尾管中强脉动排气噪声辐射的特征线解

Analysis of noise radiation of a strongly pulsating tailpipe exhaust by the method of characteristics

本文应用特征线法，对尾管中的强脉动流的传播问题进行了理论计算．尾管的强脉动排气，在流压超过临界值的情况下，在开口端处的流动将会阻塞，此时应取马赫数等于１代替流压等于大气压作为该处的边界条件．对于强脉动气流，流动的诸参数的起伏量可大于它们各自的时均值，本文将开口端向外的排气气流比作简源，令简源强度与排气气体的体积速度成正比，同时计及湍流噪声的贡献，这样得到的结果与实验可相比较．最后还计算了管长为一定时的噪声辐射与气室压力的关系，气压为一定时的噪声辐射和质量流通量与尾管长度的关系，以及不同气压下噪声辐射与尾管长度的关系，为指导气动声源和往复式发动机尾管的设计提供了计算方法．

The problem of propagation of a strongly pulsating flow in a tailpipe is discussedby the method of characteristics in this paper. For a strongly pulsating exhaust,flow maychock at the pipe open end at some part of a pulsating period,where the flow pressure exceeds its critical. In this case,one should set the Mach number there equal to 1 as the boundary condition, instead of flow pressure match condition. Moreover, the fluctuation of flowvariables may be much higher than their respective time averages. To treat with the problemwe simulate the exhaust flow out of the open end as a simple source,set its strength to beproportional to the volume acceleration of the flow and put the turbulence noise of the exhaust into consideraion. The results obtained are compared with our early experiment. Finally, we compute the pressure dependence of the noise at a given pipe length, and the pipelength dependence of the noise and mass flux at a given chamber pressure. The latter allowsone to choice a suitable length for a reciprocating internal combustion engine or an aerodynamic noise generator.