SHEAR FLOW INSTABILITY GENERATED BY NON-HOMOGENEOUS EXTERNAL FORCING
The generation of shear flow instabilities by external forcing is a subject to which considerable attention currently is being paid, as the extensive study reported in this journal by researchers at the Lockheed-Georgia Co. attests [1]. Usually, the external forcing device (e.g., a loud speaker) produces disturbances with wavelength quite different from that of the instability wave being generated. In this case, for coupling to take place between forcing and instability, it is necessary either (i) that the perturbation interact with a flow or geometric inhomogeneity [2], or (ii) that the external forcing be nonhomogeneous [3,4]. It is likely that in most instances mechanism (i) is dominant, but in some cases mechanism (ii) or both mechanisms might occur. Because controversy exists over whether Tam's [3] mechanism is viable, the experiment described in the present letter was designed to show that instability waves can indeed be generated by nonhomogeneous forcing.
Mechanism (0 includes scattering of disturbances by edges. To eliminate this mechanism, the experiment was done in the mixing layer produced by a screen stretched across the lower half of a wind tunnel test section (see Figure 1 and reference [5]). The trailing edge which exists upstream of a splitter plate mixing layer is absent in this configuration.
u.