Turbulent vortex rings

Maxworthy, T., Turbulent vortex rings. Journal of Fluids Mechanics, 64,227-239,1974. 

Figure 2 shows the direct numerical simulation of vortex ring formation at the exit of a nozzle. The vortex rings entrain surrounding fluid and travel downstream from the orifice because of self-induced velocity. The flow field of a synthetic jet might be initially comprised of a periodic array of vortex rings that are created at the exit orifice. A jet is formed by the interacting vortices. The vortices could be initially laminar or turbulent, but the final jet is often turbulent and the flow at the far field resembles that of conventional continuous and pulsed jets (see Fig. 3). 

Fig. 3 Starting synthetic jet. The first two ingestion/ expulsion cycles are visualized. In the first cycle a laminar vortex ring emanates from the orifice in a stationary domain. The following vortex ring is turbulent due to chaotic motion inside the cavity after the ingestion period, (a) Original LIT visualization, (b) After postprocessing calculation to identify sharp gradient lines...

The photographic study revealed that for (gD ) /v < 500 flow in the wake is laminar, whereas for (gD ) Zv > 900 it is turbulent. In the intermediate range of values of the dimensionless group, transition from laminar to turbulent flow is observed the vortex ring in the wake is seen to oscillate and occasionally break... 

For turbine mixers that the width of a baffle should not exceed more than one-twelfth of the tank diameter and, for propeller mixers, no more than one-eighteenth the tank diameter. With side-entering, inclined or off-center propellers, as shown in Figure 13, baffles are not required. Instead, shrouded impellers and diffuser rings may be used to suppress vortex formation. These devices contribute to flow resistance and reduce circulation by creating intense shear and abnormal turbulence... 

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