In this work, we investigate flow topology modifications produced by a swarm of large surfactant-laden droplets released in a turbulent channel flow. Droplets have same density and viscosity of the carrier fluid, so that only surface tension effects are considered. We run one single-phase flow simulation at $R{e}_{\tau }=\rho u_{\tau }h/\mu =300$, and ten droplet-laden simulations at the same $R{e}_{\tau }$ with a constant volume fraction equal to $\Phi \simeq 5.4\%$. For each simulation, we vary the Weber number ($We$, ratio between inertial and surface tension forces) and the elasticity number (${\beta }_s$, parameter that quantifies the surface tension reduction). We use direct numerical simulations of turbulence coupled with a phase field method to investigate the role of capillary forces (normal to the interface) and Marangoni forces (tangential to the interface) on turbulence (inside and outside the droplets). As expected, due to the low volume fraction of droplets, we observe minor modifications in the macroscopic flow statistics. However, we observe major modifications of the vorticity at the interface and important changes in the local flow topology. We highlight the role of Marangoni forces in promoting an elongational type of flow in the dispersed phase and at the interface. We provide detailed statistical quantification of these local changes as a function of the Weber number and elasticity number, which may be useful for simplified models.

中文翻译：

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载有表面活性剂的液滴对湍流拓扑的影响

在这项工作中，我们研究了由湍流通道流中释放的大量载有大量表面活性剂的液滴产生的流动拓扑结构的变化。液滴具有与载液相同的密度和粘度，因此仅考虑表面张力效应。我们在以下位置运行一个单相流模拟$\mathrm{[R}{Ë}_{\tau }=\rho {ü}_{\tau }H/\mu =300$，并同时进行十次含液滴的模拟 $\mathrm{[R}{Ë}_{\tau }$ 恒定体积分数等于 $\Phi \simeq 5.4％$。对于每次模拟，我们都会更改韦伯数（$\mathrm{w\; ^}Ë$，惯性和表面张力之间的比率）和弹性数（${\beta }_s$，用于量化表面张力降低的参数）。我们使用湍流的直接数值模拟结合相场方法来研究毛细作用力（垂直于界面）和Marangoni力（垂直于界面）对湍流（液滴内部和外部）的作用。正如预期的那样，由于液滴的体积分数较低，因此我们在宏观流量统计中观察到较小的变化。但是，我们观察到界面处涡度的重大变化以及局部流动拓扑的重要变化。我们强调了马兰戈尼力在促进分散相和界面处的伸长流类型中的作用。我们根据韦伯数和弹性数提供这些局部变化的详细统计量化，这可能对简化模型有用。