Turbulence suppression in free shear flows by controlled excitation

In an attempt to explain the mechanics of turbulence suppression previously observed in jets under controlled excitation, the near fields of four circular jets, a plane jet and a plane mixing layer, all with laminar efflux boundary layers, have been explored through hot-wire measurements. It is shown that controlled excitation, induced acoustically as well as by vibrating ribbons, can reduce turbulence intensities in all these flows. Reduction by as much as 80% is observed, the maximum decrease occurring at about 400 theta //e downstream from the exit; theta //e is the initial shear-layer momentum thickness. The flow-fields with and without excitation for a typical case of turbulence suppression have been studied in detail. The suppression effect is apparently a consequence of earlier transition of the shear layer vortices. Refs.