Bifurcating jets generated with stepped and sawtooth nozzles

Abstract

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.

Dynamics of Jets Issuing from Trailing-Edge Modified Nozzles

“…Interestingly though, regardless of the inclined-section relative sharpness (at least for these two studies by the authors), cross-sections of hybrid inclined jets deduced from PIV measurements appeared to correlate well with those of bifurcated jets, especially those of Longmire and Duong [37]. However, closer inspections of the cross-stream flow visualization results indicated that this is unlikely to be the case.…”

“…A more recent study by Longmire and Duong [37] offered an interesting take on the concept of stepped nozzles. In this experimental investigation, comparisons were conducted between a conventional stepped nozzle and one which was based on a "sawtooth" design, at Reynolds numbers of Re = 7600 and 22,000, as well as under unforced and forced flow conditions.…”

Turbulent jet manipulation using two unsteady azimuthally separated radial minijets

“…This is due to the unsteady minijet injections placed on the upper side. The periodic excitation may accelerate shear layer growth [19]. At x * = 1.0, the shear layer …”

“…Longmire & Duong [19] introduced the combination of both asymmetric passive technique and axial periodic excitation to manipulate a round jet by attaching a stepped trailing edge and a speaker upstream of the jet plenum. The vortex ring in the jet developed an inclined component and became asymmetrical, resulting in significant enhancement of entrainment and spreading.…”

Numerical Investigation on Flow Structures and Mixing Performances of Vector-Controlled Free Jet using DNS

“…Liner stability analyses have shown that two types of dominant modes characterize the large-scale flow structures of the near-field of a jet, namely, varicose and helical modes, and diffusion or mixing is effectively controlled through these modes. It is well known that a complex jet comprises a combination of these modes (2), (3) . For example, a flapping mode comprises a pair of helical modes with the same frequency and same amplitude.…”