Frequency Control of Unstable Disturbances in a Two-Dimensional Jet by Means of an Artificial Acoustic Loop

The frequency of acoustic sound emanating from the trailing edge of a two-dimensional airfoil is known to exhibit a ladder-like variation, displaying discontinuous jumps between discretely identifiable states as the free stream velocity varies. In order to reveal the underlying causes for this behavior, a two-dimensional jet issuing into still air with no aerodynamic sound emission is used as a model platform to study this phenomenon, because prescribed aero-acoustic sound may be readily introduced into the flow at the jet exit. When unstable disturbances growing in the shear layer of the jet are excited by a loudspeaker, an acoustic feedback loop automatically selects one frequency from the unstable frequencies present in the shear layer, and the resulting ladder-like variations are found to be similar to those present in airfoil trailing-edge noise. In addition, the observed slope of each rung of the ladder in the selected frequency behavior, and the observed jump frequency between ladder steps, show good agreement with existing empirical models. It is also discovered that when the remainder of the distance between the speaker and jet divided by the wavelength of the selected acoustic sound is equivalent to one-half wavelength of the accepted sound, the selected frequency jumps to another state.