Large-scale Motions in a Separated Turbulent Boundary layer

Proper orthogonal decomposition was used to decompose the velocity field measured using particle image velocimetry over a separated turbulent boundary layer for investigating the effect of large-scale motions (LSM) on Reynolds stresses. LSMs are defined here by the fluctuating velocity field that is responsible for $55\%$ turbulence kinetic energy. Results show that $\approx 90\%$ of the Reynolds shear stress, $\overline{uv}$, is due to the LSM. The same motions contribute about $70\%$ of the streamwise component of the Reynolds normal stress, $\overline{u^2}$. Surprisingly, both large-scale and small-scale motions equally contribute to the wall-normal component of the Reynolds normal stress, $\overline{v^2}$. Furthermore, the study reveals the characteristic length scales and frequencies of the LSMs that might be modulated to control the flow separation.