Solvers

The following right-click actions and dialogs are available for solvers in Simcenter STAR-CCM+ In-cylinder.

Solvers Right-Click Actions


Object	Right-Click Action
Solvers	Edit Opens a dialog that allows you to set the properties of the solvers. See Solvers Dialog.

Solvers Dialog

Time Step Settings

Time Step Allows you to set a constant time-step size. The supported units are s and degCA.

Stepped Table

Allows you to set the time-step size in crank angle degrees as a function of crank angle. You import the tabular values from a .csv file, which contains comma-separated or tab-separated values. Simcenter STAR-CCM+ In-cylinder interpolates the values using the step method.

Example: Decrease of time-step size during opening and closing of the valves.

Suppose the graph below shows the valve lift curves for an exhaust valve and an intake valve:

With a Closure Tolerance of 0.1 mm, you can consider the exhaust valve opening at 130 CA and closing at 360 CA. The intake valve opens at 355 CA and closes at 585 CA.

The following table reduces the time-step size of 0.1 CA to 0.05 CA during opening/closing of the valves and keeps the reduced time-step for 1 CA after opening/closing:

CA,TimeStep(CrankAngleStep)
0,0.1
129.9,0.05
131.0,0.1
354.9,0.05
356.0,0.1
359.9,0.05
361.0,0.1
584.9,0.05
586.0,0.1

Automatic

Automatically adjusts the time-step size based on valve lift thresholds, on the start and end of fuel injection (for charge-motion simulations), on the intersection between liner and intake port (for two-stroke engines), and on the ignition timing (for combustion simulations).

The Global time-step settings are valid throughout the simulation. The Valves, Liner Ports, Injectors, and Ignitors time-step settings request time-step sizes depending on specified valve lift offsets, liner-port intersection deltas, injection periods, and ignition timing, respectively. If different time-step sizes are requested, Simcenter STAR-CCM+ In-cylinder applies the minimum of any requested values.

Clicking the Settings buttons opens the Auto Time-Step Settings dialog, where you to set the following properties:

Global

Specifies the following overall time-step control properties:

Minimum Time-Step: Specifies the lower limit on the time-step size. This value also specifies the size of the first time-step in the simulation.
Maximum Time-Step Change Factor: Specifies the maximum ratio of the current targeted time-step to the previous time-step. You set this ratio to avoid large jumps in the time-step size. The specified value applies to an increasing time-step size only. If the maximum time-step change factor is exceeded, the targeted time-step size is cut off at the maximum value permitted by the change factor.

Checkpoint Times: Specifies a list of physical times (in seconds, in the cyclic time unit degCA, or an expression that uses time units) that are to be reached exactly by the automatic time-stepping.

Example: [300.0 degCA, 400.0 s, ${EndOfCycle}]

Note	For a checkpoint time in degCA, you must specify a value within the interval [ Start Angle; Start Angle + cycle length]. For a multi-cycle run, this checkpoint time is used in each cycle.

Setting checkpoint times is particularly useful, if you want to synchronize the simulation at a particular physical time for post-processing.The specified checkpoint times are reached gradually over the course of a few time-steps

Reaching a checkpoint time takes priority over any time-stepping bounds. For example, you can get time-step sizes that are below the minimum time-step. However, due to the gradual nature of approaching each checkpoint time, the deviations from these limits are small.

When the Valves, Injectors, or Ignitors time-step settings request a new time-step size, Simcenter STAR-CCM+ In-cylinder increases/decreases the time-step size by at most the specified Maximum Time-Step Change Factor per time-step until the requested time-step size is reached.

Valves

For the valves, specifies the following Lift Offset values in unit length, relative to the specified lift Closure Tolerance (LCT) of the valve:

Closed: Per Valve (read-only). In the background, Simcenter STAR-CCM+ In-cylinder sets the specified Closure Tolerance of the valve. When you change the default value of the Closure Tolerance in the Valve Motion group-box, you must click Apply to transfer the new value to this setting.
Very Low: Specifies a lift offset slightly below the Closure Tolerance.
LCT (read-only): 0.0 mm
Low: Specifies a lift offset slightly above the Closure Tolerance.
Medium: Specifies a lift offset where the valve is partially open but still near the valve seat.
High: Specifies a lift offset close to the fully open position, where the valve is far from the valve seat.

For each lift offset, you specify a corresponding Time-Step size in s or degCA.

Depending on the lift of the valve in your model, Simcenter STAR-CCM+ In-cylinder requests the following time-step sizes:


Valve Lift (VL)	Time-Step
0 $\leq$ VL $\leq$ Very Low Lift	Closed Time-Step
Very Low Lift $<$ VL $\leq$ LCT Lift	Very Low Time-Step
LCT Lift $<$ VL $\leq$ Low Lift	LCT Time-Step
Low Lift $<$ VL $\leq$ Medium Lift	Low Time-Step
Medium Lift $<$ VL $\leq$ High Lift	Medium Time-Step
VL $>$ High Lift	High Time-Step

For multiple valves, the applied time-step size is the minimum of the requested values.

In general, you need a small time-step during opening and closing of the valves, that is, at valve lifts near the specified Closure Tolerance. For this reason, it is good practice to set the Very Low and Low Lift values equal to -0.005 mm and +0.005 mm, respectively.

The following diagram displays the valve lift thresholds and the corresponding time-step sizes for an exemplary valve lift curve:

Additionally, the First Opening Event group-box allows you to specify lift offset values and corresponding time step sizes for Before and After the first valve opening event in the simulation (irrespective of intake or exhaust valve).

Liner Ports (only for two-stroke engines)

Specifies the following Delta Positions, which describe certain offsets between the plenums that represent the intake/exhaust ports and the top z-position of the ports as the piston moves down the liner, in unit length:

Before Intersection: Specifies an offset before the two volumes intersect.
During Intersection: Specifies an offset shortly after intersection where the piston is still close to the top z-position of the port.
After Intersection: Specifies an offset after intersection where the piston is far from the top z-position of the port.

where:

$z_{p i s t o n}$ is the z-position of the piston.
$z_{p o r t, t o p}$ is the top z-position of the port.

For each delta position, you specify a corresponding Time-Step size in s or degCA.

Depending on the z-position of the piston, Simcenter STAR-CCM+ In-cylinder requests the following time-step sizes:


$z_{p i s t o n}$	Time-Step
( $z_{p o r t, t o p}$ + Before Intersection Delta Position) $\geq$ $z_{p i s t o n}$ $>$ $z_{p o r t, t o p}$	Before Intersection Time-Step
$z_{p o r t, t o p}$ $\geq$ $z_{p i s t o n}$ $>$ ( $z_{p o r t, t o p}$ - During Intersection Delta Position)	During Intersection Time-Step
( $z_{p o r t, t o p}$ - During Intersection Delta Position) $\geq$ $z_{p i s t o n}$ $>$ ( $z_{p o r t, t o p}$ - During Intersection Delta Position - After Intersection Delta Position)	After Intersection Time-Step

Injectors (only if the Fuel engine model is selected)

Specifies the following Periods of time in degCA:

Before Injection: Specifies a period of time before the start of injection.
During Injection
After Injection: Specifies a period of time after the end of injection.

Simcenter STAR-CCM+ In-cylinder extracts the start of injection and the end of injection from the mass flow rate table that you import for the injector.

For each period, you specify a corresponding Time-Step size in s or degCA.

Depending on the fuel injection timing, Simcenter STAR-CCM+ In-cylinder requests the following time-step sizes:


Time (t)	Time-Step
(start of injection - Before Injection Period) $<$ t $\leq$ start of injection	Before Injection Time-Step
start of injection $<$ t $\leq$ end of injection	During Injection Time-Step
end of injection $<$ t $\leq$ (end of injection + After Injection Period)	After Injection Time-Step

In general, you need a small time-step during fuel injection to improve convergence and mass conservation. It is good practice to decrease the time-step shortly before injection. The default value is 1 degCA.

To accurately predict the fuel evaporation and air-fuel mixing process after injection, you usually keep a small time-step as long as the fuel loading and the evaporation rates are high. The default value is 20 degCA.

Ignitors (only if the Combustion engine model is selected)

Specifies the following Periods of time in degCA:

Before Ignition: Specifies a period of time before the ignition timing.
After Ignition: Specifies a period of time after the ignition timing.
Expansion: Specifies a period of time for the expansion of the gas mixture.

Simcenter STAR-CCM+ In-cylinder obtains the ignition timing from the Spark Start Time that you specify for the ignitor.

For each period, you specify a corresponding Time-Step size in s or degCA.

Depending on the ignition timing, Simcenter STAR-CCM+ In-cylinder requests the following time-step sizes:


Time (t)	Time-Step
(ignition timing - Before Ignition Period) $<$ t $\leq$ ignition timing	Before Ignition Time-Step
ignition timing $<$ t $\leq$ (ignition timing + After Ignition Period)	After Ignition Time-Step
ignition timing + After Ignition Period $<$ t $\leq$ (ignition timing + After Ignition Period + Expansion Period)	Expansion Time-Step

Note	When you select both the Stepped Table and the Automatic time-step control, Simcenter STAR-CCM+ In-cylinder applies the minimum of the requested time-step sizes.

Solver Settings

Segregated Flow

Controls the solution update and thus the convergence of the solvers that compute the intermediate velocity field and the update of the pressure field.

Velocity Under-Relaxation Factor, Pressure Under-Relaxation Factor

At each iteration, these properties govern the extent to which the newly computed solution supplants the old solution. For the theoretical background, see Eqn. (920).

The default values depend on the selected in-cylinder time model:


In-Cylinder Time Model	Velocity Under-Relaxation Factor	Pressure Under-Relaxation Factor
Implicit Unsteady	0.8	0.2
PISO Unsteady	1.0 (read only)	0.7

These values are conservative, that is they lead to a converged solution in most cases. For further guidelines, see Setting Under-Relaxation Factors for Transient Simulations.

Segregated Energy

Controls the solution update of the solver that computes the temperature field in the fluid and solid domains.

Fluid Under-Relaxation Factor, Solid Under-Relaxation Factor: In order to promote convergence, these properties under-relax changes of the solution during the iterative process. See Eqn. (920).; The default values are 0.9 for fluid energy and 0.99 for solid energy. These values achieve a stable solution in most cases.; With the PISO Unsteady model, these values are read-only.


Auto-Save Settings
Enabled	When On, Simcenter STAR-CCM+ In-cylinder saves a separate copy of the simulation file automatically at regular intervals during the run. This option is also useful for recovering from computer system crashes. The following options control how Simcenter STAR-CCM+ In-cylinder triggers the auto save operation: Delta Crank Angle Saves the simulation every time the crank angle passes the specified delta value. Delta Time Saves the simulation every time the simulation time passes the specified delta value. Number of Time Steps Saves the simulation every time the time step passes the specified value. By default, when Simcenter STAR-CCM+ In-cylinder saves copies of the simulation file, it places an @ character to separate the base file name and the number of the trigger event. Simcenter STAR-CCM+ In-cylinder saves these copies in the same directory as the original simulation file.
Number of Files to Keep	When the specified number of auto-saved simulation files is reached, the next auto-save operation deletes the oldest copy of the file. When set to 0, instructs Simcenter STAR-CCM+ In-cylinder to keep overwriting the original simulation file.

Wall Treatment

Formulation

Controls the calculation of the wall heat flux

{\dot{q}}_{w} ″

The following options are available:

Off: Calculates ${\dot{q}}_{w} ″$ as given by Eqn. (1629).
Angelberger Formulation: Calculates ${\dot{q}}_{w} ″$ as suggested by Angelberger [146].
The following Angelberger Formulation options are available:
- Gasoline: Calculates ${\dot{q}}_{w} ″$ as given by Eqn. (643).
- Diesel: Calculates ${\dot{q}}_{w} ″$ as given by Eqn. (644).
Gru-MO UniMORE Formulation: Calculates ${\dot{q}}_{w} ″$ as given by Eqn. (646).

Note	The Angelberger Formulation and the Gru-MO UniMore Formulation only apply in the gas region where it is in contact with a wall or with fluid film on a wall. For the fluid film region, Simcenter STAR-CCM+ In-cylinder applies the default heat flux calculation as given by Eqn. (1629).


Lagrangian Cell Cluster Length Fraction of Normalized Prism Thickness (only if the Injection engine model is selected)
Enabled	When On, Simcenter STAR-CCM+ In-cylinder smooths the Lagrangian sources arising from the fuel entering the continuous gas phase. The entering Lagrangian parcels can represent large mass, momentum, and energy sources to the continuous gas phase. To increase numerical stability, volume source smoothing spreads the effect of the entering Lagrangian particles over several volume cells. Simcenter STAR-CCM+ In-cylinder imposes a coarse grid for evaluating void fraction data and exchanges of momentum, energy, mass, and species, as applicable. The coarse grid is a virtual grid and is constructed by clustering the cells in the gas region. The simulation uses these larger cells for calculating parcel interactions with the fluid phase. After calculating interactions, the simulation distributes volume fraction contribution, momentum and energy source terms, and other transferred quantities evenly across the component cells. If you use volumetric mesh refinement in areas where the spray plumes grow, volume source smoothing can reduce the accuracy of the simulation results. To ensure that accuracy is not affected, you are advised to compare the results between runs with and without volume source smoothing. If the Liquid Film engine model is selected in addition to the Injection engine model, Simcenter STAR-CCM+ In-cylinder also smooths the Lagrangian sources arising from Lagrangian parcels impinging on fluid film by clustering the cells in the shell regions. The following options control the cell clustering: Cell Cluster Length Displays the absolute length of the cluster calculated as: Cell Cluster Length = Cell Cluster Length Fraction · Normalized Prism Thickness where: Normalized Prism Thickness = Total Thickness / Number of Layers Clicking the Apply button updates the value. Cell Cluster Length Fraction Specifies the ratio of the cluster length to the normalized prism thickness. The value must be greater than zero. When Off, Lagrangian sources to the gas phase are not smoothed.