Computational analysis of single rising bubbles influenced by soluble surfactant

This paper presents novel insights about the influence of soluble surfactants on bubble flows obtained by Direct Numerical Simulation (DNS). Surfactants are amphiphilic compounds which accumulate at fluid interfaces and significantly modify the respective interfacial properties, influencing also the overall dynamics of the flow. With the aid of DNS local quantities like the surfactant distribution on the bubble surface can be accessed for a better understanding of the physical phenomena occurring close to the interface. The core part of the physical model consists in the description of the surfactant transport in the bulk and on the deformable interface. The solution procedure is based on an Arbitrary Lagrangian-Eulerian (ALE) Interface-Tracking method. The existing methodology was enhanced to describe a wider range of physical phenomena. A subgrid-scale (SGS) model is employed in the cases where a fully resolved DNS for the species transport is not feasible due to high mesh resolution requirements and, therefore, high computational costs. After an exhaustive validation of the latest numerical developments, the DNS of single rising bubbles in contaminated solutions is compared to experimental results. The full velocity transients of the rising bubbles, especially the contaminated ones, are correctly reproduced by the DNS. The simulation results are then studied to gain a better understanding of the local bubble dynamics under the effect of soluble surfactant. One of the main insights is that the quasi-steady state of the rise velocity is reached without ad- and desorption being necessarily in local equilibrium

A numerical stabilization framework for viscoelastic fluid flow using the finite volume method on general unstructured meshes

A robust finite volume method for viscoelastic flow analysis on general unstructured meshes is developed. It is built upon a general-purpose stabilization framework for high Weissenberg number flows. The numerical framework provides full combinatorial flexibility between different kinds of rheological models on the one hand, and effective stabilization methods on the other hand. A special emphasis is put on the velocity-stress-coupling on co-located computational grids. Using special face interpolation techniques, a semi-implicit stress interpolation correction is proposed to correct the cell-face interpolation of the stress in the divergence operator of the momentum balance. Investigating the entry-flow problem of the 4:1 contraction benchmark, we demonstrate that the numerical methods are robust over a wide range of Weissenberg numbers and significantly alleviate the high Weissenberg number problem. The accuracy of the results is evaluated in a detailed mesh convergence study

The stressful way of droplets along single fibre strands : A computational analysis

Droplets wetting and moving on fibers are omnipresent in both nature and industry. However, little is known on the local stresses the fiber substrates experiences and, in turn, the local forces acting on those droplets while moving on vertical fiber strands. This work is concerned with disclosing the influence of droplet volume, viscosity, and chemical substrate heterogeneity on droplet motion. For this purpose, we pursue a computational simulation campaign by means of direct numerical simulations resolving all relevant spatial and temporal scales. On the basis of local simulation data, we evaluate and analyze effective viscous dissipation rates as well as viscous and capillary forces. We also assess the validity of an assumption, which is frequently used in correlations for droplets moving on single-fiber strands—neglecting the capillary force. Our computational analysis allows to falsify/verify this assumption for different scenarios and reveals that such correlations have to be applied with care, particularly when it comes to chemical heterogeneity of the fiber substrates

The stressful way of droplets along single fibre strands : A computational analysis

Droplets wetting and moving on fibers are omnipresent in both nature and industry. However, little is known on the local stresses the fiber substrates experiences and, in turn, the local forces acting on those droplets while moving on vertical fiber strands. This work is concerned with disclosing the influence of droplet volume, viscosity, and chemical substrate heterogeneity on droplet motion. For this purpose, we pursue a computational simulation campaign by means of direct numerical simulations resolving all relevant spatial and temporal scales. On the basis of local simulation data, we evaluate and analyze effective viscous dissipation rates as well as viscous and capillary forces. We also assess the validity of an assumption, which is frequently used in correlations for droplets moving on single-fiber strands—neglecting the capillary force. Our computational analysis allows to falsify/verify this assumption for different scenarios and reveals that such correlations have to be applied with care, particularly when it comes to chemical heterogeneity of the fiber substrates