Adaptive Time-Stepping
Adaptive time-stepping adjusts the time-step automatically during the run to attain a specified temporal resolution, and can improve the stability and run-time of your simulation and the accuracy of the results. This adjustment is particularly useful for cases with large variations of flow topology, large varying time scales of the physics, or when the mesh is dynamically adapted using Adaptive Mesh Refinement.
Unsteady phenomena can exhibit large variations in the significant time scales, which makes them inefficient to simulate using a constant time-step size. Although it is possible to specify a variation in time-step size in Simcenter STAR-CCM+, such as using tabular values, determining the appropriate values is time-consuming and must be repeated for each application.
The Adaptive Time-Step model automates the determination of time-step sizes. Depending on the physics models that you activate in your simulation, different time-step providers are available, using different criteria for determining the time-step size. You can use any combination of the available time-step providers. At any given point, the simulation uses the smallest time-step from the enabled time-step providers. If the Adaptive Time-Step and Adaptive Mesh Refinement models are used together, the adapted time-step proposal is based on the adapted mesh, with the calculation occurring after the Adaptive Mesh Refinement.
The time-step providers can propose very small time-steps on meshes with small or poor quality cells. To prevent an unnecessary increase in runtime of the simulation, the Adaptive Time-Step model provides a filtering option to disregard contributions from these user-specified cells to the localized time-step calculations.
The available time-step providers are listed below:
| Time-Step Provider | Description |
|---|---|
| CFL |
Limits the time-step size to meet the specified target criteria for the convective Courant number. This time-step provider is appropriate for transient flow problems. See CFL Reference. |
| Convective CFL | Limits the time-step size to meet the specified target criteria for the convective Courant number for Smoothed-Particle Hydrodynamics (SPH) multiphase simulations. |
| Max Particle Packing |
Reduces the time-step to a specified minimum value when the volume fraction is greater than the specified threshold volume fraction. This time-step provider is suitable for granular flow in the context of Eulerian Multiphase problems. |
| Free Surface CFL |
Limits the time-step size to allow for a sharp interface. This time-step provider is appropriate for free surface flows when the VOF interface crosses meshes that have different CFL numbers, such as from coarse mesh to fine mesh, or has a changing velocity field. |
| Free Surface Explicit Multi-Step |
Limits the time-step size to allow for a sharp interface when using the VOF Explicit Multi-Step (Deprecated) solution strategy. This time-step provider is appropriate for free surface flows when the VOF interface crosses meshes that have different CFL numbers, such as from coarse mesh to fine mesh, or has a changing velocity field. |
| Free Surface Implicit Multi-Step |
Limits the time-step size to allow for a sharp interface when using the VOF Implicit Multi-Step solution strategy. This time-step provider is appropriate for free surface flows when the VOF interface crosses meshes that have different CFL numbers, such as from coarse mesh to fine mesh, or has a changing velocity field. |
| Gravity CFL | Limits the time-step size to meet the specified target CFL criteria when gravitational effects are included in Smoothed-Particle Hydrodynamics (SPH) multiphase simulations. |
| In-Cylinder |
Limits the time-step size suitable for transient STAR-ICE applications. This time-step provider is used for STAR-ICE applications. |
| Melting-Solidification |
Limits the time-step size to allow for an accurate prediction of the melting/solidification progress. |
| Smoothed CFL |
The Smoothed CFL is similar to CFL above, but adds smoothing in space. This smoothing is to reduce the potentially detrimental effect of isolated and strongly locally-varying mesh length scales on the overall use of the Courant Number as the time-step size driver. |
| LSI Smoothed CFL |
This time-step provider is similar to Smoothed CFL above, with the exception that LSI Smoothed CFL only operates on the Large Scale Interface. Therefore, this option is only available when the Adaptive Interface Sharpening (ADIS) scheme is selected under the Volume Fraction Convection. |
| Thermal Diffusivity |
Limits the time-step size to meet the specified target criteria for the Von Neumann number of thermal diffusivity. This time-step provider is appropriate for problems with dominant thermal diffusion with varying magnitude, and also for supporting solid materials. |
| Solid Stress |
Optimizes the time step with respect to a variable monitor value or calculated displacement change. This time-step provider is suitable for non-linear solid stress problems such as buckling or impact phenomena. |
| Viscous CFL | Limits the time-step size to meet the specified target CFL criteria when viscous effects are included in Smoothed-Particle Hydrodynamics (SPH) multiphase simulations. |
| Volumetric Source | Limits the time-step size based on the contributions to the source terms of the continuity equation as a result of phase mass transfer, such as boiling, evaporation, cavitation and user defined sources. The proposed time-step size depends on how fast the mass transfer occurs, and the volume of the cells which inherit the transfer. This time-step provider is suitable for Mixture Multiphase (MMP) and Volume of Fluid (VOF) models. |