Abstract: 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.

Key words: S-shaped inlet, five-hole probe, vortex, separated flow, swirl distortion