背负式S弯进气道拥有较好的前向雷达隐身性能，同时有利于起落架布置、武器内埋，但其出口流场的非均匀性会严重影响发动机的稳定性。除了总压畸变、总温畸变的影响之外，旋流畸变也是流场非均匀性的一种重要体现。为研究背负式大S弯进气道的旋流畸变特性，采用美国汽车工程师协会（SAE）的旋流评估方法，利用基于五孔探针的旋转式测量段对进气道出口的强旋流场进行测量分析，入口马赫数的范围为0.2~0.6。所有马赫数下旋流方向（Swirl Directivity，SD）和旋流对数（Swirl Pairs，SP）变化不大，均显示出口旋流为对旋模式，与理论分析和数值计算结果吻合。同时，测量的对旋涡呈现出弱非对称性，最大旋流角超过40°，旋流强度（Swirl Intensity，SI）从内环的6°增加到外环的13°左右，且在马赫数小于0.5的范围内没有明显变化。本文研究表明，虽然SAE的旋流畸变计算方法中部分指标可以有效识别出大S弯进气道的旋流模式，但是旋流强度指标却明显不能表现出对旋涡的强旋状态，制约了其在该类进气道/发动机相容性评估中的应用。
The dorsal S-shaped inlet possesses an excellent ability of forward radar stealth and benefits the disposal of landing gears and the missile, but the non-uniform flow field in the outlet influences the stability of the engine seriously. Besides total pressure distortion and total temperature distortion, swirl distortion is also one important embodiment of non-uniformity. To research the characteristics of swirl distortion, this paper uses the assessment methodology of Society of Automotive Engineers (SAE) and a rotational five-hole-probe-based measurement to assess the highly swirling flow field of the dorsal serpentine inlet at Mach numbers between 0.2 and 0.6. The results indicate that the change of the Swirl Directivity (SD) and Swirl Pairs (SP) is not apparent during the appointed mach range, and there is a paired swirl in weak symmetry in the outlet. The maximum swirl angle is more than 40°. Swirl Intensity (SI) increases from 6° in the internal ring to 13° in the external ring, without apparent change at Mach numbers below 0.5. Although the assessment of SAE has identified the mode of swirl, its swirl intensity descriptor does not manifest the state of high swirling, impeding the application of SAE's assessment methodology to assessment of consistency between inlets and engines.
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