NUMERICAL STUDY ON THE MIXING OF A MIXING LAYER UNDER ACTIVE CONTROL
混合层强化混合的数值研究

Enlightened by Wang & Fiedler's experiment (1997), a two-dimensional unconfinedmixing layer and a two-dimensional mixing layer between two plane walls (a two-dimensional confined mixing layer) are systematically investigated through direct numerical simulation, in which high order compact finite difference schemes are employed. A proper periodical oscillation is introduced to the inlet of a mixing layer in order to study the mechanism for controlling and enhancing the mixing efficiency of the mixing layer. The results are as following: For a two-dimensional unconfined mixing layer, the lower the oscillation frequency, the larger the sizes of the vortices induced in the mixing layer and the vortices resemble each other when the frequency is comparatively low. For a mixing layer between two plane walls, given an actual flow equipment and the velocity ratio, there will be an optimal frequency section in which the oscillation at the inlet can yield appropriate large-scale vortex structures in the flow field. Due to the effect of the walls, these large-scale vortex structures are subject to the intense crush from the walls and are broken down into comparatively small structures. In the meantime, the similar large-scale vortices are excited in the boundary layers. The crushing and merging between these vortices enhance the mixing efficiency greatly. The confinement of the plane walls activates the mixing of the mixing layer substantially. The optimal frequency is concerned with the geometry size of the flow equipment. The confinement of the plane walls has two opposing influences on disturbance under different oscillation frequencies. When the frequency is lower than the optimal frequency, the existence of the walls inhibits the development the vortex structures in the mixing layer, and the large-scale structures do not develop completely. The mixing efficiency is not satisfactory. When the frequency is higher than the optimal frequency, the induced large-scale vortex structures by the oscillation are confined within the middle region of the mixing layer. The confinement of the walls has little effect on these vortices and the mixing result is not so efficient as the case of oscillation under proper frequency. The results correspond with Wang & Fiedler' experiment (1997) and, from the qualitative point of view, explain the mechanism that introducing oscillation at the inlet of a confined mixing layer can greatly enhance the mixing efficiency.