In this paper, the normalized acoustic input impedance of a narrow pipe with a strong axially non-homogeneous thermal profile subjected to low Mach number flow of up to $0.03\hspace{0.17em}M$ is studied. The analysis is done experimentally, using the two-microphone three-calibration technique and the results are compared with numerical simulations, using an industrial finite element package. The main application of this study is in the acoustic design of water-cooled charge-air coolers in internal combustion engines. The measurements are done on an effectively semi-infinite narrow pipe. It is shown that neglecting the flow effect in the calibration process of narrow pipes can significantly increase the measurement error. The results show that although the direct modification of the normalized acoustic input impedance due to low Mach number flow is negligible, the flow exerts an indirect but significant effect on this parameter by modifying the thermal gradient profile. Therefore, taking this effect into account can help to achieve an improved acoustical design of engine intake lines.

中文翻译：

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具有低马赫数流量和热梯度的窄管声输入阻抗的实验分析。

在本文中，具有强轴向非均匀热剖面的细管的归一化声输入阻抗在低马赫数流下达到 $0.03\hspace{0.17em}\mathrm{中号}$被研究。使用两麦克风三校准技术，通过实验进行分析，并使用工业有限元软件包将结果与数值模拟进行比较。这项研究的主要应用是内燃机水冷式增压空气冷却器的声学设计。测量是在有效的半无限窄管上完成的。结果表明，在窄管校准过程中忽略流量影响会大大增加测量误差。结果表明，尽管由于低马赫数流而对归一化声输入阻抗的直接修改可以忽略不计，但通过修改热梯度曲线，该流对该参数产生了间接但重要的影响。因此，