The effects of upstream perturbations on large-scale field and the proliferation of λ2 vortices

Abstract

The effects of upstream perturbations on large-scale field are analyzed using direct numerical simulation data of a channel flow of frictional Reynolds number, Reτ, 394. A set of three and five spanwise blowing jets are used to create the upstream perturbations. The premultiplied energy spectra show that upstream perturbations led to the increase in range of large-scale wavenumbers in contrast to the unperturbed channel flow. The energy of wavenumbers lower than inertial sub-range (−5/3 power law) shows an increase in energy due to the perturbations and sustained downstream up to x = 10D from jets. Two-point correlation based Gaussian filter is used to extract large-scale features of correlation length greater than 2h (where h is channel half-height). Premultiplied energy spectra of the Gaussian filtered large-scales demonstrate the importance of jet spacing and diameter in exciting large-scale wavenumbers. The λ2 vortices of large-scale features formed a strong ring-type vortices at y+=60. Spectral turbulence kinetic energy (TKE) production depicts a secondary peak in TKE production at y+=60, which indicates the formation of a shear layer due to the upstream perturbations.