Low Velocity Ratio Transverse Jets Influenced by Concentric Synthetic Jets

Abstract

The objective was to discover and evaluate methods of smokestack downwash reduction using unsteady forcing. The flow is essentially a low momentum jet in a crossflow. After preliminary investigations of several concepts, the final wind-tunnel model applies an annular synthetic jet coaxially to a turbulent or laminar pipe flow at the stack exit. The crosswind Reynolds number based on stack diameter is on the order of 1000 to 7000. Primary diagnostics are 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. For turbulent pipe flow, plume height increases by as much as 1.8 stack diameters. This data is collapsed using a simple model based on the idea that synthetic jet momentum dominates the plume at high forcing amplitudes.

A version of these results is presented in Diep and Sigurdson (2002).