For Simulation Limits, enter the start and end of the simulation. The extent of the simulation can be specified in terms of time or crank angle (for engine cases). You can also choose to restrict the overall number of time steps for special cases when this may be desired. In addition to transient simulations, there is an option to detect a Steady State simulation.
Time Based End Points: Specify the maximum simulation time.
Crank Angle Based End Points: Specify the initial and end points for simulation. Crank angles are assumed to be specified relative to the top-dead-center (TDC) location of the piston, such that TDC occurs at 0 crank angle degrees and increments of 360 degrees (that is, at 0, 360, 720,...). However, when the piston pin is offset relative to the crank shaft, crank angle of 0 and piston TDC do not coincide.
Initial Crank Angle: ATDC, initial crank angle for the simulation.
Final Simulation Crank Angle: Final crank angle, defining the extent of the simulation.
Engine Bore: (Automatic Mesh Generation only) Choose whether the engine bore should be calculated automatically or be user-specified. The Calculate Automatically option computes the bore value based on the dimensions of the surface(s) used to define the piston boundary. However, if the piston is split into multiple moving boundaries, the automatically calculated value is unreliable. In this case, specify the bore explicitly. This engine bore option only applies to cases using automatic mesh generation. This input is not used for cases using body-fitted meshes.
Tip: If you need to modify the simulation end points without loading the Forte project into the User Interface, you can do so using command line options. For time-based simulation, use
-tfinal time_value, wheretime_valueis a real number with unit second, for example,-tfinal 0.1. For crank angle-based simulation, use-cafinal ca_value, whereca_valueis a real number with unit degree, for example,-cafinal 180.0. The command-line input values will override the end-point values specified on the User Interface panel.Note: In a multicycle simulation, the global crank angle value will increase monotonically starting from the initial crank angle until it reaches the final crank angle. However, crank-angle-based timing parameters, such as spark timing, injection timing, and output controls, will be converted to fit in the range of [0, 720) °ATDC (for four-stroke) or [0, 360) °ATDC (for two-stroke) and treated as cyclic automatically.
Solution Constraints can be used to apply an upper bound to local flow velocity or Mach number. If the velocity limiter or Mach number limiter is enabled, local velocity magnitude will be bounded based on these user-specified limits everywhere inside the fluid domain. These limiters essentially behave as momentum sink. They allow larger CFD time steps to be reachable by sacrificing the accuracy of the local flow solution at certain high-velocity spots. They are useful in cases where the accuracy at these high-velocity spots is unimportant and it is desirable not to let these high velocities overly constrain CFD time step size. For both Velocity and Mach Number limiters, four options are provided:
No Limit: Do not apply velocity or Mach number constraints.
Constant limit: Apply a constant velocity or Mach number constraint throughout the simulation.
Crank Angle Based Profile: Engine cases must use a crank angle-based profile. The crank angle values in the profile must cover the range of [0,720] °ATDC for four-stroke engines and [0,360] °ATDC for two-stroke engines. The profile will be applied in the global time frame, and it is repeated for each engine cycle. For the duration in which you do not want to apply the limiters, use a huge velocity or Mach number value in the profile.
Time Based Profile: Non-engine cases must use a time-based profile. Similar to a crank angle-based profile, use a huge velocity or Mach number value for the duration where the limiters are intended to be unused.