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As marine geometries become more advanced, the complexity associated with mesh generation increases. Meshing complexity can be linked to multiple factors such as element type, element structure, geometry, topology, user expertise, application, and choice of meshing algorithm. As the needs of engineers have advanced, commercial meshing software has had to handle increasingly complex meshing configurations. The Cadence Fidelity CFD platform offers various meshing technologies for leading or blunt edges, free surfaces, boundary layers, viscous layers, and more. This blog post gives an overview of a few meshing strategies to ease mesh generation for complex marine geometries.
Volume to surface
Volume to surface (V2S) is a robust and parallelized meshing approach that works on complex geometries. It supports unclean geometries such as those with intersecting or non-conformal surfaces and does not require prior surface meshing. Cadence V2S meshing technology can produce both full hex and hex-dominant unstructured meshes. Full hex meshes use hanging nodes to maintain a consistent hexahedral structure, while hex-dominant meshes use tetrahedra to connect the hex sections of different sizes without introducing hanging nodes.
V2S Full Hex Mesh.
Surface to volume
The surface-to-volume (S2V) mesher is a fault-tolerant mesh generator for high-quality surface grids and viscous layers. Hence, it requires a relatively clean geometry. It generates unstructured quad-dominant meshes on surfaces and either fully tetrahedral or hex-dominant volume meshes.
S2V Hex-core mesh.
Both meshing approaches are solver agnostic. Moreover, the Cadence Fidelity platform offers dedicated mesh quality optimizers that can tune the mesh for specific solvers.
The optional surface and local refinement capabilities can increase the resolution of the mesh in target areas. Mesh uniformity, edge proximity, and local curvature are all factors that determine whether the surface mesh is further refined.
Refining each surface and checking the proximity between the surface edges can be tedious when working with complex geometries with multiple surfaces. In such cases, the global settings help refine the entire geometry. The geometry of the ship superstructure illustrated below is meshed on the Fidelity CFD platform using the V2S approach. Only global parameters are used, and no granularity is set. However, opting for surface mesh refinements in regions where the physics is crucial for the design is recommended.
Global settings were used for refining all surfaces of the ferry.
Volume refinement can be useful for marine application design studies, especially when refinement is targeted along certain volumes, such as the free surface, the actuator disk region, etc. A minimum cell number should be maintained when dealing with closed surfaces. The user can define proximity between surfaces for additional volume refinement.
When modeling the wakes produced by propellers, a refinement zone is created around the actuator, which is represented by a rectangular box, as shown in the image below. The area beyond the rectangular region is the free surface which is also refined to a certain extent but with a different refinement level.
The refinement zone is represented by a rectangular box around the propeller (top), and mesh is generated in and around the refinement zone (bottom).
There are three techniques for viscous layer insertion when generating a mesh in the Fidelity CFD platform, as follows:
1. Inflation: This technique pushes the Euler cells away from the solid boundary for viscous layer insertion. The inserted viscous layers maintain a good adjacent volume ratio. However, the layers are irregular and might not adhere to the user-defined height.
Viscous layer insertion using Inflation.
2. Extrusion: Here, the Euler cells close to the solid boundary are trimmed to insert the viscous layers and are reconnected to the far field. This technique produces viscous layers of ideal height and a perfect far-field mesh. However, the reconnection layer is often comprised of various cell types, and the first viscous layers start off fully normal to the walls.
Viscous layer insertion using Extrusion.
3. Splitting: Here, the viscous layers are inserted very fast and are done by splitting the first buffer cells. Therefore, this method is used only for extremely complex geometries. This method does not guarantee that the adjacent volume ratio will be maintained.
Viscous layer insertion using Splitting.
Despite the limitations in the viscous layer insertion techniques, 100% coverage of viscous layers around the complete geometry is assured.
V2S meshing approach is largely used for the anisotropic refinement of the free surface or the atmospheric boundary layer. The refinement is carried out at the end of the mesh generation process for an anisotropic far-field mesh.
S2V meshing produces anisotropic surface meshes for leading and blunt edges and generates high-quality viscous layers. This meshing approach can drastically reduce the cell count (compared to isotropic cells) while preserving surface capture. Moreover, the structured mesh produced using S2V is ideal for predicting cavitation. The extrusion technique in S2V ensures that the viscous layers have a smooth distribution, achieve maximum height, and guarantee that the first viscous layer is fully normal to the wall.
Structured mesh generated on the blunt edge of a hydrofoil using the S2V meshing approach.
When designing new products, it's difficult to predict the flow field beforehand and determine where mesh cells are necessary. Therefore, over-resolution is substantial to obtain a quality mesh without prior knowledge of the flow, often resulting in significant computational and memory overhead.
To overcome this, the adaptive mesh refinement (AMR) adjusts the mesh during simulations, ensuring mesh cells are only utilized where needed. This method captures all relevant physics with only a fraction of the cells. As a result, simulations can be completed 2X faster or more, especially for unsteady flows. The adaptive mesh refinement technology in the Fidelity Fine Marine flow solver can address the various unprecedented conditions and dynamically adapt the mesh to solve the involved physics.
High-speed boat free surface deformation.
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 video ‘Why Meshing Complex Marine Geometries Has Never Been So Easy!’