A simple and effective means of turbulence suppression is to introduce a small obstacle, such as a thin cylinder or wire, into the shear layer. [Strykowski & Sreenivasan (1990)] suppressed the Kármán vortex street behind a cylinder by placing a small control cylinder into one of the main cylinder shear layers. [Tong & Warhaft (1994)] suppressed the spreading and the turbulence in an axisymmetric jet by placing a thin ring behind the nozzle, and [Rajagopalan & Antonia (1998)] reduced the turbulence in a mixing layer with a small cylinder. In all these investigations the shear layer is essentially disturbed by the wake generated by the wire. A detailed stability analysis of the effect of such a wake on a shear layer was given by [Michalke & Schober (1999)].
Figure 6 shows a visualisation of the shear layer perturbed by the presence of a still wire with a diameter of d = 0.4 mm 1. The formation of shear-layer vortices is inhibited. Similar to a Kármán vortex street, vortex pairing does not occur. Laminar-turbulent transition takes place immediately behind the wire. The structure of turbulence is of a much finer scale than in the unforced case. The spreading rate of the smoke is much smaller than for the unforced case, indicating that the mixing of the wall jet with the ambient fluid decreased.
The wire has to be placed upstream of the position of natural shear-layer roll-up, since the wire can only inhibit the formation of shear-layer vortices, but cannot destroy vortices already formed.
![\includegraphics[width=0.9\textwidth]{smoke50stillwire.eps}](img10.gif)