Adaptive Mesh Refinement

Adaptive Mesh Refinement (AMR) is a dynamic method that refines or coarsens cells based on adaptive mesh criteria as they query the flow solution. Solution quantities are automatically interpolated to the adapted mesh.

Adaptive Mesh Refinement (AMR) is available as a model for a physics continuum. This location reflects the fact that the refinement takes place during the simulation run and is an integral part of the solution process.

AMR allows you to run simulations using coarser meshes yet capturing fine flow features. The initial mesh must still capture the basic flow characteristics. In addition, as the Adaptive Mesh solver only executes midpoint subdivision for volume cells, it cannot re-construct the surface mesh. The surface tessellation must be sufficiently fine for curved surfaces in the initial mesh. AMR improves the volume mesh's ability to capture the desired flow features, but it does not eliminate the need for careful mesh construction.

AMR cannot generate a coarser mesh than the one you start with. You can set a limit on the maximum number of refinements applied to any cell so that you avoid a mesh that is too fine for practical purposes.

Depending on the Prism Cell Refinement option you set, AMR can refine prism layer cells isotropically or anisotropically, or even implement an user-defined scheme for each wall boundary.

The image below demonstrates an AMR simulation where the initial mesh is refined based on user-defined criteria in the form of a field function.

AMR refines cells using midpoint subdivision. This method forms one child cell around each vertex of the parent cell. After executing one refinement, the number of the subdivided child cells is equal to the number of vertices in the parent cell.

Every time the Adaptive Mesh solver identifies a cell for adaption, the refinement level increases or decreases by 1. Only one change of refinement level is applied to any cell when the Adaptive Mesh solver runs. To achieve multiple changes of refinement level, the Adaptive Mesh solver must run for multiple times and successively increase or decrease the refinement level for a cell. When the Adaptive Mesh solver reduces the refinement level, it combines previously subdivided cells into one larger cell. The Adaptive Mesh solver does not combine cells beyond their definition in the starting mesh. The refinement level of two adjacent cells can only differ by one level.

You trigger the Adaptive Mesh solver by specifying the Trigger Mode and the corresponding child properties. In addition, you can choose to apply mesh adaption during solution initialization. See Refine in Solution Initialization. You can also trigger adaption manually when the simulation is paused.

If several adaptive mesh criteria are active at the same time, their relative influence on the adaptive mesh refinement is prioritized as follows:

Refine and Coarsen → Refine
Refine and Keep → Refine
Keep and Coarsen → Keep

By this approach a finer mesh takes precedence over a coarse mesh.

In a simulation applying the adaptive mesh method, the following sequence of steps are executed in the background:

The Adaptive Mesh solver:

Calculates the values of the adaptive mesh criteria for each cell based on the current flow solution.
Combines all the criteria to derive the final adaption request with the priorities described above.
Resets all the interfaces if applicable. For details refer to Intersecting an Interface.
Carries out the mesh adaption—subdividing the cells once per AMR trigger or keeping/coarsening the cells according to the final adaption request.
Interpolates the flow solution to the adapted mesh.
Re-initializes the interfaces.
Re-partitions the domain due to the change in the cell connectivity induced by AMR. The re-partitioning is carried out by the Partitioning solver. For details refer to 网格单元连接变化 and 分区求解器.

注	To go back to the initial mesh, select Reset Mesh and Adaption from Clear Solution. See also: 清除求解.

The Adaptive Mesh model is compatible with co-simulation models (see Co-Simulation).

AMR in Conjunction with Remesh

The Remesh solver controls the Adaptive Mesh solver to preserve current mesh granularity. After remesh is triggered, the Adaptive Mesh solver refines the remeshed mesh such that the same cell sizes as the old mesh are achieved at the corresponding locations. The Remeshing solver:

Makes a copy of the current mesh with associated refinement level and solution.
Triggers a remesh—executes the specified Mesh Operations to obtain a new mesh.
Triggers the AMR solver to refine the new mesh based on the cell sizes and the refinement level saved in the copy.
Maps the solution to the new refined mesh.

AMR Adaptive Mesh Criteria

Simcenter STAR-CCM+ supports two types of adaptive mesh criteria:

User-Defined Mesh Adaption gives you full control of the AMR at the cell level. You configure it using a field function or a table, which prescribes how the Adaptive Mesh solver drives the mesh adaption.
Model-Driven Mesh Adaptions control the AMR based on specific requirements of physics models. Simcenter STAR-CCM+ models automatically provide automatically the adaption criteria.
Currently the following options are available:
- Free Surface Mesh Refinement, which refines the free surface separating two phases to avoid smearing of the interface. This criterion is used in conjunction with the VOF multiphase model. This criterion is only valid when the Volume of Fluid (VOF) model is selected in the physics continuum.
- Lsi Mesh Refinement, which is a mesh adaptation method for resolving the interface between phases. This criterion is designed to handle the sharp interface between phases. Lsi Mesh Refinement refines the cells near the interface between phases to improve the mesh resolution. This criterion is available when the Large Scale Interface Detection multiphase interaction model is selected in a Eulerian Multiphase (EMP) simulation.
- Overset Mesh Refinement, which adapts the cell size of a lower priority region to the cell size of a higher priority region to improve the quality of overset interpolation. In the case of one background region and one overset region, the cells of the background region are refined according to the overset region. This criterion is only valid after the creation of at least one overset interface.
- Mesh Adaption for Reacting Flows, which adapts cells in flame regions by either refining cells in which the second gradient of combustion scalar fields (see Variables) is above a given tolerance, or coarsening cells in which the second gradient of combustion scalar fields is below another specified tolerance. This criterion is only valid after a combustion model is selected.
- Virtual Disk Mesh Refinement, which refines the underlying finite volume mesh to capture the virtual disk properties.
- Topology Physics Mesh Adaption, which is a mesh adaption method associated with topology optimization. This criterion defines the mesh refinement of the solid-fluid interface based on the gradient of the material indicator field. Topology Physics Mesh Adaption is only available when the Adjoint Flow with Topology Optimization models are selected in the physics continuum.