A good mesh is essential for accurate and dependable results. Autodesk Simulation CFD, formally known as CFdesign, has a long tradition of innovative meshing technologies including Mesh Enhancement, Automatic Mesh Sizing, and Automatic Refinement. In this version, we continue this tradition with the addition of Adaptive Meshing, Interactive Mesh History, and the introduction of High Performance Meshing.
Since its introduction, Automatic Mesh Sizing has greatly simplified the modeling process. Automatic Mesh Sizing defines a mesh that is optimized for the model and accurately represents every detail of the geometry.
A good representation of the geometry is only one requirement for a high fidelity solution. For example, the flow on a simple model with a uniform mesh can contain significant gradients. The results on the coarse mesh may not be highly accurate, but they do indicate flow trends. These trends can be used to identify where the elements should be concentrated to improve solution accuracy.
Mesh Adaptation uses solution results to progressively improve the mesh definition. The simulation is run several times. Each time the results in the previous cycle are used to improve the mesh in the next cycle. The result is a mesh that is optimized for the particular simulation. The mesh is finer for high gradient regions, and coarser elsewhere.
There are several regions in the valve which are not closely bounded by geometry. We can assume there will be flow activity, however, due to the momentum of the water. We will see that the flow recirculates, accelerates, and changes direction abruptly. Because the geometry does not have a great deal of curvature, the mesh may not capture the flow as well as we would like.
In the first meshing cycle, Automatic Mesh Sizing defines a default mesh distribution based only on geometric curvature. Note that the mesh is coarse upstream and downstream of the poppet where there is little geometric influence:
The resultant flow appears jagged because of the sparse mesh distribution. The flow upstream of the poppet is not well defined, and the jet through the annular throat region appears to disperse immediately downstream of the poppet before reattaching to the outlet wall:
When you define a mesh with Automatic Mesh Sizing, you define a process that the Mesher uses to generate the mesh. Unlike materials and boundary conditions, a mesh definition is a set of commands issued in a specific order. When you change the order of these commands, you often change the resultant mesh.
The Mesh Size branch of the Design Study bar lists every step in the mesh definition history. This includes when Automatic Sizing is invoked, size adjustments, and when Spread Changes is applied. Each step is listed as a separate branch, and can be modified, disabled, or deleted from the mesh definition.
All steps in the history constitute the mesh definition. Certain mesh definition commands prompt question dialogs regarding the re-institution of existing adjustment settings or the removal of existing mesh sizes. The responses to these questions affect the current mesh definition, but they do not remove earlier steps in the history. New steps are added to the definition may change the distribution and may even "undo" the effects of earlier steps.
To improve meshing performance, the Autodesk Simulation CFD mesher leverages multiple computing cores. This reduces the amount of time required to generate large meshes, and better utilizes high performance computing hardware.