Subprojects TA-C

Drop-Liquid interaction

The subprojects (SP) of TA-C focus on single and multiple drop impacts onto thin films and deep pools, on the macro as well as the micro scale, with support of multi-components visualization and numerical methods.

SP-C1: Micro and macro drop impact dynamics with miscible liquids

The utilization of bio-recyclable, blended fuels in diesel engines for attaining better emissions and replacing fossil fuels has resulted in an increased interest in binary drop collisions with miscible fluids. Although the impact dynamics is very similar to that of single component dropcollisions, new phenomena have been observed. They seem to be associated to miscibility gaps and strong differences in surface tension that may lead to a local unbalance of surface forces and hence to a premature rupture of the thin lamella. Interestingly enough, a similar phenomenology has also been observed in two-components splashing on wetted walls, where new splashing regimes have been reported, characterized by the formation of holes in the crown wall. These effects influence strongly the resulting drop size distribution and consequently the mixture formation. These preliminary considerations show clearly the need of developing new models for predicting the outcome of drop impact with miscible liquids.

A major challenge of such investigations is related to the fact that the new, observed collisional phenomena are strongly dominated by liquid motion and mixing in very thin liquid films. Therefore, to gain a better understanding of collisional dynamics with miscible fluids, it is necessary to adopt a unified approach that couples the micro-scale flow in such thin regions to the overall macroscopic flow features (e.g. crown shape). The coupling of fluid dynamic processes at different scales is exactly the focus of this sub-project.

The objectives of SP-C1 are twofold. First we aim to generate an accurate database on binary fluid collisions. Second, in close collaboration with the numerical partners, we aim to gain a better understanding on the interplay between small-scale flow features and the overall macroscopic impact dynamics.

For this purpose, a key requirement for the experimental investigations is to ensure a high temporal and spatial resolution. The high temporal resolution is required for capturing all instability details of the collisional phenomena, such as jetting or fingering. The PIs have a long-standing experience in high-speed visualisation of such impact dynamics at high Weber numbers. The high spatial resolution is required for resolving the details of the flow motion at small scale.

As mentioned earlier, thin film dynamics in the neck region controls both the thickness of the emerging lamella as well as the shape of the crown. To investigate these phenomena and their im-pact on the macro-scale fluid dynamics, we will implement two microscopic techniques in coopera-tion with SP-C5: Micro PIV and evanescent waves. Note that the ITLR group has already experience with the technique of evanescent waves applied to drop impact phenomena. The research activities in SP-C1 are organised in four main tasks:

    • High-speed visualisation of binary drop collisions. Different fluid combinations will be analysed over a wide range of surface tensions and viscosities at low and high Weber numbers. The primary objective is to derive a classification of the different impact regimes. The second objective is to formulate (when feasible) scaling laws and compare it to existing models for binary immiscible drops.
    • Development and application in cooperation with SP-C5 of micro visualisation techniques to study the thin film dynamics in splashing/deposition impact process.
    • Micro and macro scale flow visualisation of binary splashing experiments. The primary goal here is to understand how the flow dynamics in the neck region affects the global crowns morphology. Second goal is to translate this understanding into a semi-empirical model to predict the formation of holes and crown shape.

    • Investigation of the merging mechanism in thick film layers in cooperation with SP-C2.

SP-C2: Single and multiple drop impact into a deep pool

Impacts of liquid drops on liquid surfaces are involved in a large range of engineering applications and, especially, deep liquid layers are of great interest in geophysics and soil mechanics. Ferreira et al. have shown that the motion of the fluid after the impact of the drop on a thick liquid layer is responsible for an increase in soil erosion, since the receding part of crater evolution produces upward fluid motion that lift the underneath soil.

The estimation of air-sea heat and mass transfer is expected to improve the predictions of local carbon budget and cyclone development in the tropical region. These scalar transfers are affected by the underlying fluid flow behaviour. Wide qualitative information, e.g. fluid motion visualized by dying the impinging drop, is available for classical single drop impact, whereby quantitative information such as velocity fields below impacting drops are missing. Far less is available on multiple impingements of drops on liquid layers and their outcomes.

This lack of information motivates this sub-project, which focus on the understanding of the fundamental fluid mechanics governing the vorticity generation and evolution as important and complex impact outcome of liquid-liquid interaction.

The main objective of the sub-project is to contribute to the understanding of vorticity generation and to find a satisfactory mechanism for vorticity formation and evolution by single and multiple drops. A complementary measurement technique, consisting of high speed visualization and laser Doppler velocimetry, will be used to investigate the flow field. This choice will allow to draw conclusions with high temporal resolution on generation and evolution of vorticity structures. Further, Micro PIV will be utilized in coordination with SP-C1 within selected regions close to cavity, in order to extend the database obtained with Laser Doppler Velocimetry (LDV), a point wise measurement technique. The experimental conditions will take into account the main influence factor on vorticity structures (drop shape and velocity prior impact, viscosity, drop-drop distance for multiple drop impact) and the following tasks are envisioned:

    • Vortex structure map within the We-Fr ranges for single drop impacts.
    • Vortex structure map within the We-Fr ranges for multiple drop impacts.

    • Analysis of the influence of viscosity on vortex structures

SP-C3: High-order numerical methods for multi-component in-compressible flows in pools

The main goal of this sub-project is the development of a robust and accurate tool for the numerical simulation of interface problems. The nature of the physical processes considered within this sub-project is complex and involves different spatial and temporal scales. An efficient simulation tool, based on a flow model studied to correctly predict the interface evolution, and which is able to accurately solve the evolving flow structures without spoiling all the smaller scales resolution, is then mandatory.

The model involves a system of four equations, obtained from the conservation principles applied to each phase, and closed by an equation for the evolution of the volume fraction. The coupling between these classes of two-phase flow models with a high-order discretization technique such as the Discontinuous Galerkin (DG) method, known for its favourable dissipation properties, is then very appealing and will be the subject of this sub-project.

The purpose of this sub-project is the development of a new highly-accurate and efficient simulation tool for the thorough investigation of two-phase incompressible flow problems. The implementation of a diffuse interface approach in the existing code MIGALE will aim at exploiting, as much as possible, the unified and high-order, both in space and time, numerical framework currently used for compressible and incompressible flow simulations. The following workplan is foreseen during the sub-project:

    • Extension to the multi-phase case of the local incompressible Riemann problem formulation and thorough validation on simple one-dimensional flow problems.
    • High-order DG implementation within the DG code MIGALE of a five equation two-phase model inspired by the works on incompressible flows of Drew and on compressible flows of Saurel and Abgrall. Possible simplifications of the model that could lead to a set of governing equations for a sole "averaged phase" will be also considered.
    • Development of a stabilization approach, able to provide a sharp sub-cell resolution, to control numerical oscillations when volume fraction discontinuities occur inside cells.
    • Validation of the developed two-phase solver by addressing benchmark test cases and comparing the solutions with experimental and numerical results available in the literature.

    • Assessment of the solver against the theoretical and experimental findings obtained within the IRTG project.

SP-C4: Visualization of drop-liquid interaction

Within the IRTG, specially tailored visualization techniques are required to ensure better understanding of the basic processes governing the drop interaction phenomena. In SP-C4, novel visualisation techniques will be developed and existing ones will be adapted and extended in order to provide tools for the analysis of the data in all Thematic Areas (TAs), which require different visualization approaches.

Visualization of drop impact dynamics has to include high dynamics and complexity of interfaces as well as mixing of different fluids, while in the visual analysis of deep pool impact the uncertainty in the vortex analysis must be considered. Moreover, the multi-scale and multi-physics nature of the phenomena must be taken into account. With reference to porous media, new methods are required for the (combined) visualization of high-resolution micro-CT, fluid volume data, surface structures, as well as derived quantities. This poses significant challenges for the visualization both in terms of generating expressive representations, and with respect to being able to achieve this at the high performance required for interactive visual exploration. This sub-project, together with SP-B3, will deliver a set of visualization approaches for both simulation and experimental data for all TAs but its focus will be on drop-liquid interaction.

In TA-C, the investigated drop-liquid interaction processes are highly complex. On one hand, the interface dynamics leads to the generation of complicated interface formations, on the other hand, mixing of the liquid phases and their different surface tensions can result in intricate phase structures. In SP-C4, the focus will be on the development of visualization techniques that help the respective sub-projects gain insight into basic mechanisms governing these complex phenomena. For the analysis of surface dynamics and the instabilities of the interface resulting from differences in surface tension, visualization techniques based on derived quantities, such as gradients of surface tension force, will be developed to find potential breakup regions, and the interplay of velocity and surface tension will be investigated to gain understanding in the interface instability. Visualization of dye advection lends itself well to the investigation of miscible fluids. The challenge lies in the development of a robust technique that preserves intricate structures of mixing phases.

Potential approaches include texture-based and particle-based dye visualization. In both cases, numerical diffusion should be minimized, and the possibility of measuring numerical diffusion should be investigated. For the particle-based approaches that will deal with the multi-phase nature of the flow, potential problems resulting from phase change of particles will be addressed. Approaches inspired by the FTLE should be investigated to visualize thinning and folding governing the mixing processes. Abstract visualization, including glyph-based techniques, allows for a concise visualization of complex phenomena. Therefore, glyph-based visualization of droplet internal flow should help to classify various impact regimes. For a more detailed inspection of the internal flow, methods based on integral lines should be investigated that visualize the flow within the liquid phase. The velocity measurements used to analyse the vorticity structures in SP-C2 will provide a basis for the development of numerical visualization of vortex rings observed in experiments. Techniques for the identification of vortex core lines should be examined, and uncertainty of the measurements should be taken into consideration. The developed visualization methods will also allow for a comparative visualization of simulation and experimental data. For the investigation of drop evaporation in TA-A, glyph-based visualization techniques should provide an abstract representation of the phenomena investigated in molecular dynamics data and reduce the visual clutter inherent to this type of simulations. Moreover, these techniques should give insight into the dependence of droplet evaporation on interface kinetics.

The micro-CT data provided by SP-B3 will enable exploration of internal flow in surface microstructures by means of computational visualization. Interface reconstruction in porous media should be investigated, as well as methods for visual tracking of liquid interface that could increase the understanding of flow dynamics on surface structures. To ensure interactive visualization, the developed techniques will be implemented on graphics processing units (GPUs). Due to the large data sizes, utilization of massively parallel systems might also be necessary. Therefore, for the development of the techniques, parallelization on many-core systems must be taken into account.

SP-C5: Development of novel optical technique for micro-fluid dynamics (Postdoc-Project)

Liquid-liquid interactions described in SP-C1 and SP-C2 feature macroscopic flow characteristics that are governed by fluid dynamics on the micro-scale. In SP-C1, the thin film dynamics in the crown's neck region is a key parameter in the overall interaction process. Some attempts to explain the mechanism of crown rupture were made based on local gradients of shear stresses or surface tensions. To corroborate and generalize the existing models, it is necessary to build up a database in the micro-scale level. The latter includes film thickness and velocity distribution within the thin liquid film, both required with high temporal resolution. Instead in SP-C2, a volute is observed that contains a rotational spiral motion and dominates the total vortex geometry and macroscopic flow dynamics. This volute is suppressed for higher impact kinetics and transient velocity fields are absent due to the short time and space scale associated with drop impacts, but needed for gaining a better physical insight. Thereby, the focus of this sub-project is to develop and/or adapt micro-scale measurement approaches for drop-liquid interaction processes and to unify them with the established macro-scale techniques to a joined multi-scale tool applicable for the experimental oriented sub-projects in TA-C.

The main objective of this project is the adoption of micro-scale measurement approaches - Micro PIV and TIRFM - and their implementation as complementary techniques to the already established equipment of the respective sub-projects. The joined multi-scale tool is envisioned to ensure high temporal and spatial resolution, aimed at capturing instabilities and flow motions on small scales. The research activities in SP-C5 can be structured in two main tasks:

  • Implementation of Micro PIV in the sub-projects SP-C1 and SP-C2. Special attention will be devoted to: (a) optical distortion correction, (b) choice of tracer particle (type and concentration) and its effect on the flow dynamics.
  • Implementation of TIRFM in SP-C1 and its adaption on film thickness measurements for bi-nary mixtures with similar refractive indexes.

Spokesperson

Bernhard Weigand
Prof. Dr.-Ing.

Bernhard Weigand

Spokesman DROPIT, University of Stuttgart

Gianpietro Elvio Cossali
Prof. Dr.

Gianpietro Elvio Cossali

Spokesman for University of Bergamo

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