TURBULENCE SUPPRESSION IN FREE SHEAR FLOWS BY CONTROLLED EXCITATION.

In an attempt to explain the mechanics of turbulence suppression observed earlier 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 moment thickness. The suppression effect is maximum for excitation at the Strouhal number of about 0. 017.