The mechanisms of interfacial instability due to gas-liquid shear and liquid ligament acceleration that occurs in the near-field of a coaxial two-fluid atomizer play a determining role on the spray characteristics in the far-field, making understanding of these near-field physics key for spray modeling. Several metrics for these physical processes, with an emphasis on the liquid core length, are characterized in detail, using high-speed shadowgraphy, over a wide range of gas-to-liquid momentum ratios, comparing the fluids' dynamic pressures at the exit of the atomizer. A method that does not require arbitrary thresholds is proposed to rigorously define the initial spreading angle of a spray. The effect of adding azimuthal momentum to the gas co-flow (swirl) on the spray near-field is analyzed, and the possibility of periodic oscillations of the swirl is explored. Increased spreading angle is expected when swirl is present, but a decrease of the liquid core length average and standard deviation is also observed, with variations happening over timescales shorter than the actuation period. The overall effect is a wider, more dynamic spray,

Wave turbulence in quasi-1D geometry is usually not investigated experimentally since low-order resonant wave interactions are theoretically prohibited. Here, we report on the first observation of unidirectional capillary-wave turbulence on the surface of a fluid in a canal. We also show that five-wave interactions are the lowest-order resonant process subsisting at small scales, and are thus probably the one generating such quasi-1D capillary wave turbulence. We show that the wave spectrum is compatible with the corresponding dimensional analysis prediction. The main assumptions of weak turbulence theory are also verified experimentally. Quasi-1D wave turbulence could be thus highlighted in other fields of wave turbulence.

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Interfacial instabilities play a major role in breakup events in turbulent multiphase flow. Their role has been clearly identified for two-fluid atomization, and is of paramount importance in spray formation. In planar geometries, Kelvin-Helmholtz instabilities are the main mechanism of creation of a two-phase mixing layer, and information such as wavelengths and frequencies is available in the literature. In cylindrical geometries, the instabilities quickly become three-dimensional and thorough characterization is lacking, despite a wide range of applications using coaxial atomization. We conduct an experimental study of how the interfacial instabilities of a liquid jet surrounded by a turbulent gas co-flow accelerate and develop, before break-up and spray formation. We use high-speed shadowgraphy over a wide range of gas Reynolds numbers to compute the velocity of interfacial perturbations, using Lagrangian tracking, followed by a Eulerian conditioning to obtain local statistics. We identified two regimes of the gradient of the longitudinal mean velocity as a function of the gas Reynolds number: a quadratic scaling at low gas Reynolds numbers and a linear scaling at higher gas Reynolds number. In contrast, the transverse velocity gradients show a linear scaling with gas Reynolds number throughout the studied range.

When it comes to the surface, air in a pool of pure liquid does not form lasting bubbles, as it would if the bath contained surfactants. Here we describe what...

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We report on the experimental observation of a transition from a dispersive wave turbulence regime to a nondispersive regime involving shock waves on the surface of a fluid. We use a magnetic fluid in a canal subjected to an external horizontal magnetic field to tune the dispersivity of the system. For a low magnetic field, gravity-capillary wave turbulence is observed, whereas for a high enough field, random steep coherent structures arise which are found to be shock waves. These shock waves create singularities in the second-order difference of the surface elevation, leading to an ω −4 frequency power spectrum. This spectrum is also found to be controlled by the number and amplitude of the shocks and is well captured by a model based on a random Dirac-δ distribution (Kuznetsov-like spectrum). Finally, the shock-amplitude statistics exhibits a power-law distribution with an exponent close to the predictions of the one-dimensional random-forced Burgers equation. This shock-wave regime, discovered here for surface waves, thus paves the way to better explore their properties.