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Globally-unstable low-density jets
The schlieren visualization on the right shows the structure of a helium round jet issuing into air. The clean oscillation near the jet exit, which is caused by the global instability, is clearly shown. Transient side jets are also seen to emerge and decay. Side jets issuing along the optical access are obscured by the integrated nature of the visualization technique. The Reynolds number is 850.
The schlieren visualization on the right shows the structure of a helium jet issuing from an elliptical nozzle with an aspect ratio of 3. The Reynolds number is approximately 1000. The nozzle is oriented such that the major axis of the elliptical nozzle is normal to the optical axis. It is apparent that elliptical nozzles to exhibit global instability even though the momentum thickness of the boundary layer varies around the azimuthal coordinate of the jet. One of the key differences between elliptical and round helium jets is that elliptical jets show more stationary side jet behavior in time. These side jets will promote enhanced mixing compared to the round jet.
The video on the right shows a Mie scattering signal from the helium round jet in the cross section. The scattering is caused by the introduction of olive oil droplets in the jet using a Laskin nozzle. A CW laser is used to illuminate the jet cross section at an axial location of four jet diameters downstream of the jet exit. It is seen that the cross section goes through various states or geometries, with a lifespan of several (tens to hundreds) global oscillation periods.
Liquid/gas coaxial jets
Liquid/gas coaxial jets are commonly used to generate a spray. In this configuration, the liquid is generally in the center and is surrounded by an annular gas flow. A variety of mechanisms are involved in the production of the breakup of the jet, including hydrodynamic instabilities, turbulence in the liquid, and surface tension. Geometry of the coaxial jet hardware also plays a very important role. The two videos on the right show two examples, one at a low Weber number and the other at a high Weber number. In the higher Weber number case, inertial forces of the hydrodynamic instability are sufficient to overcome the stabilizing effect of surface tension.
Interactions between acoustics and liquid or liquid/gas shear flows
Liquid jets exposed to acoustic waves may undergo a transformation due to the disturbance field associated with the acoustic field. One manifestation of acoustic wave/liquid jet interactions is shown in the video on the right. In this video, the sound is initially off, and the jet is laminar with no breakup. As time advances, the speaker is activated, causing an amplification of the acoustic field, and the jet eventually undergoes a change. The jet first becomes flattened and forms a lobe. As the acoustic amplitude increases, the lobe grows and eventually leads to an atomization process.
Liquid impinging jets
Impinging liquid jets is a common method for atomizing liquids in energy and propulsion systems. The videos on the right show front and side views of two impinging water jets at moderately high Weber number. A sheet is formed that is seen to manifest impact waves, which leads to the shedding of droplet rings in the downstream region.
Temporal Shear Layers
Temporal mixing or shear layers are relevant to many fluid mechanics applications including energy and propulsion systems as well as atmospheric and oceanic flows. Due to the simple boundary conditions, the temporal shear layer is also a common flow used to simulation-based fluid dynamics studies. A study is currently underway to make particle image velocimetry measurements in a tip tank system as that shown in the video to the right. This video shows the mixing that occurs between vegetable oil on the top, and red-dyed water on the bottom. The preliminary facility was developed and tested by Troy Krizak and Ryan Boll.
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