CFD users are increasingly finding that, having taken time to create a high-quality mesh, small geometry changes or deformations require a completely new mesh to be generated. This procedure is both costly and frustrating because, while the problematic regions of mesh are typically small, the time impact on the CFD process is large.

Using CFS's utilities, it is now possible to define small regions of mesh which are then extracted. These crystal-cut regions are then easily and quickly re-meshed, and then can be seamlessly re-inserted in the main mesh. This avoids the need for manual modularisation or fixed boundaries in the mesh, or for Chimera/ overlapping meshes.

The result: A re-meshing procedure which takes minutes, rather than hours or days. An operation previously impossible using existing tools can now become routine. Component changes can now be processed quickly and painlessly. Mesh quality issues arising from mesh deformation are easily resolved, and solution quality is improved. Overall, CFD pre-processing time is substantially reduced.

Re-meshing is a common but painful experience in the CFD process where complex geometries and larges meshes are involved: having spent a considerable amount of time generating a high-quality mesh, a small component change can force an complete regeneration of the mesh, wasting previous effort. Further, tools which alter both the geometry and mesh using Free Form Deformation (FFD) techniques are becoming more widespread. These tools can degrade the cell quality to an unacceptable standard - although the number of cells affected may be small, a complete re-mesh is often required.

Recognising that re-meshing small regions of mesh is both simple and fast, CFS's solution was to develop a mesh extraction and re-insertion capability in its GigaCell suite of programs. A user is able to identify a region of mesh, either by specifying geometric limits or by flagging specific cells. Two mesh files are then produced by the process: one contains the original mesh minus the specified cells, while the other contains only the extracted cells. The extracted cells form a perfect crystal cut - our tool guarrantees fidelity in the crystal-cut boundary, including across hybrid mesh (e.g. viscous layers), to enable regeneration of such hybrid cells (for example, stitching new viscous layer growth to quad boundaries of old layers).

This smaller mesh, containing only the extracted cells, can then be imported into any standard mesh generation program, the volumes discarded and a new volume mesh quickly built. If required, it is even possible to add new geometry into the mesh at this point. Finally, the crystal-cut boundaries not having been altered, this new mesh can be re-inserted in the larger mesh using the automatic stitch utility in GigaCell Synthesis. This ensures a perfect join while merging zone names and removing any temporary boundaries.

CFS's mesh extraction and re-insertion utilities are regularly used on client meshes in excess of 100M cells. Run times for both the extraction and re-insertion parts of the process are less that 5 minutes each, for meshes of this size due to CFS's highly optimised code and fast mesh I/O routines.

To demonstrate the capability of these tools in this case study, they have been used here to extract a region of mesh from a typical shrouded turbine blade, the total mesh size being around 14M cells. The blade and mesh had been modified using a FFD tool. The geometry is shown in Figure 1.

A small cuboid region of mesh was defined, which included not only a number of fluid cells but also cut the solid boundary of the geometry the leading edge of the blade and the shroud. This region was extracted using the CFS utility - run-time for this operation on a Pentium 4 2.5GHz was 20 seconds. Figure 2 shows the section of the solid boundary that was extracted (marked in red). Figures 3 and 4 show the crystal cut boundary of the extracted region. Figure 5 shows a view of the smaller extracted mesh, with the crystal cut fluid boundary coloured red and the cut solid boundary of the leading edge and shroud coloured blue.

A subsequent re-mesh of the extracted region was performed and this was re-inserted into the larger mesh using the stitch facility in CFS's tools. This stitch operation was completed in 15 seconds on the same computer.

CFS's mesh extraction and re-insertion tools significantly reduce the amount of time required to perform a re-meshing procedure. Component changes typically only affect a small number of cells but this has previously required an entire new mesh to be generated, a costly procedure that can represent a serious bottleneck in the CFD process.

Our tools offer the capability to specify and extract smaller regions of mesh, which (independent of any particular pre-defined zones) can then be easily and quickly re-meshed using standard mesh generation packages. These re-meshed regions are then re-inserted in the mesh using a fast stitching procedure.

In this way, re-meshing can take minutes, rather than hours or days, paving the way for a rapid turnaround of geometry changes and the use of FFD, even for large meshes.