Development of Three-Dimensional Structure of Planar Mixing Layers under Pressure Gradient.

Abstract

Development of the three-dimensional structure of planar mixing layers under a pressure gradient was studied using a visualization system which enabled time-resolved two-dimensional concentration measurement. The unsteady concentration field in the transverse plane was visualized and analyzed to obtain statistical quantities of concentration. The data were also used to visualize instantaneous distribution of dissipation rate in the transverse plane and to reconstruct the three-dimensional image of the mixing layers based on Taylor's hypothesis. It was found that development of the spanwise instability was enhanced by a positive pressure gradient, as was the development of the Kelvin-Helmholtz instability, and that the development of spanwise instability was suppressed by the negative pressure gradient. The spanwise structure occurred with the same periodicity as the initially given spanwise perturbation even under the pressure gradient. Streamwise vortices caused by the spanwise instability resulted in the strong dissipation of concentration at the peak and in the valley of the spanwise structure. The spatial development of the spanwise instability could be easily understood by the three-dimensional image of the concentration field.