Hrvoje Jasak

HIGH RESOLUTION NVD DIFFERENCING SCHEME FOR ARBITRARILY UNSTRUCTURED MESHES

SUMMARY The issue of boundedness in the discretisation of the convection term of transport equations has been widely discussed. A large number of local adjustment practices has been proposed, including the well-known total variation diminishing (TVD) and normalised variable diagram (NVD) families of differencing schemes. All of these use some sort of an ‘unboundedness indicator’ in order to determine the parts of the domain where intervention in the discretisation practice is needed. These, however, all use the ‘far upwind’ value for each face under consideration, which is not appropriate for unstructured meshes. This paper proposes a modification of the NVD criterion that localises it and thus makes it applicable irrespective of the mesh structure, facilitating the implementation of ‘standard’ bounded differencing schemes on unstructured meshes. Based on this strategy, a new bounded version of central differencing constructed on the compact computational molecule is proposed and its performance is compared with other popular differencing schemes on several model problems. Copyright

Dynamic Mesh Handling in OpenFOAM

Extension of static mesh numerics in a CFD solver to cases with deforming boundaries considerably expands the scope of its use. Dynamic mesh handling includes deforming mesh cases, where the number and connectivity of mesh elements remains unchanged; and topological changes, where mesh size and connectivity varies during the simulation. Cases where the boundary deformation itself represents a part of the solution demand special attention: here, mesh handling needs to be fully automatic. This paper describes dynamic mesh support in OpenFOAM, a C++ object-oriented library for numerical simulations in continuum mechanics. Unlike other tools, where dynamic mesh support is usually retro-fitted, object-oriented dynamic mesh engine has been built up from ground-up. Emphasis is given polyhedral cell support in mesh analysis and discretisation, vertex-based automatic mesh motion techniques and hierarchical design of topology morphing engine. The paper is completed with examples of solution-dependent motion with large boundary deformation.

On ultrasound-induced microbubble oscillation in a capillary blood vessel and its implications for the blood–brain barrier

The complex interaction between an ultrasound-driven microbubble and an enclosing capillary microvessel is investigated by means of a coupled, multidomain numerical model using the finite volume formulation. This system is of interest in the study of transient blood-brain barrier disruption (BBBD) for drug delivery applications. The compliant vessel structure is incorporated explicitly as a distinct domain described by a dedicated physical model. Red blood cells (RBCs) are taken into account as elastic solids in the blood plasma. We report the temporal and spatial development of transmural pressure (P(tm)) and wall shear stress (WSS) at the luminal endothelial interface, both of which are candidates for the yet unknown mediator of BBBD. The explicit introduction of RBCs shapes the P(tm) and WSS distributions and their derivatives markedly. While the peak values of these mechanical wall parameters are not affected considerably by the presence of RBCs, a pronounced increase in their spatial gradients is observed compared to a configuration with blood plasma alone. The novelty of our work lies in the explicit treatment of the vessel wall, and in the modelling of blood as a composite fluid, which we show to be relevant for the mechanical processes at the endothelium.

RESIDUAL ERROR ESTIMATE FOR THE FINITE-VOLUME METHOD

In this article, a novel a-posteriori error estimate for the finite-volume method (FVM) measuring the absolute magnitude of the discretization error is presented. The residual error estimate is based on the cell residual, similar to the popular error estimates in the finite-element community. An appropriate normalization of the local residual creates the error estimate with the same dimensionality as the variable of interest, making it practical for engineering use. The error estimate is tested on a series of test cases and is shown to perform considerably better than the traditional Taylor series error estimates.In this article, a novel a-posteriori error estimate for the finite-volume method (FVM) measuring the absolute magnitude of the discretization error is presented. The residual error estimate is based on the cell residual, similar to the popular error estimates in the finite-element community. An appropriate normalization of the local residual creates the error estimate with the same dimensionality as the variable of interest, making it practical for engineering use. The error estimate is tested on a series of test cases and is shown to perform considerably better than the traditional Taylor series error estimates.

Viscoelastic fluid analysis in internal and in free surface flows using the software OpenFOAM

Synthetic polymer products are of great importance in several industrial sectors, such as for production of packaging, parts of appliances, electronics, cars and food processing industries. Due to the increasing demand for this kind of material, reduction of waste and increase of quality has become a key issue in polymer industry. In this sense modeling and simulation of processing operations appears as a fundamental tool, leading to better understanding of how the rheological properties of polymers affect their processability and final product quality, and reducing time and costs related to the development of processes and products. This work presents some basic results that aims to validate a developed methodology for internal viscoelastic fluid flows, which was developed in a previous work in the OpenFOAM computational fluid dynamics package and also will be showed a extension of this methodology for analysis of free surface viscoelastic fluid flows, using the VOF methodology. A classical flow phenomena used in the rheology literature to present the concept of viscoelastic effects was simulated, i.e., the die swell experiment. The results obtained using Giesekus model showed the great potential of the developed formulation, once phenomena observed experimentally were reproduced in the simulations.

Viscoelastic Flow Simulation: Development of a Methodology of Analysis Using the Software OpenFOAM and Differential Constitutive Equations

Abstract Viscoelastic fluids are of great importance in many industrial sectors, such as in food and synthetic polymers industries. The rheological response of viscoelastic fluids is quite complex, including combination of viscous and elastic effects and highly nonlinear viscous and elastic phenomena. This work presents a new Computational Fluid Dynamics (CFD) tool for the simulation of viscoelastic fluid flows, which consists of a viscoelastic fluid module to be used with OpenFOAM CFD package. The main reasons for using OpenFOAM in the development of this tool are its characteristics with relation to flexibility to deal with complex geometries, unstructured and non orthogonal meshes, moving meshes, large variety of interpolation schemes and solvers for the linear discretized system, and the possibility of data processing parallelization. Several constitutive equations, such as Maxwell, UCM, Oldroyd-B, Giesekus, Phan-Thien-Tanner (PTT), Finitely Extensible Nonlinear Elastic (FENE-P and FENE-CR) and some derivations of Pom-Pom were implemented, in single and multimode form, and in this work the results are presented with the Giesekus model. The proposed methodology was validated by comparing its predictions with experimental and numerical data from the literature for the analysis of a planar 4:1 contraction flow.

A computational method for sharp interface advection

We devise a numerical method for passive advection of a surface, such as the interface between two incompressible fluids, across a computational mesh. The method is called isoAdvector, and is developed for general meshes consisting of arbitrary polyhedral cells. The algorithm is based on the volume of fluid (VOF) idea of calculating the volume of one of the fluids transported across the mesh faces during a time step. The novelty of the isoAdvector concept consists of two parts. First, we exploit an isosurface concept for modelling the interface inside cells in a geometric surface reconstruction step. Second, from the reconstructed surface, we model the motion of the face–interface intersection line for a general polygonal face to obtain the time evolution within a time step of the submerged face area. Integrating this submerged area over the time step leads to an accurate estimate for the total volume of fluid transported across the face. The method was tested on simple two-dimensional and three-dimensional interface advection problems on both structured and unstructured meshes. The results are very satisfactory in terms of volume conservation, boundedness, surface sharpness and efficiency. The isoAdvector method was implemented as an OpenFOAM® extension and is published as open source.

Multi-dimensional modeling of the air/fuel mixture formation process in a PFI engine for motorcycle applications

The preparation of the air-fuel mixture represents one of the most critical tasks in the definition of a clean and efficient SI engine. Therefore it becomes necessary to consolidate the numerical methods which are able to describe such a complex physical process. Within this context, the authors developed a CFD methodology into an open-source code to investigate the air-fuel mixture formation process in PFI engines. Attention is focused on moving mesh algorithms, Lagrangian spray modeling and spray-wall interaction modeling. Since moving grids are involved and the mesh quality during motion strongly influences the computed in-cylinder flow-field, a FEM-based automatic mesh motion solver combined with topological changes was adopted to preserve the grid quality in presence of high boundary deformations like the interaction between the piston bowl and the valves during the valve-overlap period. The fuel spray was modeled by using the Lagrangian approach, and the spray sub-models (atomization and breakup) were tuned according to experimental validations carried out in previous works. Specific submodels were implemented to describe the impingement of fuel spray with the engine walls. The evolution of the resulting liquid film was also taken into account by solving the mass and momentum equations with the Finite-Area discretization method. The proposed methodology was applied to simulate a single-cylinder SI engine for motor-scooter applications at a low load operating condition. This operating point was chosen since these engines often run very close to idle conditions when they are used in the urban areas.

Open-source computational model of a solid oxide fuel cell

The solid oxide fuel cell is an electro-chemical device which converts chemical energy into electricity and heat. To compete in todays market, design improvements, in terms of performance and life cycle, are required. Numerical prototypes can accelerate design and development progress. In this programme of research, a three- dimensional solid oxide fuel cell prototype, openFuelCell, based on open-source computational fluid dynamics software was developed and applied to a single cell. Transport phenomena, combined with the solution to the local Nernst equation for the open-circuit potential, as well as the Kirchhoff-Ohm relationship for the local current density, allow local electro-chemistry, fluid flow, multi-component species transport, and multi-region thermal analysis to be considered. The underlying physicochemical hydrodynamics, including porous- electrode and electro-chemical effects are described in detail. The openFuelCell program is developed in an object-oriented open- source C++ library. The code is available at http://openfuelcell.sourceforge.net/. The paper also describes domain decomposition techniques considered in the context of highly efficient parallel programming

OpenFOAM Turbo Tools: From General Purpose CFD to Turbomachinery Simulations

OpenFOAM is an established object-oriented library for Computational Continuum Mechanics, with emphasis on CFD. It implements physical models of fluid flow, structural analysis, heat and mass transfer using equation mimicking, with unstructured polyhedral mesh support and massive parallelism in domain decomposition mode. In order to use OpenFOAM in turbomachinery CFD, its “general purpose” capabilities are enhanced with turbo-specific features, related to physics of rotating regions and rotor-stator interfaces. Handling for geometric simplifications of multi-blade and multi-stage rotating machines are implemented, including simple stage interfaces, non-equal pitch of blade passages, pitch-wise cyclicity and mixing plane averaging. In this paper we describe the implementation of turbomachinery-specific features in OpenFOAM, in the spirit of object orientation and C++. Emphasis is given to the basic functionality of turbo tools, software layout in OpenFOAM, numerical formulation of stage interfaces and their place in overall code design. The paper is concluded with examples of turbomachinery simulations, illustrating the capability of turbo tools on industrial cases of incompressible and compressible turbomachinery flows.Copyright