Evolving time surfaces and tracking mixing indicators for flow visualization

The complexity of large scale computational fluid dynamic simulations (CFD) demands powerful tools to investigate the numerical results. To analyze and understand these voluminous results, we need to visualize the 3D flow field. We chose to use a visualization technique called Time Surfaces. A time surface is a set of surfaces swept by an initial seed surface for a given number of timesteps. We use a front tracking approach where the points of an in initial surface are advanced in a Lagrangian fashion. To maintain a smooth time surface, our method requires surface refinement operations that either split triangle edges, adjust narrow triangles, or delete small triangles. In the conventional approach of edge splitting, we compute the length of an edge, and split that edge if it has exceeded a certain threshold length. In our new approach, we examine the angle between the two vectors at a given edge. We split the edge if the vectors are diverging from one another. This vector angle criterion enables us to refine an edge before advancing the surface front. Refining a surface prior to advancing it has the effect of minimizing the amount of interpolation error. In addition, unlike the edge length criterion which yields a triangular mesh with even vertex distribution throughout the surface, the vector angle criterion yields a triangular mesh that has fewer vertices where the vector field is flat and more vertices where the vector field is curved. Motivated by the evaluation and the analysis of flow field mixing quantities, this work explores two types of quantitative measurements. First, we look at Ottino\u27s mixing indicators which measure the degree of mixing of a fluid by quantifying the rate at which a sample fluid blob stretches in a flow field over a period of time. Using the geometry of the time surfaces we generated, we are able to easily evaluate otherwise complicated mixing quantities. Second, we compute the curvature and torsion of the velocity field itself. Visualizing the distribution and intensity of the curvature and torsion scalar fields enables us to identify regions of strong and low mixing. To better observe these scalar fields, we designed a multi-scale colormap that emphasizes small, medium, and large values, simultaneously. We test our time surface method and analyze fluid flow mixing quantities on two CFD datasets: a stirred tank simulation and a BP oil spill simulation

Visualization of Time-Varying Data from Atomistic Simulations and Computational Fluid Dynamics

Time-varying data from simulations of dynamical systems are rich in spatio-temporal information. A key challenge is how to analyze such data for extracting useful information from the data and displaying spatially evolving features in the space-time domain of interest. We develop/implement multiple approaches toward visualization-based analysis of time-varying data obtained from two common types of dynamical simulations: molecular dynamics (MD) and computational fluid dynamics (CFD). We also make application case studies. Parallel first-principles molecular dynamics simulations produce massive amounts of time-varying three-dimensional scattered data representing atomic (molecular) configurations for material system being simulated. Rendering the atomic position-time series along with the extracted additional information helps us understand the microscopic processes in complex material system at atomic length and time scales. Radial distribution functions, coordination environments, and clusters are computed and rendered for visualizing structural behavior of the simulated material systems. Atom (particle) trajectories and displacement data are extracted and rendered for visualizing dynamical behavior of the system. While improving our atomistic visualization system to make it versatile, stable and scalable, we focus mainly on atomic trajectories. Trajectory rendering can represent complete simulation information in a single display; however, trajectories get crowded and the associated clutter/occlusion problem becomes serious for even moderate data size. We present and assess various approaches for clutter reduction including constrained rendering, basic and adaptive position merging, and information encoding. Data model with HDF5 and partial I/O, and GLSL shading are adopted to enhance the rendering speed and quality of the trajectories. For applications, a detailed visualization-based analysis is carried out for simulated silicate melts such as model basalt systems. On the other hand, CFD produces temporally and spatially resolved numerical data for fluid systems consisting of a million to tens of millions of cells (mesh points). We implement time surfaces (in particular, evolving surfaces of spheres) for visualizing the vector (flow) field to study the simulated mixing of fluids in the stirred tank

The structure of a statistically steady turbulent boundary layer near a free-slip surface

The interaction between a free-slip surface with unsheared but sustained turbulence is investigated in a series of direct numerical simulations. By changing (i) the distance between the (plane) source of turbulence and the surface, and (ii) the value of the viscosity, a set of five different data sets has been obtained in which the value of the Reynolds-number varies by a factor of 4. The observed structure of the interaction layer is in agreement with current knowledge, being made of three embedded sublayers: a blockage layer, a slip layer, and a Kolmogorov layer. Practical measures of the different thicknesses are proposed that lead to a new Reynolds-number scaling based on easy-to-evaluate surface quantities. This scaling is consistent with previous proposals but makes easier the comparison between free-surface flows when they differ by the characteristics of the distant turbulent field. Its use will be straightforward in a turbulence-modeling framework

Fatigue crack propagation in microcapsule toughened epoxy

The addition of liquid-filled urea-formaldehyde (UF) microcapsules to an epoxy matrix leads to significant reduction in fatigue crack growth rate and corresponding increase in fatigue life. Mode-I fatigue crack propagation is measured using a tapered doublecantilever beam (TDCB) specimen for a range of microcapsule concentrations and sizes: 0, 5, 10, and 20% by weight and 50, 180, and 460 micron diameter. Cyclic crack growth in both the neat epoxy and epoxy filled with microcapsules obeys the Paris power law. Above a transition value of the applied stress intensity factor, which corresponds to loading conditions where the size of the plastic zone approaches the size of the embedded microcapsules, the Paris law exponent decreases with increasing content of microcapsules, ranging from 9.7 for neat epoxy to approximately 4.5 for concentrations above 10 wt% microcapsules. Improved resistance to fatigue crack propagation, indicated by both the decreased crack growth rates and increased cyclic stress intensity for the onset of unstable fatigue-crack growth, is attributed to toughening mechanisms induced by the embedded microcapsules as well as crack shielding due to the release of fluid as the capsules are ruptured. In addition to increasing the inherent fatigue life of epoxy, embedded microcapsules filled with an appropriate healing agent provide a potential mechanism for self-healing of fatigue damage.published or submitted for publicationis peer reviewe

G-CSC Report 2010

The present report gives a short summary of the research of the Goethe Center for Scientific Computing (G-CSC) of the Goethe University Frankfurt. G-CSC aims at developing and applying methods and tools for modelling and numerical simulation of problems from empirical science and technology. In particular, fast solvers for partial differential equations (i.e. pde) such as robust, parallel, and adaptive multigrid methods and numerical methods for stochastic differential equations are developed. These methods are highly adanvced and allow to solve complex problems.. The G-CSC is organised in departments and interdisciplinary research groups. Departments are localised directly at the G-CSC, while the task of interdisciplinary research groups is to bridge disciplines and to bring scientists form different departments together. Currently, G-CSC consists of the department Simulation and Modelling and the interdisciplinary research group Computational Finance

Macroinstability and Perturbation in Turbulent Stirred Tank Flows

Impeller stirred tank reactors (STRs) are commonly used in the chemical processing industries for a variety of mixing and blending technologies. In this research, a numerical study of flow and mixing inside turbulently agitated STRs are carried out. An immersed boundary method (IBM) is utilized to represent moving impeller geometries in the background of multi-block structured curvilinear fluid. The IBM This curvilinear-IBM methodology is further combined with the large eddy simulation (LES) technique to address the issue of modeling unsteady turbulent flows in the STR. Verification of the combined IBM-LES implementation strategy in curvilinear coordinates is done through comparisons with the measurements of laminar and turbulent flows in baffled STRs with pitched blade impellers. Flow structures are studied inside a dished bottom pitched-blade baffled for different impeller rotational speeds in the turbulent regime to observe the formation of trailing edge vortices which are associated with higher levels of turbulent kinetic energy relative to the remaining flowfield. Instabilities occurring at a frequency lower than the frequency of impeller rotation are identified from the time signal of velocity components. The role of these low frequency macro-instabilities (MI) is explored by observing changes in the three-dimensional circulation pattern within the STR. Significant amount of kinetic energy is observed to be associated with the dynamics of the trailing edge vortices during MI cycles. Flow inside an unbaffled Rushton impeller STR is perturbed using time-dependent impeller rotational speeds at a dominant MI frequency. Perturbation increased the mean radial width of the impeller jet-stream and enhanced overall turbulent kinetic energy compared to the constant rotational speed cases. Large-scale periodic velocity fluctuations due to perturbations produced large strain rates favoring higher turbulence production. Fluctuations in power consumptions are shown to correlate with the perturbation amplitude. Study on the mixing of a passive scalar inside STR showed that the growth rate of unmixed tracer is influenced by the MI oscillations. Perturbation of the STR flow resulted into significant reduction of mixing time. The spatio-temporal behavior of the large-scale mixing structures revealed that fast mixing is promoted due to the break-up of unmixed segregated zones during a perturbation cycle

Développement, validation et application de modèles de bilan de populations sur des réacteurs à écoulement de bulles

Abstract: In order to optimize and design new bubbly flow reactors, it is necessary to predict the bubble behavior and properties with respect to the time and location. In gas-liquid flows, it is easily observed that the bubble sizes may vary widely. The bubble size distribution is relatively sharply defined, and bubble rises are uniform in homogeneous flow; however bubbles aggregate, and large bubbles are formed rapidly in heterogeneous flow. To assist in the analysis of these systems, the volume, size and other properties of dispersed bubbles can be described mathematically by distribution functions. Therefore, a mathematical modeling tool called the Population Balance Model (PBM) is required to predict the distribution functions of the bubble motion and the variation of their properties. In the present thesis, three different types of reactors are modeled using the open-source Computational Fluid Dynamic (CFD) package OpenFOAM. Furthermore, the Method of Classes (CM) and Quadrature-based Moments Method (QBMM) are described, implemented and compared using the developed CFD-PBM solver. These PBM tools are applied in two bubbly flow cases: bubble columns (using a Eulerian-Eulerian two-phase approach to predict the flow) and a water electrolysis reactor (using a single-phase approach to predict the flow). The numerical results are compared with measured data available in the scientific literature. It is observed that the Extended Quadrature Method of Moments (EQMOM) leads to a slight improvement in the prediction of experimental measurements and provides a continuous reconstruction of the Number Density Function (NDF), which is helpful in the modeling of gas evolution electrodes in the water electrolysis reactor. In the last case of the project, oxygen distribution in a laboratory scale (3 liters) bioreactor in the presence of an axial impeller is modeled using the CFD-PBM solver. The combined effect of the bubble breakup and coalescence in the tank is accounted for EQMOM. The three-dimensional simulation is made using a Multiple Reference Frame (MRF), a well-established method for the modeling of mixers. The model is used to predict the spatial distribution of gas phase fraction, Sauter mean bubble diameter (d32), NDF, Dissolved Oxygen (DO) evolution and flow structure. The numerical results are compared with experimental data and good agreement is achieved. The results are discussed based on four rotational speeds of impeller with different volumetric mass transfer coefficients.Dans l’optique d’optimiser et de concevoir des réacteurs à écoulement de bulles, il est nécessaire de prédire le comportement et les propriétés des bulles dans le temps et l’espace. En temps normal, la distribution de taille des bulles est bien définie et les bulles s’écoulent de manière uniforme et homogène. Toutefois des agrégats et les grosses bulles se forment rapidement si le l’écoulement n’est plus homogène. Il possible d’appuyer l’analyse de ces systèmes à l’aide de modèles mathématiques d’écrivant l’évolution de la distribution du volume, de la taille et d’autres propriétés dans le réacteur. La présente thèse explore l’application d’un de ces modèles mathématiques, appelés modèle de bilans des populations (PBM), dans différents cas impliquant deux colonnes à bulles rectangulaires et une cellule d’électrolyse. Les travaux ont été effectués à l’aide d’un ensemble d’outils numériques libres en CFD, mécanique des fluides numérique appelé OpenFOAM. C’est dans cet ensemble que la méthode des classes (CM) et des méthodes basées sur la quadrature des moments (QBMM) furent implémentées pour être comparées, par la suite, avec d’autres méthodes de bilans des populations précédemment publiées. La méthode de quadrature étendue des moments (EQMOM)5, dans cette thèse, mène à une amélioration de la prédiction des échanges gazeux, lorsque comparés aux résultats en laboratoire. De plus, la méthode permet la reconstruction des distributions statiques des bulles, ce qui s’avère particulièrement utile pour une modélisation précise des écoulements dans les cellules d’électrolyse. Une application plus complexe de l’implémentation des solveurs CFD-PBM àaussi été faite dans un bioréacteur de 3 litres en présence d’une hélice marine placée sur l’axe. Dans ce cas, l’effet combiné de la fission et de la coalescence des bulles a été pris en compte. L’écoulement tridimensionnel est réalisé en utilisant des référentiels multiples (MRF), une méthode bien établie dans la modélisation de réacteur de ce type. Le modèle a été utilisé pour prédire la distribution du gaz, le diamètre moyen de Sauter (d32), la fonction de densité (NDF), l’oxygène dissous et l’évolution de l’écoulement. Les résultats de la simulation numérique impliquant 4 vitesses de rotation concordent de manière satisfaisante aux résultats obtenus en laboratoire

Hydrodynamics, dissolution, and mass transfer effects in different dissolution testing apparatuses and laboratory systems

Dissolution testing apparatuses and shaker flasks agitated by shaker tables are laboratory systems routinely found in many laboratories at most companies and agencies, and especially in pharmaceutical companies. These devices are commonly used in a number of applications, from drug development to quality control. Despite their common use, these systems have not been fully investigated from an engineering perspective in order to understand their operation characteristics. For example, the hydrodynamics of many of these systems have received little attention until relatively recently, and only over the last few years have some of these systems, such as the USP dissolution testing Apparatus 2, been studied in greater detail by a few research groups, including our group at NJIT. Meanwhile, a number of modifications have been introduced in industry to simplify the practical use of these devices and to automate many of the processes in which they are utilized. This, in turn, has resulted in the introduction of variability in the way these devices are operated, with possible implication for the results that they generate in laboratory experiments and tests. Therefore, this work was aimed at studying some of these devices in order to quantify how such systems operate and what the implications for their use in the laboratory are. More specifically, the systems that were examined here included the USP Dissolution Testing Apparatus 2 with and without automatic sampling probes, dissolution testing mini vessel apparatuses, and baffled shaker flasks. In order to study all these phenomena, a number of tools were used, including Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD) to investigated the hydrodynamics of these systems; experimental tablet dissolutions under a number of controlled environments; and a combination of experimental, computational and modeling approaches to study mass transfer and solid suspension effects. The issues that were investigated depended on the specific apparatus. For the case of the USP Dissolution Testing Apparatus 2, the effects of the presence of different probes on the hydrodynamics in the dissolution vessel and on the dissolution profiles using solid dosage forms were investigated in this work. The results indicate that in most cases, the presence of the probe resulted in statistically significant increases in the dissolution curves with respect to the curves obtained without the probe, and that tablets at, or close to, the center of the vessel were more significantly affected by the presence of the probe, and so were tablets located immediately downstream of the probe. The hydrodynamic effects generated by the arch-shaped fiber optic probe were small but clearly measurable. The changes in velocity profiles in the dissolution vessel resulted in detectable differences in the dissolution profiles, although not high enough to cause test failures. However, these differences could contribute to amplify the difference in dissolution profiles in those cases in which tablet has an intrinsically higher release rate. In addition, the minimum agitation speed, Njs, to achieve particle suspension was investigated. A novel method to determine Njs was first developed and then applied to determine Njs as a function of different operating variables. Similarly, the hydrodynamics of smaller dissolution apparatuses termed “minivessels” were studied here and compared with the standard USP 2 system. The flow pattern in minivessels was obtained by both CFD simulations and PIV velocity measurements for four different agitation speeds in the mini vessel, and it was shown to result in flow patterns qualitatively similar to those in the standard USP 2 system. The velocity profiles were also compared on several iso-surfaces for the mini vessel system and the standard system, showing difference between two systems. In the most important zone, i.e., the inner core zone at the vessel bottom, the velocities were similar on the lowest iso-surface, especially for the axial velocity at 100rpm and 125rpm in the mini vessel compared with 100rpm in the 900mL USP 2 system. This was not clearly the case for iso-surfaces above the bottom zone. Finally, the hydrodynamics of baffled shaker flask was investigated. These baffled “trypsinizing” flasks are similar to the typical Erlenmeyer-type conical shaker flasks commonly used in biological laboratories but with a major difference, in that they are provided with vertical indentations in the glass flask so as to create vertical baffles that promote better mixing when shaken. Measurements of the velocity in the flask were obtained using PIV for seven rotation speeds of 100, 125, 150, 160, 170, 200, and 250 rpm. Two vertical cross sections were measured to obtain the velocity profiles in the flask: the one with largest diameter of the flask, and the one with the smallest diameter. The 1D energy spectra indicate nearly isotropic flow in the BF for all rotation speeds and the existence of inertial subrange, which validate the use of dimensional argument analysis for the estimation of energy dissipation rate. The results obtained in this work will contribute to increase our understanding of the performance of a number of very common and important laboratory apparatus thus contributing to a more appropriate use of all these devices in both industry and federal and state agencies