【作者】 吕祥翠；

【导师】 罗纪生；

【作者基本信息】 天津大学 ， 流体力学， 2004， 硕士

【摘要】 流体中脉动产生噪声的机理是气动声学研究飞行器噪声的关键问题之一。现代喷气式飞机在起飞和着陆时将发出强大的噪声。其主要噪声源之一,是在机翼的尾缘、喷流发动机的出口处,流体流过尾缘时产生的强烈脉动。边界层中的扰动在尾缘处可能产生较强的散射;上游中的扰动会导致尾迹流中的不稳定波产生;边界层中的扰动与尾迹流产生的不稳定波相互作用,这些都是可能的噪声源。目前这类流动产生噪声的机理人们还不太清楚。本文用直接数值模拟的方法,研究了二维亚音速的情况下,上下来流相同的流体流过一无限薄平板时,平板单边的T-S 波和尾缘及尾迹流相互作用产生声场的可能性。在计算定常的基本流时,为消除机器误差,在尾缘以后对应中心线上采用了限制条件:若|v| ≤10^-12,则将v 赋成0。我们分别对四种情况进行数值研究,得出如下结论: （1）平板尾缘以后对应中心线上全部加限制条件。当T-S 波传过尾缘以后,尾缘对其基本上没有影响;T-S 波的频率和波长都没变,也没有声波产生。（2）在平板尾缘以后对应中心线上某点以后用限制条件。由于流动中的扰动在流场中加条件的位置上被限制,这相当于在该点加了一个强的扰动。这个扰动将产生类似点源产生的声场,声场的波动特征与加在入口处的T-S 波没有直接的关系,而与尾迹流中的不稳定波有关系。（3）不加限制条件,初始时在平板上侧加入T-S 波,然后在计算域的入口处加相应的T-S 波。这时,流动中初始阶段出现了类似由点源产生的声场,但是该声场不能持续维持。其原因是由于初始时刻T-S 波在尾缘处突然间断。（4）不加限制条件,只在计算域的入口处加T-S 波。尾迹流中的Lighthill应力张量能在远场产生声场。不过这时声源不再是一个点,而是一个区域;该声场的波动特征也主要依赖于定常尾迹流的速度分布产生的不稳定扰动。更多还原

【Abstract】 The mechanism of generation of noises due to the fluctuation in the flows is one of the key problems in studying the noises generated by aircraft in aeroacoustics. Modern jet plane would generate strong noises when it flies off or lands fall. One of the main causes of the noises is the strong fluctuation generated by the flows passing throng the trailing edge of the airfoil and the outlet of jet engine. The probable strong disperse generated by the disturbance in boundary layer flows at the trailing edge, the instable waves in the wake flows generated by the upstream disturbance and the interactions between the disturbance in the boundary layer flows and instable waves in the wake flows might be main sources of the cause. Whether or not the sound field would be generated by the interactions among the T-S wave introduced at the entrance of one side of the plate, the trailing edge and wake flows is studied by direct numerical simulation,when the same 2-D subsonic incoming flows pass through a infinitely thin flat plate. To eliminate the error caused by computer, we adopt a restrictional condition that if the absolute value v is not greater than 10^-12, we give zero to v to gain a stable basic flow. Four different instances are considered and the numerical results are listed below: （1） The restrictional condition is used for the symmetrical line from the trailing edge of the plate to the outlet of the computational domain. The trailing edge has almost no influence on the T-S wave after the T-S wave passes through it. There are no changes in the frequency and wavelength of the T-S wave and no sound wave emerges. （2） The restrictional condition is used for the symmetrical line where the x values from x_z to x_l. The control of the disturbance in the flows generated a strong artificial disturbance at x_z. A sound field that is similarly like the single source would be generated due to the presence of the artificial disturbance. And it is not the T-S waves but the instable waves that have the relations with the characters of sound field. （3） Under the conditions that T-S wave is introduced initially on the upper side of the plate and then the T-S wave is introduced at inlet all the time without the restrictional condition, the disturbances lead to a sound wave like the one generated by single source because of the discontinuation at the rear edge of the T-S wave existed in all the field on the upper side of the plate. But this sound source can’t last. （4） No restrictional conditions are used and the T-S wave is introduced only at the inlet of the computational domain. The Lighthill stress tensors in the wake flows could generate sound field in the far field. However, the sound source is not a single source, but locates in a region. The characteristics of the sound field mainly rely on the disturbance generated by the instability of the volicity’s distribution of the steady wake flows.更多还原