Control Panel
Model Options Introduction
The Model Options panel enables you to set Shared and Module-specific options. Depending on the modules you have selected in the PT panel, other tabs may be displayed. If so, they are described in the Modules section. The following sections appear on the Model Options Shared tab:
• Simulation Description
• Polar/Axisymmetric
• Transient Conditions
• Body Forces
• Rotation
Model Options Panel
Since the settings in the Model Options page are Module-specific, except the Shared tab, information pertaining to them is in the Modules section.
Shared Tab
The Shared tab of the Model Options panel controls parameters that affect all Modules and all grid regions in the simulation. They include the polar nature of a 2D problem, time dependence, body forces and rotation reference are controlled through this panel. The Model Options figure shows the Model Options page in the Shared settings mode.
Control Panel
Model Options-Simulation Description
The Simulation Description section enables you to enter a name for your simulation.
Control Panel
Model Options-Polar/Axisymmetric
The Polar/Axisymmetric option is valid for any 2D grid systems. Selecting the axisymmetric option indicates that the complete geometry is defined by an angular rotation of the model one radian about the x-axis. The x-y coordinate system is recast as a x-r coordinate system.
Control Panel
Model Options-Transient Conditions
The Transient Conditions (or time dependence) section of the Model Options panel contains time dependence, transient time step, and time accuracy.
Time Dependence
You can set the Time Dependence of the problem by selecting one of the following options from the pull-down menu:
• Steady - for steady-state simulations
• Transient - for time-accurate or unsteady simulations Transient Time Step
When you select the Transient option in the Time Dependence section, the Transient Time Step panel appears and enables you to pick an evaluation method for calculating the time step. There are four evaluation methods to choose from:
• Standard - allows you to pick the number of time steps and each time step size (the time step size will remain constant throughout the simulation).
• Parametric Input - allows you to set the time step size based on a parameter defined in the Tools->Parametric Input section. See Tools Menu for more information regarding input of parameters.
• User Sub (udt) - allows you to program your own user subroutine (udt) to control the timestep size based on your coding in the subroutine.
• Auto Time Step - is available for any problem except those in which the Stress Module has been activated. This evaluation method enables you to specify the start and end time of the simulation an Error Criterion (values in the range of 1e-4 to 1e-3 are usually suitable) and an initial time step size. When the VOF Module is selected you must a target CFL number instead of an Error Criterion. When this time step evaluation method is employed, the solver will automatically adjust the timestep size in order to try to match either the Error Criterion or the Target CFL number (in the VOF context, the target CFL number represents the percent of a cell over which the VOF front can traverse). You will also be able to set minimum and maximum timestep sizes to constrain the automatic timestep calculation.
where:
ε represents error criteria
d represents weighted RMS Norm for all the variables solved and calculated as
where:
u and v are two components of velocity N is the total number of cells (vector length) umax = max|(u)| and similarly for other variables o represents values for old time step
Time Accuracy
The Time Accuracy (temporal differencing) section provides a pull-down menu with the following options:
• Euler(1st Order) The default is backward Euler. The differencing is performed in the following manner
where n is the timestep and i is the current cell in which the computation is being performed for variable f. k corresponds to the diffusion coefficient.
• Crank-Nicolson (2nd Order) - Enables you to enter a blending factor. A blending factor of 0.5 produces pure Crank-Nicolson differencing and a blending factor (
β
) of 1.0 produces pure backward Euler. The default value is 0.6. The differencing is performed in the following mannerwhere n is the timestep and i is the current cell in which the computation is being performed for variable f. k corresponds to the diffusion coefficient.
Control Panel
Model Options-Body Forces
Activating Body Forces (or gravity) can be done in one of two ways:
• Non-Boussinesq Approximation - relies on a change in fluid density to provide gravitational source terms,
• Boussinesq Approximation - can be used with constant density fluids.
For either case, you must enter the gravity vector to determine the direction and strength of the gravity field.
The Non-Boussinesq Approximation produces a source term to the momentum equations of the form:
Si = gi(ρ -ρref)Vol (1-1)
The ρref value can be input by the user or calculated automatically by the code in some situations.
ρref should be chosen so as to remove the need to specify a hydrostatic pressure variation at the exit boundaries (i.e., if ρref = density at the exits then the source term at the exits goes to zero and you can apply a constant pressure at the exit boundaries rather than a hydrostatic variation.) In effect you are acknowledging that the hydrostatic variation is a constant in the solution and subtracting this variation from the solution.
For the Non-Boussinesq approximation to work, density must vary (otherwise the source term is a constant). So you must ensure that density is a function of temperature or use ideal gas law.
The Boussinesq approximation also adds a source term to the momentum equations, but the source term is independent of the density variation. The source term is a function of the temperature variation as follows:
Si = giρVol( T-Tref)Beta (1-2)
where Beta is the thermal expansion and is normally set to 1/Tref. As you can see, the source term is dependent on the density, but density does not have to vary to cause an effect. This approach can be used for (almost) incompressible liquids.