3.6.2. Time Step Panel

Ansys Forte simulations use advanced adaptive time-step control to allow the most efficient solution while maintaining accuracy during dynamic simulations where sharp gradients in solution variables may occur. Often, default time-step controls are acceptable, but in some cases modifying the time-step controls may aid in convergence or allow faster simulation times. There are three major ways in which the time-step is controlled: the initial time step, the maximum time step, and the algorithm by which the time-step is increased under different conditions. These controls are described below:

Initial Simulation Time Step: The starting value of the time step. The time step will be determined adaptively starting from the specified time-step value, based on rate of change of solution gradients. Generally, the adaptive time-step algorithm will gradually increase the step size until a limit is encountered or solution gradients become too large.
Restrict Time Step by Crank Angle: This determines an overall maximum time-step size for engine cases and is set to 1.1 by default.
Maximum Time Step Option: The actual transport time step in Ansys Forte is adaptively determined, but the maximum time step constrains time-step increase, which can help convergence during times of rapid change. Such rapid changes can occur, for example, when droplet breakup and fuel vaporization occur or during valve opening or closing. The maximum time step may be defined as a constant value or as Time Varying, that is, a profile that is a function of crank angle or time. The latter option allows you to specify a large time step for parts of a simulation where you don't expect sharp gradients and restrict it to a smaller value during critical simulation periods, such as fuel injection.
If the maximum time step ("dtmax") profile is a function of crank angle, the Repeat Profile Each Cycle option shifts the profile to fit within 0-–360 (for 2-stroke) or 0–720 (for 4-stroke) crank angles. The adjusted dtmax profile will then be treated as cyclic and repeated on a 720-degree schedule (4-stroke) or 360-degree schedule (2-stroke). You may choose to Use Global Crank Angle Limits to impose a global crank angle range for the cyclic repetition of the dtmax profile. If the Repeat Profile Each Cycle option is not selected, then the crank angle provided in the dtmax profile will not be converted to cyclic. For a time-based simulation, a time-based dtmax profile must be used. The time-based dtmax profile can also be treated as cyclic. See Profile Editor for the description of how to define the cyclic feature of the profile.
Specify Minimum Time Step: You can specify the minimum simulation time step explicitly by checking this box. If not specified, the minimum time step will be set to the Initial Simulation Time Step multiplied by 1.E-7. The simulation will be terminated when the adaptively controlled actual CFD time step drops below the minimum time step value.

Advanced Time Step Control Options: The adaptive time-stepping algorithm within Ansys Forte adjusts the time-step size to optimize solution efficiency and accuracy. In most cases, the default parameters that control this algorithm should be retained. However, if convergence issues are encountered it may be advantageous to reduce the values on some of these controls. The tool tips provide a handy reference for each parameter, describing the recommended range and the anticipated effect of adjusting the parameter away from its default values.

In Ansys Forte calculations, the CFD time step is limited by the user-specified maximum value (dtmax) or several other factors, whichever is smaller. The current limiting factor is reported in the log file and refers to the following:

Table 3.2: Description of time-step-limiting factors

Name	Description
Pressure Iteration	Limits the number of pressure iterations in the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm. This type of iteration is referred to as big iteration. The max number of pressure iterations is controlled to be less than 50. If this limit is exceeded within a time step, this time step will be recycled with a reduced time step (reduced by a half). If the number of iterations is larger than 3 in the previous time step, the current time step will be reduced by a half also.
Fluid Acceleration	Limiting factor based on fluid acceleration.
Rate of Strain	Limiting factor based on the rate of strain of fluid. It limits the amount of cell distortion that can occur due to mesh movement in the Lagrangian stage.
Fluxing volume	Limits the magnitude of fluxing volume in any coordinate direction to a value less than the cell volume.
Chem heat release	Time-step growth limiting factor when time-step is limited by rate of change of internal energy due to chemical heat release. This factor is a limit of the rate of change of energy in each cell over a time step. A larger value makes time-steps less constrained by rate of change of energy, but may affect the accuracy of the overall solution
Vaporization	Similar to Chemistry, this factor limits the internal energy change due to vaporization to be smaller than a percentage of the cell's current internal energy.
Valve opening or Region changing	This limit is applied when a valve is first opening. When a valve is opening, the flow velocity across the valve tends to be large, thus a smaller value is applied.
Injection timing	Used for precise control of injection timing. If the current time step is so large that the next time point will pass an injection timing, then the time step will be reduced to make the next time point match the timing precisely.
Spark ignition	Similar to Injection, used for precise control of spark ignition timings.