Effect of surfactant-laden droplets on turbulent flow topology

TL;DR

This study uses direct numerical simulations to explore how surfactant-laden droplets influence turbulence and flow topology in a channel, revealing significant local modifications despite minimal changes in overall flow statistics.

Contribution

The paper introduces a detailed analysis of the effects of surfactant-laden droplets on turbulence, emphasizing the roles of capillary and Marangoni forces through advanced simulations.

Findings

Marangoni forces promote elongational flow at interfaces

Droplets cause major local flow topology changes

Macroscopic flow statistics are minimally affected

Abstract

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 $Re_\tau=\rho u_\tau h / \mu = 300$, and ten droplet-laden simulations at the same $Re_\tau$ with a constant volume fraction equal to $\Phi \simeq5.4\%$. 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.