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

本文对高速射流进行了数值模拟和实验研究。实验中采用微射流作为激励器来增强射流混合，实验结果如下：（1）扑动激励方式的混合效果最好，其势核的轴向长度比基准激励方式减少了35%。(2)随着驱动频率变高，射流在更上游处分解。(3)由于欠膨胀射流具有固有的稳定性，与最佳射流相比，它不易受外界扰动的影响。(4)对于噪声，只有稳定的驱动才能降低噪声水平。另一方面，关于数值模拟，获得了以下结果。(1)在射流的轴对称计算中，观察到了射流剪切层的周期性卷起。这些涡流相互融合，并显示出加速运动，从而导致声音发射。(2)在三维模拟中，上游产生的某些不稳定性在下游不能保持轴对称状态，出现三维不稳定模态。其结果是，射流破碎的程度减弱，这减少了马赫波状的声发射。(3)沿射流轴线的速度分布沿着显示出良好的一致性之间的三维数值模拟和实验数据。但是，在下游，它们之间略有不同。(4)在欠膨胀射流中，射流剪切层中的旋涡与激波单元相互作用，产生一种噪声。

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.