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Ship designers and naval architects increasingly use computational fluid dynamics (CFD) tools for more accurate solutions, detailed physics, and quicker results. Marine ship design studies in the past relied mainly on scaled-down models in towing tanks for insights into ship resistance, seakeeping, propulsion, and maneuvering. However, these models had discrepancies in their Reynolds and Froude numbers due to scaled-down models. With the help of CFD simulation technology, it is now possible to test full-size models. But the mesh generation step in CFD often requires expertise and experience, especially when dealing with complex geometries. Such complex geometries may have cavities, overlapping surfaces, extrusions, and saddle corners that impede quality meshing and precise CFD solutions. For easier meshing, the Fidelity CFD platform has various geometry cleaning tools and features to help streamline mesh generation and enhance overall marine CFD simulation.
The first step in a CFD simulation is to create or import a geometry to be tested. Multiple file formats can be imported into the Fidelity CFD platform, and a list of the acceptable file formats is tabulated below:
Geometry can be imported in parallel from both local and remote machines. As seen in the image below, a CAD tree can simultaneously import 12 parts of a car geometry on a 12-core computing system.
A DrivAer configuration is imported in parallel on 12 cores.
Once the Fidelity CFD platform imports the geometry, a thorough analysis is conducted to find free edges, non-manifold edges, and feature curves. After tessellation, the triangle aspect ratios and distance from the original CAD are calculated. It provides tools for cleaning the geometry, including ensuring geometry conformality, improving tessellation, and capping holes.
Stitching or sewing ensures geometric conformity (left), while retriangulation improves tessellation (right).
The Fidelity CFD platform includes AutoSeal, an advanced geometry cleaning tool that utilizes parallel algorithms to generate closing surfaces on CAD and STL formats. It can handle dirty geometries with holes, gaps, and cavities. The processing time varies from a few seconds to minutes for standard geometries and up to an hour for complex ones, such as a complete car. It doesn't modify the initial geometry; instead, it produces a set of triangulated surfaces that make the geometry watertight, ensuring it is fit for volume-to-surface (V2S) meshing.
To make a geometry water-tight and ready for mesh generation, the user only needs to provide the internal and external points. AutoSeal will take care of the rest. In the example of the ferry image below, five internal points and one external point were defined. Using this input, Autoseal created over 250 surfaces in under 90 seconds, successfully covering all the holes in the geometry. The resulting geometry is now fully-sealed and ready for mesh generation.
Autoseal was able to quickly generate over 259 surfaces in just 80 seconds based on the user's input of 5 inside points and 1 outside point.
When a user plans to mesh a geometry using the surface-to-volume (S2V) approach, it is crucial to ensure that the geometry is clean and has superior conformality in comparison to the geometry utilized for volume-to-surface (V2S) meshing. In such cases, the wrapping tool can quickly generate a clean, tessellated surface, enabling the speedy generation of an S2V mesh.
The group surfaces tool in the Fidelity CFD platform scans the geometry and automatically categorizes it into groups based on surface tangency and/or fillet radius. The container ship hull shown below originally had every surface in a single group with no useful organizational information. After automatic regrouping, the geometry was assembled into six different groups, allowing users to work with individual parts of the geometry as required.
Initially, all the surfaces of the container ship hull were grouped together (left). After undergoing automatic regrouping, the geometry surfaces were divided into 6 distinct groups (right).
Sharp corners can be problematic for meshing, and the automatic defeaturing tool in the Fidelity CFD platform is ideal for handling this issue. The tool can identify and remove sharp corners or angles on surfaces. By locally merging triangles with neighboring surfaces, the sharp corners are defeatured. As a result, the mesh quality is improved.
Sharp corner is defeatured and merged with the neighboring surfaces for easy meshing.
When working with boats, you may have noticed that the sharp edges of the hull can be challenging while meshing. To address this, you can utilize the defeature sharp angles tool. This tool allows you to set a threshold for the angle of sharp edges, which will then be merged with the adjacent surfaces for easier meshing.
The saddle corners tool can detect saddle-type corners and generate spheres of customized diameters at those locations. Saddle-type corners are vertices where multiple surfaces meet and form successive convex and concave angles. In other words, saddle corners are attached to four or more edges, and these surfaces have their normals pointing in different directions. Placing a sphere at these corners can prevent mesh generation at these locations and improve mesh quality. This tool is particularly beneficial for V2S meshing.
A sphere is placed at a saddle corner to prevent mesh generation at that location.
To learn more about the different tools and features on the Fidelity CFD platform for geometry cleaning and easier meshing for marine CFD, watch the CadenceTECHTALK on ‘Why Meshing Complex Marine Geometries Has Never Been So Easy!’