What are the benefits and challenges of using overset grids in CFD for complex naval geometries?

1 What are overset grids?

Overset grids are a technique for generating and coupling multiple grids or meshes that overlap each other in a domain. Each grid can have different resolution, topology, and boundary conditions, and can move independently of the others. The overlapping regions are handled by interpolation methods that exchange information between the grids and ensure continuity and conservation of the flow variables. Overset grids can be used with any type of grid, such as structured, unstructured, or hybrid, and with any type of solver, such as finite volume, finite element, or finite difference.

2 Why use overset grids for naval geometries?

Overset grids offer several advantages for simulating naval geometries, especially those that involve complex shapes, relative motions, and large deformations. These advantages include increased flexibility and control over grid quality and resolution, reduced computational cost and time due to the elimination of re-meshing or grid adaptation for changing geometries, simplified grid generation process avoiding difficulties of creating conformal grids for complex or moving geometries, and the ability to use different solvers or models for different grids.

3 What are the challenges of using overset grids for naval geometries?

Overset grids, although useful for naval geometries, can present some difficulties and limitations. For instance, additional steps and tools are necessary to generate, couple, and interpolate the grids, which can complicate the simulation workflow. Errors and uncertainties may arise in the overlapping regions due to interpolation methods that could cause numerical diffusion, discontinuities or inconsistencies in the flow variables. Furthermore, the accuracy and stability of the solution can be affected by the grid spacing, alignment, and orientation in the overlapping regions. Lastly, some numerical schemes or physical phenomena may not be compatible with overset grids or require special treatment or modification.

4 How to use overset grids for naval geometries?

Using overset grids for naval geometries requires following certain steps and guidelines to ensure a successful and reliable simulation. To begin with, it is important to select a suitable grid generation tool, such as Pointwise, GridPro or Hexpress, which can create and modify grids for the geometry and application. Additionally, a suitable solver must be chosen that can handle and couple overset grids for the flow problem; some examples are OVERFLOW, CFD++ or STAR-CCM+. Similarly, an interpolation method must be selected that can exchange information between the grids, while ensuring continuity and conservation of the flow variables; some examples are donor-cell, least-squares or implicit hole-cutting. Furthermore, a suitable grid resolution and overlap ratio should be chosen to balance accuracy and efficiency of the solution; generally, 10-20% overlap between the grids with at least 3-5 grid points in the interpolation stencil is recommended. Finally, a suitable grid motion and deformation method should be chosen that can update the grid positions and connectivity when the geometry changes or moves; some examples are rigid-body motion, spring analogy or mesh morphing.

5 What are some examples of using overset grids for naval geometries?

Overset grids have been used for various naval geometry applications, such as simulating the flow around ships with appendages, submarines with control surfaces, propellers with complex blade shapes, and hydrofoils with large deformations. These simulations can take into account different resolution, motion, or loading than the main hull, body, fluid, or supporting structure.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?