This blog post explores the critical inputs of minimum size, maximum size, and growth rate in the Generate Surface Mesh task of the Watertight Geometry Workflow in Ansys Meshing, detailing their significance, how to choose appropriate values, and their impact on surface mesh quality for CFD analysis.
In this blog post, we will delve into the essential inputs of the ‘Generate Surface Mesh’ task within the Watertight Geometry Workflow in Ansys Meshing. Specifically, we will focus on the minimum size, maximum size, and growth rate parameters, discussing how to select appropriate values and their influence on the resulting surface mesh.
The ‘Generate Surface Mesh’ task is a pivotal step in the Watertight Geometry Workflow. This task is responsible for creating a mesh over the surfaces of an imported CAD geometry or re-meshing an existing surface mesh using predefined size fields or control files. Surface meshes are crucial as they define the computational region for Computational Fluid Dynamics (CFD) analysis and serve as the foundation for generating a high-quality volume mesh.
It is vital to not only generate a good quality surface mesh but also to refine it in key areas to accurately capture the flow physics. Unlike the ‘Add Local Sizing’ task, which allows for local control over the mesh on selected entities, the inputs under the ‘Generate Surface Mesh’ are applied globally to all surfaces, regardless of whether they are fluid or solid regions.
Before importing CAD models into the workflow, it is essential to clean, simplify, and remove unnecessary features. This practice helps reduce mesh generation time and optimizes computational resource usage. Additionally, this task aids in identifying regions in the imported CAD or mesh file that will later be utilized to create the volume mesh.
To begin, launch a fresh instance of Ansys Fluent Meshing in double precision, as this is the default setting. Select the “Watertight Geometry Workflow” from the drop-down menu. For demonstration purposes, we will use the solid CAD geometry of a stop valve with inlet and outlet elbows. We will keep the units at their default settings and import the geometry into the workflow without adding any local sizing tasks, focusing solely on the ‘Generate the Surface Mesh’ task.
The next critical inputs are the ‘Minimum Size’ and ‘Maximum Size’. These parameters define the minimum and maximum sizes of the elements for the surface mesh, specifically referring to the tangential sizes along the surface. By default, these values are pre-populated based on the size of the bounding box for the domain, which is often sufficient for generating a good surface mesh.
When selecting the minimum size, it is crucial to set it small enough to allow for resolution on smaller CAD features of interest. It is recommended that the global minimum size be at least 0.01 to avoid numerical issues during mesh generation. Proper scaling of the model by selecting the appropriate units during import is also essential.
The maximum size value is contingent upon the geometry being meshed. For internal flow problems, such as flow through the valve, it is advisable that the maximum size be less than 10% of the distance across the cross-section to ensure adequate flow resolution. This value may vary based on the specific application and desired accuracy.
To determine this distance, users can utilize the node selection filter to select two nodes and measure the distance using the Distance tool. For our example, we will use a maximum size value of 5mm based on the measured distance.
After setting the minimum and maximum sizes, we can proceed to generate the surface mesh. It is important to note that these size parameters are applied to the Curvature and Proximity size functions, which will be discussed in a future video. Users can adjust the minimum and maximum sizes to refine or coarsen the surface mesh, but they must monitor the quality of the surface mesh, which is reported in the Fluent Meshing Console after each mesh generation.
Next, we will explore the growth rate option, which defines the increase in element edge length with each successive layer of elements. This parameter dictates how the surface mesh elements expand along each layer. To illustrate this, we will create three surface meshes with growth rate values of 1.05, 1.2, and 1.3.
A lower growth rate value, closer to 1, results in finer mesh resolution further from the boundary, but this refinement can significantly increase computational costs. If a growth rate lower than 1.05 is desired, the use of a body of influence is recommended. Conversely, increasing the growth rate beyond 1.2 will coarsen the surface mesh and should be approached with caution, as values greater than 1.4 should be rarely used.
In summary, we have explored the importance of generating a high-quality surface mesh and its impact on the volume mesh. We discussed the minimum size, maximum size, and growth rate options, providing guidance on how to select these values effectively. Additionally, we demonstrated how to use size boxes to preview the relative minimum and maximum sizes in the graphics window. Understanding these parameters is crucial for achieving optimal results in CFD analysis.
