TransAT uses three different strategies for meshing:
- Cartesian single and multiblock,
- Body-fitted coordinates (BFC), also single and multiblock,
- and the coupled Immersed Surfaces Technology with Block mesh refinement (IST/BMR).
This novel strategy -unique worldwide- avoids to have to deal with complex geometries using unstructured and BFC grids.
Briefly, it has the following advantages:
- Reduces grid generation from days/weeks to a couple of hours
- No need to change discretization
- Suitable for rigid body motion
- Suitable for conjugate heat transfer
- Local Defect Correction (LDC) – fully conservative
- Easy to handle detailed complex geometries
- Automated refinement per block
- Scalable parallel multi-block method
- Save up to 70% cells in 3D
Immersed Surface Technique (IST)
The Immersed Surfaces Technology (IST) has been developed by ASCOMP GmbH. The idea is to represent solid walls by a Level Set function representing the exact distance to the surface, which is zero at the surface, positive in the fluid(s) and negative in the solid. The fluid(s) and the solid have their own material properties, based on the Level Set function: density, heat capacity and thermal conductivity,
The technique has the major advantage to solve conjugate heat transfer problems, in that conduction inside the body is directly linked to external fluid convection. In practice, the CAD file of the solid is first immersed into a cubical grid covered by a Cartesian mesh. The Navier-stokes equations are modified to acount for the presence of the solid level set function. The treatment of viscous shear at the solid surfaces is handled very much the same way as in conventional CFD codes, where the wall normal vector needed to estimate the wall shear is obtained using
Block-based Mesh Refinement (BMR)
The BMR technique was developed in the TransAT code to help better solve the boundary layer zone when use is made of the IST technique discussed above. In BMR, more refined sub-blocks are automatically generated around solid surfaces; with dimensions made dependent on the Reynolds number (the sub-block scale should always be set such that it covers the boundary layer thickness). An unlimited number of sub-blocks of different refinement can be generated, with connectivity between the blocks matching up to 1-to-8 cells. This method can save up to 75% grid cells in 3D, because it prevents clustering structured grids where unnecessary.
Adaptive Mesh Refinement (AMR)
Adaptive mesh refinement (AMR) as used in TransAT is a method of adaptive meshing around evolving fluid-fluid interfaces. The method is based on discretizing the continuous domain of interest into a grid of many individual elements of smaller size, automatically clustered around moving interfaces. The grid is dynamic in that it tracks the resulting interface topology as the computation progresses.
The general advantages of dynamic gridding schemes are: increased computational savings, Increased storage savings, and complete control of grid resolution, compared to a static grid approach. In TransAT, AMR for interface tracking returns a better resolution for interface topology, for curvature determination, for contact angle treatment, and for mass conservation within the level set approach.