Unthrottled Flows with Complex Background Waves in Curved Isolators

The unthrottled flows in curved isolators are studied experimentally and numerically. Wind-tunnel tests are conducted at a freestream Mach number of 4.92. The internal flowfield is visualized and measured. Large-scale separation bubbles in the inlet are observed, and the downstream duct curvature is found to be able to decrease the separation bubble size via lowering the local pressure downstream of the separation bubble. Complex background waves appear in these isolators, and the increase of duct curvature tends to intensify the reflected shocks. Particularly, the strength distributions along the shock direction of the background shocks behave differently under the influence of duct curvature. For the right-running shocks, the strength distribution changes from a descending trend to an ascending trend with the increase of duct curvature, whereas the strength of left-running shocks always descends with an increasing speed. The strength of background shocks is particularly related to the left-running expansion waves, which get intensified and become the dominant expansion waves in the isolators as the duct curvature increases. The schlieren images indicate a significant transverse density gradient and pressure gradient in the curved isolators. The wall pressure distributions further show that the radial pressure gradient in small curvature isolators switches between centripetal and centrifugal repeatedly; whereas it remains centripetal consistently in large curvature isolators, which is also determined by the left-running expansion waves. As a result, the secondary flow gets stronger as the duct curvature increases, resulting in a larger low-momentum flow region near the bottom wall. In general, the compression capability of the isolator gets attenuated with the increase of the duct curvature.