An overset mesh combines two different meshing forms in a CFD simulation.
Each mesh type is applied in different regions of the simulation such that the meshes overlap.
An overset mesh is finished by enforcing continuity across the transition region between each mesh in the system.
Selecting a mesh style and grid style that conforms to the entirety of your system in a CFD simulation involves tradeoffs. When looking at images of meshes, it sometimes seems like you only have the freedom to choose between structured or unstructured meshes throughout the entire system to find the best balance of computation time and accuracy. Fortunately, you can get the best of both worlds when you use an overset mesh to define shapes and surfaces used in your CFD simulation.
An overset mesh provides a few important advantages in certain geometries that are found in many real systems. This meshing style can also be used in plane cut cross-sections along the body of a complex system so that a more complex mesh can be targeted to specific regions with greater curvature and complexity. In this article, we’ll look more at overset mesh styles and how an overset mesh is created for a CFD simulation.
What Is an Overset Mesh?
All meshes are used to approximate flow paths between points in a standard 2D or 3D grid in a CFD simulation system. An overset mesh is constructed from multiple grids in a complex system and it may resemble a typical hybrid mesh. Together, the grids form a mesh that plays a similar role as adaptive meshing; high node density is applied in regions with high curvature (and thus high gradient), while low node density is applied everywhere else.
An overset mesh is normally applied to describe flows with a complex shape embedded in a background with a very simple structure. An overset mesh is generated using two types of overlapping grids:
On-body grid: This grid is applied around the exterior of the more complex object and is used to capture the shape of the object as accurately as possible. This grid could be structured or unstructured, with higher node density near curved surfaces to capture higher flow gradients.
Off-body grid: This grid is also called the background grid. This grid would exist without any other objects that have significant structural complexity or high curvature, and this grid defines flow characteristics in all other regions away from the on-body grid. This grid is normally a structured linear grid.
Together, these two types of grids and the resulting mesh can capture varying levels of detail within the simulation system and ensure high accuracy in the simulation results.
To best understand an overset mesh, it helps to look at an example. The image below shows an overset mesh example with an airfoil and a coarse background grid, and the grid types are overlapping around the outside of the airfoil. Together, these grids are combined to form an overset mesh for the system.
This image shows two pairs of overlapping unstructured grids around an airfoil. The central circular grid is the on-body grid, while the coarser grid is the background grid. Together, these can be used to generate an overset mesh.
The above example is typical in CFD simulations involving overset meshing, where two structured meshes with different connection styles are applied in different regions of the system. We can see in the image that a structured grid is defined in different regions with different curvatures; a simple linear mesh is often enforced in the background, as it has zero curvature, although the background grid could be unstructured as shown above. The curved regions could have a structured or unstructured grid with high density in regions of high curvature in order to capture the surface features of the object. Although the above example shows a mesh conforming to a circular cross section, the same type of mesh could be developed in 3D for a spherical object.
When to Use an Overset Mesh
The typical application of an overset mesh is to capture curvature of one or more complex bodies, where each body has its own on-body grid embedded in the off-body grid. Once the grids are applied to each section of the system, the grids are fused in their transition to ensure continuity in the solution to the main CFD equations.
From looking at the above image, the advantages of an overset mesh in a CFD simulation should be clear:
- High resolution in critical regions: The flow gradient in critical regions of the design, which tends to correspond to complex shapes or high curvature, can be more accurately modeled using an overset mesh.
- Lower computation time where it matters: Similar to a hybrid unstructured grid, the accuracy can be increased in critical regions without major increases in computation for the entire simulation.
Once the grid is created, it will be used to generate the overset mesh, and finally to develop a numerical scheme to solve the full Navier-Stokes equations or a reduced flow model. Most CFD simulation platforms have trouble generating an overset mesh automatically and may require an additional 3rd party program to visualize the generated mesh. Newer, more sophisticated CFD simulation applications are incorporating more advanced overset mesh tools that can expedite and visualize the generated mesh and immediately use it to build a CFD model for simulations.
The most advanced CFD simulation suites allow you to implement an overset mesh with ease. When you need to investigate flow behavior in your design, use the best meshing utilities in Pointwise and the complete set of fluid dynamics analysis and simulation tools in Omnis 3D Solver from Cadence. These two applications give systems designers everything they need to build and run CFD simulations with modern numerical approaches.