Anatomy of a turbulent spot

Abstract

Measurements have been made in the plane of symmetry of a turbulent spot to study the issue of entrainment. The mean flow is assumed to be similar in coordinates (x/t, y/t), and laser Doppler velocimeter data at several stations are used to determine the effective origin in x and t and to establish particle trajectories with respect to the stationary interface. Entrainment occurs over most of the rear interface and also close to the wall at the front of the spot. Flow visualization has also been used to obtain additional information about entrainment and sublayer structure. Pictures of the underside of spots, wedges, and boundary layers were taken through a glass wall, using a very heavy concentration of aluminum flakes, so that only motions occurring for y+ less than about 30 were visible. Sublayer streaks have essentially the same streaky, knotted appearance in all three types of flow. Optical correlations confirm the value λ+~100. An attempt has been made to estimate the strength of sublayer streaks as streamwise vortices, assuming that the streaks are a manifestation of local Taylor–Görtler instability. The results account for many of the phenomena actually observed in the sublayer.