CONVERGE FAQs

Some cases may be able to use larger pseudo-time-steps and achieve faster solutions by using the SIMPLE algorithm and pressure-based solver in CONVERGE 3.0. Another improvement is that in CONVERGE 3.0 you can have a split steady-state monitor approach: you can define looser criteria for auto-grid scaling and auto start of Adaptive Mesh Refinement than the final convergence criteria. For example, you can have convergence on NOx and CO only in your final convergence criteria so you don’t spend time during grid scaling waiting for those variables to reach steady values. CONVERGE 3.0 scales significantly better than previous versions, and these improvements will help accelerate your parallel steady-state simulations.

The steady-state solver in CONVERGE 2.4 is a density-based pseudo-time-stepping solver that can be used for solving a wide range of steady flow simulations (internal/external flows, combustion, sprays and films, CHT, MRF, surface chemistry, etc.). The solver allows the use of higher CFL numbers and also automated solver control for certain simulation parameters (CFL numbers, solver tolerances, and grid sizes). Both of these features help reduce the computation cost of your simulation.

Please consult the CONVERGE 2.4 Manual for recommended parameters for steady-state simulations. Remember that these values may require modification for some cases. Some general recommendations are given below.

We recommend initiating your steady simulation with a relatively coarse grid (grid_scale = -1 or -2 in inputs.in), so as to allow the initial transients to be rapidly flushed out of the domain. Time-based or automated grid scaling should be used, although care should be taken to ensure that the grid always remains adequately refined in regions in which the flow is complex.  The maximum CFL number can be set to approximately 20 to 30 for non-reacting flows and approximately 10 to 15 for reacting flows. If the solver has excessive recoveries, you can reduce the maximum CFL number. If automated control is activated, the solver will perform this action on its own.  Monitoring a few flow variables is useful for determining convergence in a steady simulation and can be used for automatically controlling grid scaling or solver settings. We recommend monitoring variables at OUTFLOW boundaries (temperature, mass flow rate, and species concentration), within the domain (maximum, minimum, and mean pressure and temperature; species concentration; and spray mass), or at monitor points (velocity, pressure, and temperature). The initial velocities and pressures and the corresponding INFLOW boundary conditions should be as consistent as possible. For instance, if the inflow velocity is 1 m/s, then the initial condition should be 1 m/s as well.

We recommend CONVERGE BiCGSTAB with the SOR preconditioner as the go-to pressure solver option in steady-state simulations.

CONVERGE 3.0 includes three different varieties of the MUSCL scheme (Monotonic Upstream-Centered Scheme for Conservation Laws). The MUSCL_CVG option includes a 3D gradient-based slope limiter (the minmod method of Barth and Jespersen or the venkatak method of Venkatakrishnan). The MUSCL scheme can be very useful for supersonic flows and for obtaining second-order upwinding.

Setting Numerical_schemes > implicit_fraction = 0.5 in solver.in yields the Crank-Nicholson time-marching scheme, which is second-order in time for the momentum equation. Please be sure temporal_control > max_cfl_u in inputs.in is less than 1.0.

CONVERGE 3.0 input files are YAML-compliant. YAML is a standardized plain-text format that offers more flexibility than the input file format of CONVERGE 2.4-. See the CONVERGE 3.0 Manual (Chapter 23 – Input and Data Files) for more details about YAML-compliant input files and for details about each input and data file. Because of the change in file format from CONVERGE 2.4 to CONVERGE 3.0, we strongly recommend using CONVERGE Studio 3.0 to  set up new cases or to convert input files from previous versions.

We recommend using CONVERGE Studio to automatically convert input files from an older version of CONVERGE. Open CONVERGE Studio 3.0 and go to File > Import > Import input file(s). CONVERGE Studio will convert your files to version 3.0. You can then export these version 3.0 files in CONVERGE Studio and use them in a CONVERGE 3.0 simulation.

CONVERGE 3.0 stores the surface geometry on each compute node rather than on each core. Node-based storage reduces the memory requirement without affecting computational performance. This memory savings can be significant for geometries with a large triangle count and will be more significant in HPC systems with a larger number of cores per node.

Starting in CONVERGE 3.0 you can set up a moving WALL boundary and link to it the motion of other WALL boundaries.

Starting in CONVERGE 3.0, the periodic matching directions do not need to be coordinate-aligned, periodic faces do not need to be planar, and a case can have multiple periodic matching directions. In CONVERGE 2.4, the rotational periodic boundaries were limited by z as the axis of rotation, xz as symmetry plane, and planar faces. Also, periodicity could be enforced in only two directions in translational periodic cases.

CONVERGE uses the first entry and ignores any subsequent entries for that species. If you validate your therm.dat file in CONVERGE Studio before running a simulation, CONVERGE Studio will offer several ways to resolve duplicate entries.

Each version of CONVERGE contains enhancements and bug fixes, and these changes may affect simulation results. Please see the release notes (available on hub.convergecfd.com/downloads) for specific information about changes to each version of CONVERGE. If you have specific questions about why results may have changed or how to more closely match results from a previous version, please contact the Convergent Science Applications team [[email protected] (US), [email protected] (EU), or [email protected] (India)].

Yes. A well-resolved unsteady RANS simulation does not necessarily eliminate all perturbations and thus can predict cyclic variations. An example of this phenomenon is GDI engines that show high cycle-to-cycle variation in measured cylinder pressure data. The following publications contain details on this topic.

By changing some numerical settings, you can force predictions to be more repeatable. Increasing numerical viscosity in the solution will dampen perturbations. Increasing cell sizes and using lower-order discretization schemes can increase the repeatability of a solution. It is important to note, however, that these changes may reduce accuracy.

It is important to simulate the induction in order to accurately characterize the velocity field. It is possible to run the intake simulation and map that solution at IVC for the closed-cycle simulation rather than assuming constant initial flow conditions.

Please refer to the example cases. In CONVERGE Studio, go to File > Load example case. These cases are also available at hub.convergecfd.com/downloads (login required).

No. An LES simulation will usually require smaller cell sizes.

Check the following items.

CONVERGE contains a sealing test utility (converge –l) that allows you to quickly identify errors in the sealing setup. For more information about this tool, please consult the CONVERGE Manual.

We recommend the real gas equation of state for all simulations.

CONVERGE does not allow multiple boundary embeddings for a single boundary. You can, however, accomplish the same effect by adding a box or cylinder embedding.

Yes. CONVERGE 2.4+ contains a monitor points option in which points assigned to a moving boundary will move with that boundary. To set up this feature in CONVERGE Studio, go to Output/Post-Processing > Monitor points.

CONVERGE Studio 2.4+ contains a compression ratio calculator (go to Applications > IC engine > Compression Ratio). You can also use this tool to move the piston to a location that yields the desired compression ratio. If you are using CONVERGE Studio 2.3 or earlier, please consult Chapter 19 of the CONVERGE 2.3 Manual for directions on calculating the compression ratio and moving the piston to the desired location.

Please note that the CR calculation tool assumes that all the valves are closed at both TDC and BDC. If any of the valves are open at these times, the calculated CR will not match the experimental data.