Overset mesh (CFD) vs. Generic local remeshing

Overset mesh is a mesh method used majorly in CFD, the concept is turning a complex geometry into decomposed geometrically simple overlapping grids, where boundary data is exchanged between meshes through mapping intepolation.

In my view, overset mesh is a subset of generic local remesh. Some CFD packages have done some good job(as to May 2015) , but something can be done better for robustness and handling current limitations.

Figure 1: CFD overset mesh

How it works

- Active cells

- Passcive cells

- Acceptor and Donor cells at overset boundary and background mesh

Workflow

- Two regions and corrsponding mesh

- Overset mesh interface

- Moved overlapping zone

- Data mapping

Tips

- keep cell size and size gradient

- min 4 cell layers between bodies and overset boundaries

- 8 cells between wall boundaries in multiple overlkapping regions

- mapping (linear interpolation )

Limitation

- CD v8.06

- No strict mass conservation at interface

Benifits

- Do impossible works with very complicated geometry in past

- Parametric studies

- Moving parts

+ lifeboat, relative motion

+ store separation

+ 6DOF motion

+ DFBI( dynamic fluid body interaction)

- Process automation

- Constraint motion

Knowning issues

- Multiple overset grids and overlapping each other

- Avoid failures due to inadaquate grids in overlapping zone

- Sizing

- Cell layers in gaps (can'tr allow zero layer?)

- Motion over large distance (refining, coarzening)

Discussions

- Regular discretized equation are solved in active cells only

- Donor and acceptor cells : interface cells on two sides of the interface

- 2D tri and 3D tet interpolation elements are used

- change of cell status is controlled by solver

- Allow each grid deform in FSI cases.

- Allow grid fall out of solution domain

- overset grid can't move more than one cell per time step in overlapping zone

Other altenative approaches ( you find the Pros/cons)

- Sliding grids

- Morphing mesh

What I can do more through generic local remeshing?

- Adptive layers of body regions

- Allow buffer regions for mesh transiting

- Absolute confirming mesh for mass conservation

- Allow large motion

- Structure analysis

Figure 2: Active sells and passitive cells

Figure 3: Pressure contour (check out imperfection across the boundary)

Again more discussions

- Unstructured mesh (limitations ... boundary layers, accurancy,

-OGA(overset grid assembly)

- DCI( domain coonectivity information)

- vs. Multi-block connected mesh

Refered CFD packages

- NASA pegasus (Suhs 2002)

- CD adapco (2015)

- Chalmesh (peterson 1999)

- Suggar++

Videos

- CCM+ youtube (v9.020

- Overset Mesh - The Acrobatic Duck

- Zero overlap technology

References:

(1) best practice workshop

(2) Star south east Asian conference 2013

(3) Overset Mesh with Zero Gap Interface (2014)

(4) Easy Meshing with Structured Accuracy