Evolution of confined mixing layer under optimal excitation frequency

The exceptionally rapid fluid mixing that is achievable in confined mixing layers through external excitation is studied through both flow visualization and particle image velocimetry at a moderate Reynolds number. Compared with free mixing layers, confined mixing layers exhibit a twofold larger spreading rate in the absence of excitation. When they are excited, a significantly larger V component of the mean velocity can be generated. This broadens the mixing layer on large scales, meanwhile generating a large |∂V/∂y| around the centerline. This large |∂V/∂y| further stretches the spanwise vortices, induces earlier collapse, and indirectly promotes small-scale mixing. Thus, the mean flow field plays an important role in the evolution of a confined mixing layer with excitation. Under the optimal excitation frequency, the turbulent kinetic energy primarily attributed to acoustical excitation is rapidly consumed by inverse energy transport. This further explains the formation of the mean field from the perspective of kinetic energy transfer. The results obtained here can be potentially applied to the design of high-efficiency and compact active mixers/reactors for chemical engineering and biomedical applications.