MasterNanosciences, nanotechnologies

The destabilization of a liquid jet by a coaxial gas jet is a useful setting to study fundamental mechanisms of two-phase flows, such as interfacial instability or break-up. In the case of zero gas velocity, the mechanisms are well described along the Rayleigh-Plateau instability, which becomes modified as the liquid velocity increases and entrains the surrounding atmosphere [1]. With the addition of a coaxial gas jet, the physics differ drastically. The velocity difference between the liquid and gas jets leads to a Kelvin-Helmholtz instability, forming waves whose crest gets elongated by the gas flow, forming ligaments [2]. Beyond a threshold velocity, the acceleration is such that the ligaments can suffer break-up through a Rayleigh-Taylor instability [3]. This phenomenological framework can shed light on the underlying physical mechanisms involved in destabilization and break-up phenomena in turbulent two-phase flows, but it remains fairly qualitative overall, which prevents a direct predictive power for the application.
 
An experimental setup has been assembled at LEGI, consisting of a two-fluid coaxial atomizer. The attached visualization illustrates the spray formation phenomena that result from the destabilization of the gas-liquid interface (highlighted in red). Using ultra-high-resolution imaging, we can focus on the formation and transport of the interfacial perturbations [4]. This corresponds to a factor of approximately 5 compared to the provided visualization, reaching spatial and temporal resolutions of respectively 1 µm and 0.02 µs. This framework can be used to evaluate quantitatively the change of physics as the gas Reynolds number is increased (by change of velocity). In addition to this situation, the role of two other phenomena in the instability will be investigated. The first one is the onset of turbulence in the liquid flow, presenting small-scale perturbation in the initial conditions. The second one is modifications of the large-scale structure of the gas jet, by adding angular momentum or increasing the turbulence intensity. Measurements with multiple high-speed cameras will also be considered to study how the local interfacial perturbation can travel around the liquid jet.
 
This work requires skills in fluid mechanics and will involve image and signal processing tools that will be gained during the internship.

References

1] Villermaux, E. (2007). Fragmentation. Annu. Rev. Fluid Mech., 39, 419-446.
[2] Matas, J. P., Delon, A., & Cartellier, A. (2018). Shear instability of an axisymmetric air–water coaxial jet. Journal of Fluid Mechanics, 843, 575-600.
[3] Marmottant, P., & Villermaux, E. (2004). On spray formation. Journal of fluid mechanics, 498, 73-111.
[4] Ricard, G., Machicoane, N., Osuna-Orozco, R., Huck, P. D., & Aliseda, A. (2021). Role of convective acceleration in the interfacial instability of liquid-gas coaxial jets. Physical Review Fluids, 6(8), 084302.