Types and properties of local secondary instabilities of the boundary layer of a swept wing
V.I. Borodulin1,2, A.V. Ivanov1,2, Yu.S. Kachanov1,2, D.A. Mishchenko1,2
1S.A. Khristianovich Institute of Theoretical and Applied Mechanics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow, Russia
Keywords: swept wing boundary layer, low-turbulence transition, stationary vortex cross-flow instability, local unsteady secondary instabilities
Abstract
An experimental study of the primary and secondary unsteady instabilities of the boundary layer of a swept wing was conducted under conditions of dominant primary crossflow instability in the presence of a localized three-dimensional roughness element on the streamlined surface. Measurements were performed using a hot-wire anemometer in a low-turbulence wind tunnel at low subsonic free-stream velocity under conditions of uncontrolled ("natural") unsteady disturbances. Four types of amplified unsteady boundary layer disturbances were detected and studied in detail: low-frequency disturbances associated with primary flow instability, mid-frequency disturbances (type III secondary instability), and high-frequency disturbances (types I and II secondary instability). The properties of these disturbances were studied in detail, and an analysis of the position and shape of their localization regions in the plane normal to the wall and flow was performed relative to the layers of strong shear of the longitudinal component of the mean flow velocity along the spanwise and normal to the wall. The complex nature of the secondary disturbances is demonstrated, which is difficult to explain by the simplified concepts of the existence of z- and y-modes of secondary instability considered in previous studies.
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