EXCELLERAT: Enabling parallel mesh adaptation with Treeadapt
A Use Case by 
Short description
CFD remains highly dependent on mesh quality. Advanced meshing software is generally limited to sequential or shared memory architectures. Therefore, meshing requires tens of hours to generate highly complex grids and it is highly dependent on user experience. Refinement zones are also bounded by standard geometrical shapes. To bypass these bottlenecks, codes have turned to mesh adaptation as a solution but massively parallel mesh adaptation workflows remain scarce and require efficient load balancing, interpolation and remeshing techniques.
Results & Achievements
So far, simulations using AVBP relied on static commercial software generated meshes. This bounded the quality of results to the experience and insight of the user performing the simulation. Furthermore, these mesh generation tools are mostly non-parallel and require days to mesh complex cases.
Recent work on efficient load balancing within the EPEEC project allowed CERFACS to use the open source library Treepart to create a new tool/library called Treeadapt that allows massively parallel mesh adaptation. Treeadapt uses its mesh partitioning and load balancing algorithms based on the ZOLTAN library to efficiently decompose the domains taking into account the architecture intricacies of the system it is running on. These load balancing algorithms coupled with MMG allow faster and more efficient remeshing.
Then, Treeadapt has been used to improve the simulation of a 42 injector rocket engine demonstrator simulation called BKD in the PRACE project: the prediction of combustion instabilities in liquid rocket engines (RockDyn). This simulation in collaboration with EM2C Centrale Supelec (T. Schmitt) reaching one Billion tetrahedras in less than 30 minutes, using 4,096 AMD Epyc 2 cores (compared to 70 hours with a standard meshing tool).
Objectives
Building on previous experience with the INRIA MMG library and a partnership with the EPEEC project during which an efficient mesh partitioning and load balancing library called Treepart was developed, EXCELLERAT is looking to develop a new application/library called Treeadapt for massively parallel mesh adaptation. Currently operating in full tetrahedral grids, Treeadapt generates a first partitioned domain where MMG can be called independently while freezing the parallel interfaces. A rebalancing and adaptation iterative process then occurs until the whole domain is within a user provided tolerance with regards to the error estimator (example: gradient of density, Hessian of the velocity).
