>

3.3 The wall jet manipulated by an oscillating wire

During the experiments with the still wire we noticed that under certain conditions the wire performed self-excited oscillations and a wire tension was necessary to inhibit these oscillations. When the tension was lowered with the wire positioned in the shear layer, the oscillations started. [Vandsburger & Ding (1995)] reported a similar phenomenom in a mixing layer, where an oscillating ''music'' wire greatly enhanced the mixing.

Figure 7: Visualisation of shear layer structures (oscillating wire) (movie 500kB).

Figure 7 shows a visualisation of the shear layer structures generated by the oscillating wire. The wire frequency was f_w $\approx$ 175 Hz and thus approximately six times smaller than the natural shear layer instability frequency. The very large coherent structures generated by the wire are clearly visible. Also, several stages of vortex pairing can be observed.

The wire sheds a vortex every time it traverses through the shear layer, thus generating two vortices per cycle. The first vortex, generated when the wire traversed into the potential core, is very small and convects downstream at almost the speed of the wire. It is produced by transporting fluid particles with low kinetic energy into a region with high kinetic energy. The second vortex, generated when the wire moved out of the potential core, is larger than the first one. This second vortex is produced by transporting fluid particles with high kinetic energy into a region with low kinetic energy. Since the sign of the vorticity of both vortices is the same and since they are very close together, pairing takes place immediately at x/b $\approx$ 4. The next stage of vortex pairing occurs at x/b $\approx$ 9. The spreading rate of the wall jet is largely enhanced by the oscillating wire.

The vortex shedding frequency solely depends on the oscillation frequency of the wire and the Kelvin-Helmholtz instability is unimportant for the vortex shedding. The wire frequency can be adjusted by varying the wire parameters, most conveniently by changing the wire tension. Choosing low oscillation frequencies results in small vortex shedding frequencies and thus leads to large vortices. Since low frequencies are accomplished by a small wire tension, the amplitude of the wire becomes larger. Since the wire must not touch the nozzle, the amplitude is limited by the distance betweeen the wire and the nozzle.

Drupal 7 Appliance - Powered by TurnKey Linux