arXiv subjects

Bernhard Weigand

Publications and source records attributed to Bernhard Weigand.

13 recordsLinked to original sources

arXiv2026

Retraction dynamics of polymer-laden droplets impacting a smooth solid surface

Hypothesis: The presence of a polymer in a droplet plays a significant role in contact line dynamics following droplet impact onto a solid substrate. We hypothesize that the polymer concentration modifies the post-impact wetting behavior. Experiments: In our experiments, we visualize the contact line dynamics of polymer-laden droplets impacting onto a flat sapphire substrate. Using high-speed side and bottom-view imaging, we analyze droplet spreading and retraction over a broad range of Weber numbers (10-700) and polyethylene oxide (PEO) concentrations (0-400 ppm). To systematically characterize the contact line dynamics, we used image processing to quantify the maximum spreading factor, receding velocity, and receding contact angle. Findings: The effect of the polymer on the spreading phase is negligible, as the maximum spreading factor follows classical inertial-capillary scaling, showing that polymer addition produces no measurable change in the maximum spreading under the present conditions. In contrast, the receding phase exhibits a strong dependence on the polymer concentration: increasing the PEO content leads to a significant reduction in the receding velocity and dynamic contact angle, while the spreading dynamics remain water-like. These results are consistent with an additional resistance associated with the receding contact-line region, while the microscopic origin of this resistance remains unresolved. To capture this behavior, we propose a scaling model that relates the normalized retraction velocity to the receding contact angle and impact conditions. The model effectively collapses the experimental data across all investigated concentrations and Weber numbers.

physics.flu-dyn↗

arXiv2026

The Role of Interfacial Tension in Direct Numerical Simulations of Drop-Film Interaction for Immiscible Fluids

Many experimental studies have reported variations in interfacial tension. Isolating all the geometric and fluid material parameters and varying the interfacial tension can be useful to check their influence. Numerical investigations using Free Surface 3D (FS3D), have been conducted to compare varying values of interfacial tension and evaluate the sensitivity. A grid independence study compared the compound crown height of a splash to determine the required resolution for validation. A qualitative validation showed FS3D could correctly capture the impact morphology while varying the viscosity ratio of the drop and film liquid when compared to the experimental results. A quantitative validation for a water drop impacting onto an oil film shows a good match for the crown heights of the numerical and experimental data. The same setup was then extended to study the variation of interfacial tension, where the deviation of the overall compound crown height and spreading diameter of the internal crowns was compared. Results revealed minor changes in the compound crown height and spreading diameter of the drop liquid, but the internal crown composition showed significant differences. In order to run FS3D efficiently on the new supercomputer Hunter, which has a new APU architecture-based system, extensive work had to be done. To adapt to the new hardware architecture, large parts of FS3D have been ported to utilise the AMD Instinct MI300A accelerated processing units (APUs) at HLRS using OpenMP. Implementation of Umpire memory pools improved performance for larger workloads per APU. The GPU-accelerated code achieves a 4 times speedup compared to CPU-only execution on the same hardware. Strong and weak scaling tests have been conducted, showing good strong scaling for up to 4 APUs, and linear weak scaling for up to 512 APUs, resulting in a total of 4096**3 cells for the first time.

physics.flu-dyn↗

arXiv2025

Direct Numerical Simulations of Droplet Impact onto Heated Surfaces using the Program Free Surface 3D (FS3D)

Droplet impact onto heated surfaces is a widespread process in industrial applications, particularly in the context of spray cooling techniques. Therefore, it is essential to study the complex phenomenon of droplet spreading, heat removal from a hot surface, and flow distribution during the impact. This study focuses on Direct Numerical Simulation (DNS) of the initial stage of a water droplet impact onto a highly conducting heated surface, below the saturation temperature of the liquid. The maximum spreading diameters at different impact velocities in the presence of a heated surface, are analysed. Free Surface 3D (FS3D), an in-house code developed at the Institute of Aerospace Thermodynamics, University of Stuttgart, is used for this work. A grid independence study investigates the resolution required to resolve the flow field around the droplet. As evaporation effects during the initial stage of the droplet impact process are negligible, they are ignored. However, for longer simulation times, evaporation plays a significant role in the process. Preparing for such simulations, an evaporating droplet in cross flow is simulated to study the performance gain in the newly implemented hybrid OpenMP and MPI parallelisation and red-black optimization in the evaporation routines of FS3D. Both the scaling limit and efficiency were improved by using the hybrid (MPI with OpenMP) parallelisation, while the red-black scheme optimization raised the efficiency only. An improved performance of 23% of the new version is achieved for a test case investigated with the tool MAQAO. Additionally, strong and weak scaling performance tests are conducted. The new version is found to scale up to 256 nodes compared to 128 nodes for the original version. The maximum time-cycles per hour (CPH) achieved with the new version is 35% higher compared to the previous version.

physics.flu-dyn↗

arXiv2024

JAX-based differentiable fluid dynamics on GPU and end-to-end optimization

This project aims to advance differentiable fluid dynamics for hypersonic coupled flow over porous media, demonstrating the potential of automatic differentiation (AD)-based optimization for end-to-end solutions. Leveraging AD efficiently handles high-dimensional optimization problems, offering a flexible alternative to traditional methods. We utilized JAX-Fluids, a newly developed solver based on the JAX framework, which combines autograd and TensorFlow's XLA. Compiled on a HAWK-AI node with NVIDIA A100 GPU, JAX-Fluids showed computational performance comparable to other high-order codes like FLEXI. Validation with a compressible turbulent channel flow DNS case showed excellent agreement, and a new boundary condition for modeling porous media was successfully tested on a laminar boundary layer case. Future steps in our research are anticipated.

physics.flu-dyn↗

arXiv2023

An investigation of information flux between turbulent boundary layer and porous medium

The interaction between boundary layer turbulence and a porous layer is the cornerstone to the interface engineering. In this study, the spatial resolved transfer entropy is used to assess the asymmetry of the causal interaction next to a permeable wall. The analysis was based on pore-resolved direct numerical simulation of turbulent channel flow over a cylinder array. The spatial map of transfer entropy reveals the information flux between the porous medium and arbitrary nearby position.

physics.flu-dyn↗

arXiv2023

Modeling of Coupled Turbulent Channel Porous Media Flow through a Deep Autoencoder Echo State Network Framework

In this study, we propose a novel approach, namely the combined Convolutional Deep Autoencoder Echo State Network (CDAE ESN) model, for the analysis and forecasting of dynamics and low order statistics in coupled turbulent channel porous media flows. Such systems find wide applications in industrial settings, including transpiration cooling and smart interface engineering. However, the complex geometry of coupled flow systems presents additional challenges for purely data-driven models. Our results demonstrate that the integration of deep autoencoder and echo state network techniques enables effective modeling and prediction of dominant flow behaviors, particularly within the porous domain exhibiting laminar regimes. To enhance the model s applicability across a broader range of data domains, we further employ fine-tuning on a dataset encompassing varying porosities. The achieved average statistics exhibit a reasonable agreement, underscoring the efficacy of our proposed approach.

physics.flu-dyn↗

arXiv2023

DNS of the Early Phase of Oblique Droplet Impact on Thin Films with FS3D

Spray impacts occur in several environmental and technical applications. The impact of droplets at different angles onto walls covered with a thin film of the same liquid can be regarded as an elementary process here. Direct Numerical Simulations (DNS) provide an important contribution to the understanding and modeling of the impact outcome, which might be associated with the formation of a crown and ejection of secondary droplets. Thus, we gain detailed information about, e.g. the flow field and shape of the interface, which are not accessible in experiments. This chapter presents a DNS study of the early crown formation mechanisms present at an oblique droplet impact on a thin film, as well as a grid study showing the resolution required to resolve the impact's details. Highly resolved simulations in large domains require continuous development of the numerical solver's efficiency. The performance of different cycles of the multigrid (MG) solver for the solution of the pressure Poisson equation was compared, and a F-cycle was added. Furthermore, we implemented a hybrid MPI and OpenMP parallelisation, which both increases the scaling limit further. Additionally, studies on the strong and weak scaling are conducted. The choice of the F- and V- cycle in the MG-solver and the additional hybrid parallelisation increased the achieved computed cycles per hour (CPH) by a factor of 12.4 compared to the formerly employed setup. The nodes efficiently usable were increased by a factor of 16. Both, the close to linear scaling regime for the strong scaling and the almost constant performance regime for the weak scaling were increased by this factor.

physics.flu-dyn↗

arXiv2023

Simulation and analytical modeling of high-speed droplet impact onto a surface

The fluid dynamics of liquid droplet impact on surfaces hold significant relevance to various industrial applications. However, high impact velocities introduce compressible effects, leading to material erosion. A gap in understanding and modeling these effects has motivated this study. We simulated droplet impacts on surfaces and proposed a new analytical model for impact pressure and droplet turning line, targeting at predictions for enhanced cavitation. The highly compressed liquid behind the droplet expands sideways, causing lateral jetting. As the droplet encounters a shock wave, it reflects as a rarefaction wave, leading to low-pressure zones within the droplet. These zones converge at the droplet's center, causing cavitation, which, upon collapse, induces another shock wave, contributing to erosion. Using the well-established model for the low-velocity impact shows a significant discrepancy. Hence, an analytical model for the turning line radius is introduced, incorporating the lateral jetting's characteristic length scale. Comparing our model with existing ones, our new model exhibits superior predictive accuracy.

physics.flu-dyn↗

arXiv2022

Towards DNS of Droplet-Jet Collisions of Immiscible Liquids with FS3D

In-air microfluidics became a new method for technical production processes with ultra-high throughput formerly performed in micro channels. Direct Numerical Simulations (DNS) provide a valuable contribution for the fundamental understanding of multiphase flow and later application design. This chapter presents a feasibility study with first DNS results of droplet-jet collisions of immiscible liquids using the in-house software Free Surface 3D (FS3D). Two cases were investigated with a setup comparable to experiments by Baumgartner et al. [1], where a droplet chain of a glycerol solution hits a jet of silicon oil which encapsulates the droplets. The droplets' shapes present are observed to be more complex than comprehensible from the two-dimensional images from the experiments. Thus, DNS with FS3D can provide additional information like the surface area or the velocity contributions in order to find analytical models of such collision processes in the future. Simulations of such increasingly complex systems require constant improvement of the numerical solver regarding the code's performance. Thus, the red-black Gauss-Seidel smoother in the multi-grid solver, the iterative red-black scheme to compute the viscous forces as well as the momentum advection method were enhanced with a cache- and memory usage optimization. An overall performance gain of up to 33% was obtained for a representative test case.

physics.flu-dyn↗

arXiv2022

Influence of liquid miscibility and wettability on the structures produced by drop-jet collisions

Collisions between a stream of drops and a continuous jet of a different liquid are experimentally investigated. In contrast to previous studies, our work focuses on the effects of liquid miscibility and wettability on the collision outcomes. Thus, miscible and immiscible liquids providing total and partial wetting are used. We show that, as long as the jet surface tension is smaller than the drop surface tension, the drops can be encapsulated by the jet, providing the so-called drops-in-jet structure. The transitions between the different regimes remain similar in nature with a capillary fragmentation responsible for the jet break-up and an inertial fragmentation causing the drops (and then possibly the jet) to break up. The dimensionless numbers proposed in the literature to model the inertial fragmentation thresholds do not bring the results obtained with different liquids at the same critical value. We explain the reason via a detailed analysis of the collisions, accounting for the drop and jet extensions and their kinetics. The drop fragmentation is found to occur during the recoil phase, leading us to propose a new dimensionless parameter that successfully reproduced all our experimental data obtained with immiscible liquids. Finally we demonstrate that the most dramatic change of the collision outcomes is produced by using drops that totally wet the jet. In this case, the encapsulation of the drops cannot be achieved, constituting a true limit to some applications based on the solidification of the drops-in-jet structure.

physics.flu-dyn↗

arXiv2020

Information transfer between turbulent boundary layer and porous media

The interaction between the flow above and below a permeable wall is a central topic in the study of porous media. While previous investigations have provided compelling evidence of the strong coupling between the two regions, few studies have quantitatively measured the directionality, i.e., causal-and-effect relations, of this interaction. To shed light on the problem, we use transfer entropy as a marker to evaluate the causal interaction between the free turbulent flow and the porous media using interface-resolved direct numerical simulation. Our results show that the porosity of the porous medium has a profound impact on the intensity, time scale and spatial extent of surface-subsurface interactions. For values of porosity equal to 0.5, top-down and bottom-up interactions strongly are asymmetric, the former being mostly influenced by small near-wall eddies. As the porosity increases, both top-down and bottom-up interactions are dominated by shear-flow instabilities.

physics.flu-dyn↗

arXiv2017

Visualization of Feature Separation in Advected Scalar Fields

Scalar features in time-dependent fluid flow are traditionally visualized using 3D representation, and their topology changes over time are often conveyed with abstract graphs. Using such techniques, however, the structural details of feature separation and the temporal evolution of features undergoing topological changes are difficult to analyze. In this paper, we propose a novel approach for the spatio-temporal visualization of feature separation that segments feature volumes into regions with respect to their contribution to distinct features after separation. To this end, we employ particle-based feature tracking to find volumetric correspondences between features at two different instants of time. We visualize this segmentation by constructing mesh boundaries around each volume segment of a feature at the initial time that correspond to the separated features at the later time. To convey temporal evolution of the partitioning within the investigated time interval, we complement our approach with spatio-temporal separation surfaces. For the application of our approach to multiphase flow, we additionally present a feature-based corrector method to ensure phase-consistent particle trajectories. The utility of our technique is demonstrated by application to two-phase (liquid-gas) and multi-component (liquid-liquid) flows where the scalar field represents the fraction of one of the phases.

cs.GR↗

arXiv2017

Experimental and Numerical Investigation of Phase Separation due to Multi-Component Mixing at High-Pressure Conditions

Experiments and numerical simulations were carried out in order to contribute to a better understanding and prediction of high-pressure injection into a gaseous environment. Specifically, the focus was put on the phase separation processes of an initially supercritical fluid due to the interaction with its surrounding. N-hexane was injected into a chamber filled with pure nitrogen at 5 MPa and 293 K and three different test cases were selected such that they cover regimes in which the thermodynamic non-idealities, in particular the effects that stem from the potential phase separation, are significant. Simultaneous shadowgraphy and elastic light scattering experiments were conducted to capture both the flow structure as well as the phase separation. In addition, large-eddy simulations with a vapor-liquid equilibrium model were performed. Both experimental and numerical results show phase formation for the cases, where the a-priori calculation predicts two-phase flow. Moreover, qualitative characteristics of the formation process agree well between experiments and numerical simulations and the transition behaviour from a dense-gas to a spray-like jet was captured by both.

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