Bifurcating jets generated with stepped and sawtooth nozzles

A simple round jet is perturbed by attaching either a stepped or a sawtooth trailing edge that acts as a form of passive control. Flow visualization, laser Doppler anemometer, and hot-wire measurements are used to document the flow behavior in both water and air for Reynolds numbers of 7600 and 22 000, respectively. When the jet is forced, the flow bifurcates. The bifurcation requires a low forcing frequency that encourages the formation of tilted primary vortex rings with significant vorticity and axial spacing. In the step nozzle flow, the rings result from closed vortex loops. In the sawtooth nozzle flow, they develop from sections of a helix. A powerful secondary jet initiates in straining regions between primary ring cores, where closed-loop or helical structures with opposite tilt are present. The radial momentum in this jet is large enough to divert the path of the primary jet in the opposite direction. The bifurcating effect was found to be strongest when the forcing frequency was less than or equal to one-third of the natural roll-up frequency.