Coherent structures associated with interscale energy transfer in turbulent channel flows

We analyze the energy spectra, production spectra, and interscale energy transfer spectra for direct numerical simulation data of turbulent channel flows at Re-tau = 180 and 550. The transfer spectra are characterized by a negative region and a positive region. We use the negative region as a spectral mask to separate the flow fields into large-scale and small-scale fields. The former are characterized by streaks and quasi-streamwise vortices, and the latter are characterized by hairpin-like vortical structures that are similar to the original flow fields. Proper orthogonal decomposition and linear stochastic estimation are utilized to further extract the flow structures of the decomposed fields. The outer large-scale low-momentum regions are flanked by large quasi-streamwise vortices that are attached to the wall. This coherent structure presents a high degree of similarity to the structure of the near-wall cycle. This result indicates that the outer large scales may present a self-sustaining mechanism similar to that of the near-wall region. We also extract the coherent structures associated with the real-space energy transfer. The formation of small-scale hairpin-like vortices is related to the large-scale shear layer. Within this process, the energy is transferred from large scales to small scales