Energy characteristics of large coherent structures in turbulent boundary layers*

While length scales in a turbulent boundary layer scale with distance from the wall, coherent structures with scales typically much larger than the integral length scale have a large modulating effect on the spectral characteristics and velocity distributions. Some highly probable motions, such as sweeps and ejections, are the result of packets of hairpin vortices. Furthermore, there are very long meandering structures: large coherent structures and very large coherent structures with scales of 2δ - 3δ and 3δ - 10δ, respectively, where δ is the boundary layer height. These large-scale features are embedded in the flow, modulate its surroundings, and affect smaller scales. The contributions of these coherent structures to the flow and kinetic energy can be quantified from turbulent fluctuations through a triple decomposition, where velocity is split into a mean, coherent, and random component. Using wall-modeled large-eddy simulations, spectral-based model decomposition of instantaneous velocity snapshots is used to filter the flow field and quantify the coherent kinetic energy. The characteristics of large structures such as the bispectrum are elucidated. The spatial organization of the large-scale flow structures is analyzed to understand the relationship between large-scale fields and smaller vortex-dominated scales.