Turbulence suppression in free shear flows by controlled excitation

In an attempt to explain the mechanics of turbulence suppression previously observed by us 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 400θ e downstream from the exit; θ e is the initial shear-layer momentum thickness. The suppression effect is a maximum for excitation at the Strouhal number St θ(≡ f θ e / U e ) of about 0.017. In the jets, the turbulence suppression is observed over the range 0·75 ≤ x / D ≤ 8, while for the plane mixing layer it could be detected as far downstream as x ≅ 6000θ e . The flow-fields with and without excitation for a typical case of turbulence suppression have been studied in detail. Spectra of the u signal and time-averaged field data indicate that excitation at St θ ≅ 0·017 suppresses the formation of naturally occurring energetic vortices - an observation confirmed by flow-visualization experiments and by study of the large-scale coherent structures of the shear layer, educed through conditional-sampling measurements. Excitation at St θ ≅ 0·017 produces a rapid growth of the shear layer instability mode, and consequently, its saturation, roll-up and breakdown occurs much earlier in x than is found to occur naturally (at St θ ≅; 0·012). The suppression effect is apparently a consequence of earlier transition of the shear layer vortices, which otherwise naturally grow to larger sizes and survive for larger x, as well as being due to the prevention of successive pairing of these structures.