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A comparison of angular velocity to the number of vortex coreline segments within a droplet. These two quantities behave inversely, i.e., higher velocity yields a lower number of coreline segments
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We present a data-driven visual analysis approach for the in-depth exploration of large numbers of droplets. Understanding droplet dynamics in sprays is of interest across many scientific fields for both simulation scientists and engineers. In this paper, we analyze large-scale direct numerical simulation datasets of the two-phase flow of non-Newto...
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... seen by looking at the velocity magnitudes and the surface of the jet base in the first time step of the simulation (Fig. 6). However, it has previously been unclear how this setup affects droplet velocities further within the simulation domain. Next, we look at the correlation between angular velocity and the number of vortex coreline segments (Fig. 7). Originally, our hypothesis was that a high angular velocity in the sense of rigid body rotation could be seen as a vortex, but in fact the plot shows exactly the opposite. We see that only droplets with a relatively small angular velocity have a high number of vortex core line segments, while droplets that have at least one vortex ...
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... each droplet by merging a cube of 2 Â 2 Â 2 cells into a single cell and repeated the quantity calculation thereon. As expected, the calculations are quite stable for large droplets containing many grid cells and less stable for smaller droplets consisting of fewer cells. An example for this behavior in the form of the velocity value is shown in Fig. 17a. Further, we can look at the temporal development of droplet quantities. Due to the relatively high temporal resolution of our dataset, we expect smooth changes in the quantities. Figure 17b shows that this only holds for the highresolution grid, while the downsampled grid introduces numerical errors due to the coarse ...
Citations
... Then it should design an implementation that, for example, allows to robustly ignore interactions that are irrelevant or not yet implemented and take part in the rest of the communication. This effectively describes a generalization of the implementation in Heinemann et al. [14]. A first internal draft of this is outlined by G. Reina. ...
... Furthermore, we applied the density-based spatial clustering of applications with noise (DBSCAN) algorithm [25,26] on the iso surface of the phase fraction field to isolate and analyse the droplets of a Taylor cone jet, as we see on Figure 5. By employing DBSCAN on the iso-surface of the field, we could accurately segment the droplets from the continuous phase of the flow, even when they were in close proximity or overlapping. ...
Taylor cone liquid jets occurs when a conductive liquid is placed on a capillary nozzle and a strong electric field is applied. The electric field causes the surface of the droplet to deform into a conical shape, and a liquid jet is ejected from the tip of the cone. This phenomenon has a wide range of applications, such as in inkjet printing, drug delivery, and electrohydrodynamic propulsion. An understanding of the underlying physics of the Taylor cone jet is essential for optimizing the performance of devices that utilize this phenomenon. Computational fluid dynamics (CFD) has become a powerful tool for studying the Taylor cone jet, and in this paper, we propose the utilization of a full three-dimensional model to study the complete dynamics of the Taylor cone jet. These electrohydrodynamic jets are a method to accomplish the controlled emission of microdroplets, with applications from constructing nanofibers to micro-propulsion. For the numerical computations, we use the interIsoFoam solver on OpenFOAM, which resolves an immiscible two-phase flow, and coupled it with a transport equation for the electric charges as well the simplified version of the Maxwell equations for an electrostatic field. The advection equation of the phase fraction is solved by a geometric Volume-Of-Fluid (VOF). Moreover, the hydrody-namic momentum equation incorporates electrically generated body forces using the Maxwell Stress Tensor (MST). While axisymmetric simulations are computationally less expensive, they fail to capture an important behavior of this type of jet, such as the whipping effect and the tiny droplets emitted during the receding of the jet emission cycle. In contrast, the three-dimensional simulations used in this study offer a more accurate representation of the physics involved in the jet formation process, including the formation of instabilities and the resulting complex jet shapes. As we show in our results the droplets are radially scattered on the target collector due to the formation of the ionic wind, which we also show the three-dimensional structures. The current study begins with the numerical validation of the Taylor cone formation, by comparing the cone shape with the experimental results of the literature. Then simulations were performed for different electric potentials and inlet flow rates, which showed that the stable window is narrowed by the applied electric potential. The results revealed that the instability of the jet is due to the concentration of the electric charges, which led to a breakup of the jet into droplets, in the direction of the electric field. Overall, this study emphasizes the value of using three-dimensional numerical simulations to study Taylor cone jet instabilities because they provide a more accurate depiction of the physics at play and can offer useful information for optimizing Taylor cones jet-using equipment like inkjet printers and electrospray systems.
... We exemplify the utility of our technique using multiphase flow simulation data. The investigation of droplets is an active area of research (Focke and Bothe 2012;Heinemann et al. 2021;Jadidi et al. 2019;Tryggvason et al. 2011;Yokoi 2008), as is the analysis of liquid jets (Ertl and Weigand 2015;Li et al. 2010). We refer the reader to for a detailed introduction to multiphase flow simulations and Lefebvre (1989) for a thorough description of liquid atomization and sprays. ...
In multiphase flows, the evolution of fluid-fluid interfaces is of interest in many applications. In addition to fluid dynamic forces governing the flow in the entire volume, surface tension determines droplet interfaces. Here, the analysis of interface kinematics can help in the investigation of interface deformation and the identification of potential breakups. To this end, we developed a visualization technique using metric and shape tensors to analyze interface stretching and bending. For interface stretching, we employ the eigenpairs of the metric tensor defined for the deformation rate of the fluid surface. For interface bending, we present a technique that locally captures the interface curvature change in terms of a shape tensor, extracting its principal directions and curvatures. We then visualize interface deformation by combining both representations into a novel glyph design. We apply our method to study multiphase flow simulations with particular emphasis on interface effects. These include the interplay between fluid dynamics and surface tension forces leading to breakup processes following droplet collisions, as well as droplet-droplet interactions of different fluids where Marangoni convection along the surface is explicitly taken into account.
Graphical abstract
... Generally, a rendering module within MegaMol is provided with a render target and a camera, both of which can be modified by other modules before the renderer is executed, and the render target provides access to the rendering result for further processing afterward. Our technique was implemented as a module that we can simply insert in front of any 2D renderer into existing projects (see also Fig. 1 Iris is taken from the UCI Machine Learning Repository (Dua and Graff 2017), Concrete Beam is from an FEM structural mechanics simulation of a beam (containing nodes and stress tensors) (Kelleter et al. 2020), and Droplets are extracted physical quantities per droplet of a multiphase jet simulation as used by Heinemann et al. (2021). We picked Parallel Coordinates Plot and Scatterplot Matrix renders as typical representatives of 2D visualizations. ...
Data visualization relies on efficient rendering to allow users to interactively explore and understand their data. However, achieving interactive frame rates is often challenging, especially for high-resolution displays or large datasets. In computer graphics, several methods temporally reconstruct full-resolution images from multiple consecutive lower-resolution frames. Besides providing temporal image stability, they amortize the rendering costs over multiple frames and thus improve the minimum frame rate. We present a method that adopts this idea to accelerate 2D information visualization, without requiring any changes to the rendering itself. By exploiting properties of orthographic projection, our method significantly improves rendering performance while minimizing the loss of image quality during camera manipulation. For static scenes, it quickly converges to the full-resolution image. We discuss the characteristics and different modes of our method concerning rendering performance and image quality and the corresponding trade-offs. To improve ease of use, we provide automatic resolution scaling in our method to adapt to user-defined target frame rate. Finally, we present extensive rendering benchmarks to examine real-world performance for examples of parallel coordinates and scatterplot matrix visualizations, and discuss appropriate application scenarios and contraindications for usage.
Graphical Abstract
In a cylindrical chamber, the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate was used to compress gaseous hydrogen from 220 to 752.3 bar, concurrently raising its temperature from 298.15 to 394.2 K over approximately six seconds. A three-dimensional liquid-piston compressor model was established and validated against the experimental data. Two-phase flow was simulated using the finite volume method and the volume of fluid model in ANSYS Fluent software. A novel heat transfer enhancement technique using cooling pipes was investigated inside a liquid-piston compressor to achieve near-isothermal compression. Multiple cooling scenarios were explored to enhance the compression and thermal performance, considering the number of pipes, cross-sectional shape, diameter, and pipe temperature. This approach provided a more comprehensive understanding of the flow regimes and heat transfer behaviors of working fluids throughout the compression process. To achieve the desired pressure ratio, using four circular pipes reduced the hydrogen temperature from 394.2 to 355 K, marking 40.8% improvement in thermal efficiency. Furthermore, compression performance reached 95.8% with a power density of 3221.4 kW·m−3, compared to 90.9% and 4550.6 kW·m−3 without cooling.
Physics of supercritical fluids is extremely complex and not yet fully understood. The importance of the presented investigations into the physics of supercritical fluids is twofold. First, the presented approach links the microscopic dynamics and macroscopic thermodynamics of supercritical fluids. Second, free falling droplets in a near to supercritical environment are investigated using spontaneous Raman scattering and a laser induced fluorescence/phosphorescence thermometry approach. The resulting spectroscopic data are employed to validate theoretical predictions of an improved evaporation model. Finally, laser induced thermal acoustics is used to investigate acoustic damping rates in the supercritical region of pure fluids.
High-voltage composite insulators are specially designed to withstand different environmental conditions to ensure a reliable and efficient electric power distribution and transmission. Especially, outdoor insulators are exposed to rain, snow or ice, which might significantly affect the performance of the insulators. The interaction of sessile water droplets and electric fields is investigated under various boundary conditions. Besides the general behavior of sessile droplets, namely the deformation and oscillation, the inception field strength for partial discharges is examined depending on the droplet volume, strength and frequency of the electric field and the electric charge. Particularly, the electric charge is identified to significantly affect the droplet behavior as well as the partial discharge inception field strength. In addition to ambient conditions, the impact of electric fields on ice nucleation is investigated under well-defined conditions with respect to the temperature and electric field strength. High electric field strengths are identified to significantly promote ice nucleation, especially in case of alternating and transient electric fields. Different influencing factors like the strengths, frequencies and time constants of the electric fields are investigated. Consequently, the performed experiments enhance the knowledge of the behavior of water droplets under the impact of electric fields under various conditions.
The present paper aims at developing a generally valid, consistent numerical description of a turbulent multi-component two-phase flow that experiences processes that may occur under both subcritical and trans-critical or supercritical operating conditions. Within an appropriate LES methodology, focus is put on an Euler-Eulerian method that includes multi-component mixture properties along with phase change process. Thereby, the two-phase flow fluid is considered as multi-component mixtures in which the real fluid properties are accounted for by a composite Peng-Robinson (PR) equation of state (EoS), so that each phase is governed by its own PR EoS. The suggested numerical modelling approach is validated while simulating the disintegration of an elliptic jet of supercritical fluoroketone injected into a helium environment. Qualitative and quantitative analyses are carried out. The results show significant coupled effect of the turbulence and the thermodynamic on the jet disintegration along with the mixing processes. Especially, comparisons between the numerical predictions and available experimental data provided in terms of penetration length, fluoroketone density, and jet spreading angle outline good agreements that attest the performance of the proposed model at elevated pressures and temperatures. Further aspects of transcritical jet flow case as well as comparison with an Eulerian-Lagrangian approach which is extended to integrate the arising effects of vanishing surface tension in evolving sprays are left for future work.
A fundamental understanding of droplet dynamics is important for the prediction and optimization of technical systems involving drops and sprays. The Collaborative Research Center (CRC) SFB-TRR 75 was established in January 2010 to focus on the dynamics of basic drop processes, and in particular on processes involving extreme ambient conditions, for example near thermodynamic critical conditions, at very low temperatures, under the influence of strong electric fields, or in situations involving extreme gradients of the boundary conditions. The goal of the CRC was to gain a profound physical understanding of the essential processes, which is the basis for new analytical and numerical descriptions as well as for improved predictive capabilities. This joint initiative involved scientists at the University of Stuttgart, the TU Darmstadt, the TU Berlin, and the German Aerospace Center (DLR) in Lampoldshausen. This first chapter provides a brief overview of the overall structure of this CRC as well as a summary of some selected scientific achievements of the subprojects involved. For further details the reader is referred to the subsequent chapters of this book related to the individual subprojects.
