Evidence for streaks and the lift-up mechanism in turbulent bluff body wakes

Interrogation of the LES database of a circular disk wake at Re = 50,000 shows another coherent structure - large-scale streaks - in addition to the energetically dominant vortex shedding (VS) mode. These streaks appear as coherent elongated regions of streamwise velocity throughout the wake. They exhibit a near-zero frequency St → 0 and, correspondingly, a near-zero streamwise wavenumber k_x → 0. Surprisingly, the streak mode energetically dominates the VS mode in the far wake (x > 70D). Spectral proper orthogonal decomposition (SPOD) reveals that most of the streak energy resides in the azimuthal m = 2 mode. Conditionally-averaged vorticity fields demonstrate that streamwise vortices lift up low-speed fluid from the wake core and push down high-speed fluid from the outer wake. The lift-up mechanism transfers energy from the mean to the streaks. Bispectral mode decomposition (BMD), which is employed to quantify the intensity of triadic interactions, shows that the m =1 and St = ± 0.135 interaction to form the m = 2, St → 0 streak is active. Our results suggest that the VS mode interacts with its conjugate to generate the m = 2, St → 0 streamwise vortices, leading to streak formation through the lift-up process. To our knowledge, this is the first report of large-scale streaks and the associated lift-up mechanism in a turbulent wake.