The objective was to discover and evaluate methods of smokestack downwash reduction using unsteady forcing. The flow was essentially a low momentum jet in a crossflow. After preliminary investigations of several concepts, the final wind-tunnel model applied an annular synthetic jet coaxially to a turbulent or laminar pipe flow at the stack exit. The cross-wind Reynolds number based on stack diameter was of the order of 1000. Primary diagnostics were hot-wire measurements and image processing of smoke flow visualization photographs of the plume gas. For a laminar pipe flow and increasing forcing amplitude, the initially turbulent plume can be made to completely relaminarize and then become turbulent again at higher amplitudes. Therefore an increased forcing does not always increase mixing. At higher amplitudes vortices appear at the forcing frequency. An abrupt transition of the vortices' arrangement occurs at a particular amplitude that coincides with an apparent jump in mixing.
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