On boundary-layer transition in transonic flow

Abstract

Boundary-layer transition in transonic external flow is addressed theoretically. The transonic area is rich in different flow structures, and transition paths, and the work has wide potential application in transonic aerodynamics, including special reference to the example of flow transition over an engine nacelle. The investigation is intended partly to aid, compare with, and detect any limitations of, a quasi-parallel empirical methodology for design use in the area, especially with respect to the transonic range, and partly to develop an understanding and possible control of the nonlinear natural or by-pass properties of the compressible transition present. The mechanisms behind three major factors, (a) substantial external-flow deceleration, (b) rapid boundary-layer thickening, (c) three-dimensional nonlinear interactions, are identified; these three are involved in the specific application above and in more general configurations, depending on the disturbance background present. It is found also that some similarities exist with the phenomenon of buffeting on transonic airfoils, and the relevant physics and governing equations throughout are identified. Sensitive nonlinear effects are important in all the factors (a)-(c), especially a resonance linkage between shock buffeting and boundary-layer thickening, and nonlinearly enhanced three-dimensional growth triggered by slight three-dimensional warping for instance, peculiar to the transonic range. The latter enhanced growth is perhaps the most significant finding. The implications, in the general setting as well as for the nacelle-flow context in particular, are also presented.