Influences of background noise on auto-generation of near-wall vortical structures in a channel flow

We examine the auto-generation process by which new hairpin vortices are created from a sufficiently strong hairpin vortex, leading to the formation of a hairpin packet. The initial conditions are given by conditionally averaged flow fields associated with the second quadrant (Q2) event in the fully turbulent channel flow DNS database at Re_t=395. The nonlinear evolution of the initial vortical structure is tracked by performing a spectral simulation. Background noise is introduced by adding small amplitude perturbations to the initial field or by imposing momentum forcing. The background noise gives rise to chaotic development of a hairpin packet. The hairpins become asymmetric, leading to much more complicated packet structures than are observed in the symmetric hairpin vortex train of the flow with a clean background. However, the chaotic packets show the same properties as the clean packet in terms of the rate of growth of vertical and spanwise dimensions and the distance between successive vortices, suggesting that the auto-generation mechanism is robust. The background noise leads to a decrease in the minimum value of the Q2 strength required to trigger auto-generation, indicating that background noise enhances auto-generation, especially in the buffer layer. The auto-generation process is more enhanced by the background noise with wavenumbers k_x < k_z. Finally, the auto-generation process was tested in a real turbulent environment taken from an instantaneous field of a turbulent channel flow DNS.