UNC_2 25–25 800.00 0.985 1.53E + 04 32.5

UNC_3 50–0 1600.00 1.059 3.05E + 04 87.5

UNC_4 0–0 800.00 0.659 1.53E + 04 30.0

Fig. 8. P1 V Image at the Nozzle Outlet, 150 frames/section, UNC_1, z = 0.00 D. Color scale from 0.00 to 12.00 m/s, increment 0.8 m/s.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this

article.)  

Therefore, a trigger variation was tried to obtain traces of other singularities inside of the flow. However this led to similar results to those found earlier.

Triggering points at 90%, 67% and 34% of the highest peak after 5 min of free running were used. No significant differences to the 90% trigger level results were found.

Re-construction of the 3D body was performed as shown in Fig. 9. The PVC behaves as a semi-helix. The twist is only seen up to 0.60 D from the outlet, spiralling for some 70–90 from its origin, then straightening as identified in Fig. 10.

 Fig. 9. 3D image processing. Flow signal (Yellow), accompanied by a TTL Pulse, (Blue), which triggers the Lasers (Green and Purple). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

 Fig. 10. Detailed PVC, Case UNC_1.

Little difference was observed between experiments at different triggering points. However, another process became evident. A higher resolution analysis showed more detail of the region where the PVC stops spiralling and enlargement of the PVC section occurs (at 0.45–0.60 D). The PVC appears to keep rotating up to 0.45 D without any constraint. At this point, two smaller vortices are apparent inside of the PVC envelope. They then increase in size and strength to a point where they dominate the PVC. At this point, both vortices join and merge as a single stronger vortex. This vortex then becomes roughly axi-symmetric in the flow. Fig. 11 shows the development of both structures until they merge as a single stronger vortex, Cala et al who observed similar mechanisms in their studies.

As the flow was unconfined, no External Recirculation Zones (ERZ) were observed. Although some external shear flow eddies were detected, their appearance was attributed to Kelvin–Helmholtz instability. These structures were not considered in the current study. Fig. 12 shows a typical ‘‘axial” plot.

In the lower regions of the PVC the axial velocity fluctuated between 1.64 and 2.765 m/s. At the upper section the velocities ranged from 1.261 and 2.21 m/s. This was probably due to wobble of the PVC which was quite evident as shown in the amplitude modulation in the PVC traces recorded by the HWA. This in turn is transmitted to the CRZ giving rise to maximum velocity deviations at its boundary of 3.214 m/s, (these are located in the lowest region of the structure where a small vortex appears). Values are similar to those at the edge of the shear layer, Fig. 13. The lowest value was 0.959 m/s, located at the top of the CRZ. Elsewhere, velocities ranged from 1.2 to 1.8 m/s. The CRZ boundary appears to be wobbling matching that of the PVC. The end and bottom of the CRZ appears to be especially unstable.