The parametric study on the development of the large-scale coherent structures in a mixing layer

The theoretical investigation of the role of three-dimensional large-scale coherent structures and their mutual interactions in a developing plane mixing layer subjected to external forcing is presented. Large-scale coherent structures are decomposed into 3 fundamental and 2 subharmoic wave modes. A parametric study is carried out examining effects of a multitude of initial conditions. It is found that the evolution of the forced three-dimensional shear layer and the associated local entrainment can be influenced greatly by the initial amplitudes and phases of the large-scale modes. The presence of three-dimensional modes may have a profound effect on shear layer growth when forced at amplitudes comparable or larger than those of the two-dimensional ones. This effect is more pronounced at low frequency. Nonlinear interactions between the fundamentals and subharmonics indicate that subharmonics of the most amplified frequency of the shear layer are usually produced during the early stages of flow development, while its harmonics are always produced far downstream, regardless of initial conditions. The results of this study provide useful parametric information for the control, through multi-mode forcing, of shear layers in practical applications, aiming at mixing and transport augmentation.