Rao V. Garimella

Boundary layer mesh generation for viscous flow simulations

Viscous flow problems exhibit boundary layers and free shear layers in which the solution gradients, normal and tangential to the flow, differ by orders of magnitude. The generalized advancing layers method is presented here as a method of generating meshes suitable for capturing such flows. The method includes several new technical advances allowing it to mesh complex geometric domains that cannot be handled by other techniques. It is currently being used for simulations in the automotive industry. Copyright

A comparative study of interface reconstruction methods for multi-material ALE simulations

In this paper we compare the performance of different methods for reconstructing interfaces in multi-material compressible flow simulations. The methods compared are a material-order-dependent Volume-of-Fluid (VOF) method, a material-order-independent VOF method based on power diagram partitioning of cells and the Moment-of-Fluid method (MOF). We demonstrate that the MOF method provides the most accurate tracking of interfaces, followed by the VOF method with the right material ordering. The material-order-independent VOF method performs somewhat worse than the above two while the solutions with VOF using the wrong material order are considerably worse.

Polygonal surface mesh optimization

A procedure has been developed to improve polygonal surface mesh quality while maintaining the essential characteristics of the discrete surface. The surface characteristics are preserved by repositioning mesh vertices so that they remain on the original discrete surface. The repositioning is performed in a series of triangular-facet-based local parametric spaces. The movement of the mesh vertices is driven by a nonlinear numerical optimization process. Two optimization approaches are described, one which improves the quality of elements as much as possible and the other which improves element quality but also keeps the new mesh as close as possible to the original mesh.

A second-order accurate material-order-independent interface reconstruction technique for multi-material flow simulations

A new, second-order accurate, volume conservative, material-order-independent interface reconstruction method for multi-material flow simulations is presented. First, materials are located in multi-material computational cells using a piecewise linear reconstruction of the volume fraction function. These material locator points are then used as generators to reconstruct the interface with a weighted Voronoi diagram that matches the volume fractions. The interfaces are then improved by minimizing an objective function that smoothes interface normals while enforcing convexity and volume constraints for the pure material subcells. Convergence tests are shown demonstrating second-order accuracy. Static and dynamic examples are shown illustrating the superior performance of the method over existing material-order-dependent methods.

Interface Reconstruction in Multi-fluid, Multi-phase Flow Simulations

An advanced Volume-of-Fluid or VOF procedure for locally conservative reconstruction of multi-material interfaces based on volume fraction information in cells of an unstructured mesh is presented in this paper. The procedure employs improved neighbor definitions and topological consistency checks of the interface for computing a more accurate interface approximation. Comparison with previously published results for test problems involving severe deformation of the materials (such as vortex-in-a-box problem) show that this procedure produces more accurate results and reduces the “numerical surface tension” typically seen in VOF methods.

Polyhedral Mesh Generation and Optimization for Non-manifold Domains

We present a preliminary method to generate polyhedral meshes of general non-manifold domains. The method is based on computing the dual of a general tetrahedral mesh. The resulting mesh respects the topology of the domain to the same extent as the input mesh. If the input tetrahedral mesh is Delaunay and well-centered, the resulting mesh is a Voronoi mesh with planar faces. For general tetrahedral meshes, the resulting mesh is a polyhedral mesh with straight edges but possibly curved faces. The initial mesh generation phase is followed by a mesh untangling and quality improvement technique.We demonstrate the technique on some simple to moderately complex domains.

Conformal Refinement of Unstructured Quadrilateral Meshes

A multilevel adaptive refinement technique is presented for unstructured quadrilateral meshes in which the mesh is kept conformal at all times. This means that the refined mesh, like the original, is formed of only quadrilateral elements that intersect strictly along edges or at vertices, i.e., vertices of one quadrilateral element do not lie in an edge of another quadrilateral. Elements are refined using templates based on 1:3 refinement of edges. It is demonstrated that by careful design of the refinement and coarsening strategy, high quality elements can be maintained in the refined mesh. The method is demonstrated on a number of examples with dynamically changing refinement regions.

Efficient Algorithm for Local-Bound-Preserving Remapping in ALE Methods

The remapping algorithm is an essential part of the ALE (Arbitrary Lagrangian-Eulerian) method. In this talk we present such an algorithm based on linear function reconstruction, approximate integration and mass redistribution.

An intermediate-scale model for thermal hydrology in low-relief permafrost-affected landscapes

Integrated surface/subsurface models for simulating the thermal hydrology of permafrost-affected regions in a warming climate have recently become available, but computational demands of those new process-rich simu- lation tools have thus far limited their applications to one-dimensional or small two-dimensional simulations. We present a mixed-dimensional model structure for efficiently simulating surface/subsurface thermal hydrology in low-relief permafrost regions at watershed scales. The approach replaces a full three-dimensional system with a two-dimensional overland thermal hydrology system and a family of one-dimensional vertical columns, where each column represents a fully coupled surface/subsurface thermal hydrology system without lateral flow. The system is then operator split, sequentially updating the overland flow system without sources and the one-dimensional columns without lateral flows. We show that the app- roach is highly scalable, supports subcycling of different processes, and compares well with the corresponding fully three-dimensional representation at significantly less computational cost. Those advances enable recently developed representations of freezing soil physics to be coupled with thermal overland flow and surface energy balance at scales of 100s of meters. Although developed and demonstrated for permafrost thermal hydrology, the mixed-dimensional model structure is applicable to integrated surface/subsurface thermal hydrology in general.

Guest editorial: 17th International Meshing Roundtable Special Issue

The committee of the 17th International Meshing Roundtable is pleased to publish this special edition of Engineering with Computers based on papers presented at the conference in October, 2008. The conference was one of the best attended with over 50 papers submitted and 36 papers accepted. In addition, the conference attracted 12 research notes. Of the 36 papers, the committee invited 10 authors with the best reviews to submit their work for this special issue. Mesh generation as field has matured considerably over the last decade or two and researchers are beginning to address the more challenging problems that remain in mesh generation. This was reflected amply in the conference which had cutting edge topics such as real-time tessellation of NURBS surfaces, GPU assisted surface reconstruction and meshing for biological models and granular processes. At the same time, the papers reflected the continuing interest in and challenge of automatically generating hexahedral meshes. Finally, the conference saw evidence of ample research continuing into mesh optimization often based on solution metrics. The papers in this issue reflect the complexity and diversity of mesh generation and we hope they will educate and inform the reader about the latest advances in this field.