Effects of background noise on generating coherent packets of hairpin vortices

Abstract

We examine the autogeneration process by which new hairpin vortices are created from a sufficiently strong hairpin vortex, leading to the formation of a hairpin packet. Emphasis is placed on the effects of background noise on packet formation. The initial conditions are given by conditionally averaged flow fields associated with the second quadrant (Q2) event in the fully turbulent channel flow direct numerical simulation (DNS) database at Reτ=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 autogeneration mechanism is robust. The background noise leads to a decrease in the minimum value of the Q2 strength required to trigger autogeneration, indicating that background noise enhances autogeneration, especially in the buffer layer. The autogeneration process is more enhanced by the background noise with wavenumbers kx<kz. Conditionally averaged flow fields around the tall attached vortices in the hairpin packet show that they are associated with elongated low-momentum structures in the streamwise direction. Finally, the autogeneration process was tested in a real turbulent environment taken from an instantaneous field of a turbulent channel flow DNS. The generation of secondary hairpin vortices is clearly observed upstream of the primary hairpin.