CONVERGE 4, the latest major release of our CFD software, includes many exciting new features and enhancements that expand both the capability and usability of the code.
Speed & Accuracy Improvements
Under-Relaxation Steady Solver
CONVERGE 4 introduces a new Under-Relaxation Steady (URS) solver, which offers a number of benefits for steady-state simulations. Instead of using time marching to reach steady state, as the pseudo-transient solver does, the URS solver uses under-relaxation in place of the transient term. With this scheme, the residuals often converge much faster, significantly reducing runtime. Depending on the application, the URS solver can be approximately 3–100 times faster than the pseudo-transient solver.
2D Axisymmetric Solver
With the new 2D axisymmetric solver, you can solve 3D problems with axisymmetric characteristics using a cylindrical framework. Compared to solving these problems in Cartesian coordinates with sector geometries, the 2D axisymmetric solver offers improved spatial accuracy and substantially reduced computational cost. You can use this feature with the transient, pseudo-transient, or URS solvers.
Cross-Stream Synchronization
Previous versions of CONVERGE allowed you to use different physical models and solver settings in different streams, i.e., different sub-domains of the simulation. CONVERGE 4 adds the capability to use different time-steps in different streams, which are then synchronized at a user-specified time interval. This enhancement can accelerate transient simulations in which the time-scales are very different for different streams and strong coupling between the streams is not required to adequately predict flow behavior, for example, in some conjugate heat transfer simulations.
Enhanced Modeling Capabilities
Combustion & Chemistry Modeling
CONVERGE 4 includes enhancements to several combustion models. The Extended Coherent Flame Model (ECFM) and 3-Zone Extended Coherent Flame Model (ECFM3Z) have been expanded to be compatible with hydrogen and ammonia fuels. Furthermore, the thickened flame model (TFM), previously available for use with large eddy simulations (LES), can now be used with Reynolds-Averaged Navier-Stokes (RANS) simulations as well. For hydrogen combustion, where the flame thickness is very thin, and for other fuels operating under high pressures, this feature allows you to track the flame front at a much lower computational cost.
In addition to these model enhancements, CONVERGE 4 also includes a new spark model, the Lagrangian-Eulerian Spark Ignition (LESI) model. The LESI model provides a more realistic approach to simulate spark ignition by modeling the arc/glow phase as a segmented line that interacts with the flow. Users also have the option to include sub-models to predict blowout, restrike, and short-circuiting.
In CONVERGE 4, the SAGE detailed chemistry solver can be used to model liquid-phase and solid-phase chemistry. Combining liquid-phase chemistry modeling with the volume of fluid (VOF) dissolved gas model allows you to study processes such as carbon sequestration, ocean acidification, and pollution control devices for industrial gas streams. Using the solid-phase chemistry feature, you can model a reactive solid that undergoes thermochemical decomposition when exposed to an external heat source. Applications of solid-phase chemistry include biomass fuel pyrolysis (e.g., wildfires) and battery reactions occurring during thermal runaway.
CONVERGE’s chemistry tools have also been improved in version 4. The surrogate blender tool can help you find the optimal mixture composition of a multi-component surrogate to match the properties of a complex target fuel. Updates have been made to the tool to increase the accuracy of the surrogate fuel properties. The mechanism tuning and reduction tools, which have been available for several years with limited 0D and 1D targets, have been upgraded to allow unlimited targets, including soot emissions. Furthermore, CONVERGE 4 includes a new chemistry tool, the 1D spherical flame solver. This solver calculates the Markstein length, an important parameter for modeling the effects of curvature on laminar flamespeed. Modeling these effects may be important for some premixed combustion cases, such as hydrogen combustion.
Multi-Phase Modeling
The Multi-Fluid Multi-Field (MFMF) model in CONVERGE 4 allows you to model multiple interspersed phases, which can be liquids, gases, or even small solid particles that can be reasonably modeled as a continuum. All phases share a single pressure field but can have separate velocity fields. Continuity and momentum equations are solved for each phase, and they are coupled by interphase momentum exchange terms such as drag, lift, and virtual mass. The MFMF model is useful for many applications in the oil and gas industry, for example, simulating oil separators, fluidized beds, granular flows, and the complexities of multi-phase pipe flow regimes.
The drift flux model, introduced in CONVERGE 3.1, can now be used in porous media. This feature can model two-phase flow within heterogenous porous media, where each phase responds differently due to inertial, viscous, capillary, and interfacial forces. This model can be applied to applications including polymer electrolyte membrane (PEM) fuel cells, geologic carbon sequestration, and gas hydrate formation in sediment.
CONVERGE 4 also includes a new boiling model, the Lee model, which was implemented in collaboration with IFP Energies nouvelles. This model is implemented in the VOF framework, and treats the liquid-vapor-gas mixture as a single fluid in the CFD cell. The Lee model is inexpensive and capable of simulating large-scale industrial problems.
Several popular homogeneous mixture-based cavitation models have been implemented in CONVERGE 4: the Saito, Singhal, and Schnerr and Sauer models. These models are relatively simple and efficient, assuming a homogeneous mixture in each CFD cell. They span the spectrum of stability and accuracy, so you can choose the model that works best for your case. These models are particularly useful for marine applications, such as ship propellers, as well as pumps, compressors, and other turbomachinery.
A variety of tools for wind and wave specification, available as UDFs in previous versions, have been integrated into the main code in version 4. With these tools, CONVERGE can generate both regular and irregular water waves using various wave theories. To reduce numerical wave reflections, you can include wave relaxation zones at the inlet and outlet that act as sink terms for momentum and mass. In addition, you can introduce a wind profile above the water, and CONVERGE will couple the wind and waves using its VOF method for multi-phase modeling. These tools are critical for conducting realistic marine and offshore simulations.
