CONVERGE is a fundamentally different CFD code. Given a triangulated surface and several parameters, CONVERGE automatically generates the grid at runtime. You do not need to spend any time creating a mesh! For simulations with moving boundaries or varying embedding, CONVERGE recreates the grid at each time step.
CONVERGE offers several ways to control the grid during a simulation.
- Base grid size. You can specify dx, dy, and dz values for the base grid size
- Grid Scaling. Use grid scaling to refine or coarsen the entire grid. Refinement ensures that you capture critical phenomena in your simulation. Coarsening saves computational time during less-important times or locations in your simulation.
- Fixed Embedding. Refine the grid at a specific location via fixed embedding. This features helps ensure that CONVERGE captures important phenomena such as flow through valves or near nozzles.
- Adaptive Mesh Refinement (AMR). With AMR, CONVERGE automatically refines the grid based on fluctuating and moving conditions such as temperature or velocity. You can specify the important variables (e.g., velocity for a flow case or temperature for a combustion case) and CONVERGE will refine the grid based on these variables.
To include these grid control methods in a CONVERGE simulation, you simply specify the relevant parameters in a set of simple input files
CONVERGE generates the grid automatically at runtime. Prior to running a CONVERGE simulation, you need to verify that the surface geometry is clean and closed and you need to assign the surface triangles to boundaries.
CONVERGE represents the surface as a series of connected triangles. The CONVERGE Studio pre-processor reads in an STL file, which is a triangulated surface file format that can be written from any CAD program. CONVERGE Studio has tools for cleaning the surface and repairing surface defects Once you have repaired the surface and marked the boundaries, your geometry is ready to go.
The surface triangles must be organized into boundaries, which are identified with identification numbers. You can assign different boundary conditions to each boundary. For example, you can add embedding (grid refinement) to selected boundaries. In the image below, the valve is divided into two boundaries. We can add embedding for the yellow boundary when it is at low lift to help resolve the flow in the valve-seat area.
True Geometry Representation
Given a triangulated surface, CONVERGE creates the grid automatically at runtime. There is an inherent, albeit not obvious, advantage to providing the surface geometry as an input to CONVERGE. When the grid resolution is changed due to Adaptive Mesh Refinement (AMR) or other embedding, the refined cells near the geometry walls are re-cut using the original surface definition.
In traditional CFD codes in which you must provide the full grid as an input, the actual surface geometry is no longer available and thus the accuracy of the surface location is limited by the resolution in the original grid. Any grid refinement performed during the simulation will cut the existing cells. Any resolution added near walls will not result in a better representation of the actual geometry.
Only by including the original surface information, as is done in CONVERGE, can the additional embedding result in improved accuracy in the near-wall flow predictions.
Structured, Cartesian Mesh
CONVERGE uses a perfectly orthogonal, structured mesh for high accuracy and simplified numerics.
The image below shows the Cartesian mesh used for a port, valve, and cylinder geometry. The grid includes one level of local refinement around the valve.
Fixed Grid Embedding
Fixed grid embedding allows you to easily specify regions in the domain where additional resolution is needed. To improve the accuracy of the simulation while minimizing the computational time, CONVERGE includes options for you to control the location and duration of each embedding. For example, you can have a boundary embedding that refines the grid near a boundary for the entire simulation and a sphere embedding that refines a spherical portion of the grid only for a short time period.
Grid Size Scaling
In CONVERGE, you can coarsen or refine the grid on the fly (as the simulation is running) or at pre-determined times. This feature is useful for transient problems in which portions of the simulation do not require as much accuracy. Additional, grid scaling helps accelerate convergence in steady flows, allowing rapid convergence at low resolutions, followed by successive refinement of the entire mesh.