The effects of the S-layer on nonplanar microflows: A critical view on the accuracy of slip models

It is generally accepted that slip boundary conditions that are higher-order in Knudsen number can extend the validity of slip models beyond the conventionally-accepted upper limit of Kn = 0-1. The present paper investigates oscillatory rarefied flows over nonplanar surfaces and demonstrates that the effects of the Knudsen layer cannot explain why slip models perform poorly for nonplanar flow cases in comparison to the equivalent planar case; nor the presence of Knudsen layers can explain why slip models show larger discrepancies over convex surfaces. The paper highlights the dramatic effects of curvature; and the profound influence of the S-layer in microscale.