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Stability and Breakup of Oscillating Liquid Jets

Figure 1: Oscillating jet, emanated from a fluidic oscillator, under experimental conditions [1].
Lupe

Fluidic oscillators have shown improved performance over a wide range of conventional nozzle types, e.g. hole or fan nozzles. The device emits a continuous jet, subjected to a spatiotemporal oscillation, transversal to its penetration direction while being supplied by a steady pressure source. Oscillation of the jet is caused by a hydrodynamic instability within the device. Current investigations show strong indications for superior performance with liquid fluids in spray formation. These arise from broader spatial distribution of the ejected liquid as compared to axisymmetric jets. Scalability of the devices over a wide range of pressures and oscillation frequencies enables possible usage in a variety of fields ranging from combustor technology, over cooling applications to the usage in health related applications.

Figure 2: The spatial evolution of an oscillating liquid jet [1]. Shown is a 3D- rendering of the liquid/gas interface. The jet enters the domain from the left and breaks up under the influence of hydrodynamic instabilities.
Lupe

The forced oscillatory movement transversal to its penetration direction is significantly altering the jet's breakup behavior as compared to non-oscillating, axisymmetric jets. The breakup length reduces, indicating increased presence of aerodynamic forces acting on the liquid/gas interface. The research project is investigating the development of hydrodynamic instabilitiesin the jet and their influence on the jet's breakup dynamics.In axisymmetric jets, hydrodynamic instabilities in the shear layer of the moving liquid phase and the quiescent gas phase act destabilizing on the jet surface resulting in formation of surface waves along the interface that are amplified as they travel in downstream. The presence of instabilities results in increasing corrugations of the liquid surface and eventually leads to disintegration and breakup of the liquid core into ligaments and droplets. The same mechanisms also act in case of oscillating jets but are accompanied by additional baroclinic instabilities, introduced by periodic acceleration of the liquid interface in normal direction due to the oscillation. The twofold destabilization of the liquid jet in presence of transverse oscillation is the main mechanism of breakup and droplet formation of oscillating liquid jets. Understanding and controlling these instabilities is vital to improve nozzle design for increased performance in a wide field of applications.  

Literature:

[1] S. Schmidt, O. Krüger, K. Göckeler, and C.O. Paschereit, 2018, Numerical Investigation of the Breakup Behavior of an Oscillating Two-Phase Jet. Phys. Fluids, 30(7), 072-101

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