Response of artificial longitudinal vortex pair embedded in the boundary layer to acoustic excitation

As a flow model for so-called 'secondary instability' in a three-dimensional boundary layer, a flat plate boundary layer with artificially generated embedded streamwise vortices embedded in it is examined using a hot-wire anemometer and flow visualization in order to determine the onset conditions of the secondary instability and its response to acoustic excitation. The results show that the nondimensional parameter, with which the onset conditions of the secondary instability can be predicted, is the product of critical inflection point height and inverse of maximum shear at critical conditions. Transition seems to start when this value reaches about 0.2. Acoustic disturbance seems to excite the secondary instability at certain conditions. Direct response to the disturbance frequency was observed within a frequency range of 290 Hz-450 Hz. Below 280 Hz, the spectral peak of velocity fluctuation was held constant at 345 Hz.