DOUBLE-FREQUENCY FORCING ON SPATIALLY GROWING MIXING LAYERS

Spatially growing mixing layers are simulated numerically using a two-dimensional vortex method. Special attention is paid to the effect of double-frequency forcing on the development of a mixing layer. Two different types of forcing are considered: superposition of a fundamental frequency (F) on one of its subharmonics (Case I), and superposition of two frequencies of a resonance type, F +/- DELTA-F (Case II). The effects of forcing amplitude and relative phase shift between the two forcing frequencies are also examined. Instantaneous plots of discrete vortices and various statistics up to the second-order moment are obtained to see the variation of coherent structures. Results show that the number of merging vortices and thus the growth of a mixing layer can be effectively controlled by double-frequency forcing if forcing frequencies, phase shifts and forcing amplitudes are suitably selected.