D numerical simulation and experimental data. However, there is a slight difference between them in the downstream. (4) In under-expanded jets, vortices in the jet shear layer interact with the shock cell, which makes a kind of noise.

本文对高速射流进行了数值模拟和实验研究。实验结果表明：(1)扑动驱动方式对混合效果最为显著，与基线相比，扑动驱动方式可使势芯轴向长度减小35%；(2)驱动频率越高，射流越上游破裂。(3)由于膨胀不足的射流本质上是稳定的，因此与最佳射流相比，它不容易受外界干扰而改变。(4)噪声方面，只有平稳驱动才能降低噪声水平。另一方面，在数值模拟方面，得到了以下结果：(1)在射流轴对称计算中，观察到射流剪切层周期性卷起。这些涡旋相互融合，表现出加速度运动，从而产生声发射。(2)在三维模拟中，上游产生的一些不稳定在下游不能保持轴对称状态，出现三维不稳定模态。因此，射流的破裂程度被削弱，从而减少了类似马赫波的声发射。(3)三维数值模拟与实验数据吻合较好。但是，它们在下游有细微的差别。(4)在欠膨胀射流中，射流剪切层内的涡与激波单元相互作用，产生一种噪声。