For the unforced wall jet, which emanates laminar from the wall-jet nozzle, the Kelvin-Helmholtz instability leads to the formation of shear-layer vortices. These vortices undergo several stages of pairing processes leading to relatively large-scale turbulent structures.
Figure 8 compares the structures without, with a still, and with an oscillating wire. The arrows at the right-hand side are of equal length and represent the spreading of the smoke for the unforced case.
A thin wire, placed directly behind the nozzle into the shear layer prevents the shear-layer roll-up. Pairing processes can no longer be observed. The wire significantly reduces the size of the turbulent structures. The spreading of the wall jet is reduced, the mixing with the ambient fluid reduced.
A self-excited oscillating wire introduces structures which do not depend upon the shear-layer properties but on the wire frequency. Choosing low oscillation frequencies leads to the formation of large vortices, which increase in size over several stages of vortex pairing. The spreading rate and the mixing is dramatically increased.
![\includegraphics[width=0.9\textwidth]{smoke50mod4fer.eps}](img12.gif)