How to manage restarts and overcome challenges effectively?

This article provides a comprehensive guide on managing restarts in thermal analysis. It lists changes that can and cannot be made while performing a restart, discusses common challenges, steps to ensure consistency in restart options, detailed instructions on selecting and configuring restart options, and best practices.

Introduction

In certain cases, after a solution has run or has been paused, you may want to reuse data that was calculated. For example, for a model with the same geometry but different thermo-optical properties, the black body view factors in enclosures are the same. View factor calculation is an expensive computational process, hence reusing previously calculated view factors saves you time.

Supported modifications to the model

You can make some modifications to your model and still perform a restart using a previous analysis.
Can change Cannot change
  • Thermal boundary condition values
  • Solver attributes and parameters
  • The geometry
  • The mesh
  • Modeling objects
  • The solution units

Issues linked to restart

Restart-related issues can arise during simulations involving radiative conductances, solar and orbital heat fluxes, and specific file formats.

The challenges often arise from:
  • Restarting radiative conductances in a MODLF when reverse or Oppenheim element offsets are not defined in either the initial or restart run.
  • Elements defined for solar heating that are part of an open enclosure, causing poor convergence of radiative heat flux balance in the visible wavelength within the POWER module.
    • Use a restart to calculate solar and orbital heat fluxes and radiative conductances separately for the involved elements.
    • Calculate radiative conductances separately, as they remain consistent across runs.
  • Restarting radiative conductances when space elements are involved, as the element labels such as SPACEX+, SPACEX-, SPACEY+, SPACEY-, SPACEZ+, SPACEZ- can vary between the initial run and restart, with no option to force consistency. This requires recalculating the cavity involving space elements in every run.
  • File format inconsistencies, where crashes may occur if GPARAM 0 17 -1 is defined in generic entities output in ASCII format, while binary MODLF or VUFF files are used as input.

Restart process overview

To manage a restart effectively, ensure consistency between:
  1. Restart options such as location of restart files, thermal items to be reused or recalculated, and thermal items to be processed or skipped.
  2. Solution definitions. Activate or deactivate the thermal boundary conditions in the solution according to the restart options. Avoid the recalculation of previously computed data.
  3. Provided files. Place VUFF, MODLF, INPF and tmggeom.dat files in the correct location.
The following scheme shows the thermal solver run.

Defining restart options

To perform a restart in the Thermal Flow, Space Systems Thermal, and Electronic Systems Cooling environments in the Solution dialog box, select the Perform Restart check box and specify the files location directory:
  • Run Directory to use the files form the current run directory.
  • Specify to use the files located in a previous run directory.
Select the Restart Options check box, and select the following options:
Restart Options Reuse or calculate Description
Conduction, Capacitance and Duct Network Delete previous - Calculate new Recalculates the specified data in the COND module when you change the conductivity, specific heat, density, physical property or fluid property of at least one material in the model.
Reuse previous - Do not calculate Reuses the specified data from the MODLF file.
Thermal Couplings Delete previous - Calculate new Recalculates the specified data in the VUFAC module when you modify at least one of the following parameters of your existing Thermal Coupling:
  • The type or value of your thermal coupling.
  • The primary or secondary region.
  • The coupling resolution.
Reuse previous - Do not calculate Reuses the specified data from the MODLF file.
Convection Couplings Delete previous - Calculate new Recalculates the specified data in the VUFAC module when you modify:
  • Any parameters of your existing Thermal Coupling — Convection simulation object.
  • Ambient conditions when your model includes a Convection to Environment constraint.
Reuse previous - Do not calculate Reuses the specified data from the MODLF file.
Black Body View Factors Delete previous - Calculate new Recalculates the black body view factors in the VUFAC, HEMIVIEW, or VFRTGPU modules when you change the selection or modify parameters in the Radiation simulation object.
Reuse previous - Do not calculate Reuses the specified data from the VUFF file.
Reuse previous - Calculate new Reuses the black body view factors from the previous solution and calculates black body view factors for all active elements in the current solution.
Note:
You must deactivate previously calculated regions using the Deactivate Radiation option in the Deactivation Set simulation object if you do not want to double the black body view factors for these regions.
Radiative Conductances Delete previous - Calculate new Recalculates the radiative conductances in the VUFAC, GRAYB, POWER, GPURT (if GPU is used) modules when you modify Radiation or Thermal Coupling — Radiation, or the emissivity of any element.
Reuse previous - Do not calculate Reuses the specified data from the MODLF file.
Reuse previous - Calculate new Reuses the radiative conductances from the previous solution and calculates radiative conductances for all active elements in the current solution.
Note:
You must deactivate previously calculated regions using the Deactivate Radiation option in the Deactivation Set simulation object if you do not want to double the black body view factors for these regions.
Note:
You should always recalculate radiative conductances if you recalculate black body view factors.
Solar and Orbital View Factors Delete previous - Calculate new Recalculates the solar, IR, and orbital view factors in the VUFAC module when you modify the parameters or the selection of the following simulation objects:
  • Orbital Heating
  • Radiative Heating
  • Solar Heating
Reuse previous - Do not calculate Reuses the specified data from the VUFF file.
Reuse previous - Calculate new Reuses the solar, IR, and orbital view factors from the previous solution and calculates the solar, IR, and orbital view factors for all active elements in the current solution.
Note:
You must deactivate previously calculated regions using the Deactivate Radiation option in the Deactivation Set simulation object if you do not want to double the black body view factors for these regions.
Solar and Orbital Heat Fluxes Delete previous - Calculate new Recalculates the solar and orbital heat fluxes from the VUFAC, GRAYB, POWER, GPURT (if GPU is used) modules when you:
  • Modify the solar spectrum properties of one of your materials.
  • Modify the source fluxes.
  • Calculate new solar, IR, or orbital view factors.
Reuse previous - Do not calculate Reuses the solar and orbital heat fluxes from the MODLF file.
Reuse previous - Calculate new Reuses the solar and orbital heat fluxes from the previous solution and calculates solar and orbital heat fluxes for all active elements in the current solution.
Note:
You must deactivate previously calculated regions using the Deactivate Radiation option in the Deactivation Set simulation object if you do not want to double the black body view factors for these regions.

Defining restart options in the Simcenter 3D Thermal Multiphysics environment

Simcenter 3D Thermal Multiphysics environment does not currently provide a user interface for restart options. However, you can use a thermal include file to reuse previously calculated data.

To do this:
  1. Create a customized input file in the INPF format to include in the thermal analysis. The following example shows how to reuse the black body view factors.
    
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RESTART ON
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$
$ -----------------------------
$ Solution Attributes - Restart
$ -----------------------------
$
PARAM RESTARTDIR D:\pathToFiles\solutionDirectory\\
PARAM RESTART COND_CAP_FLUID DELETE CALC
PARAM RESTART THERMAL_COUPLINGS DELETE CALC
PARAM RESTART CONVECTION_COUPLINGS DELETE CALC
PARAM RESTART BLACK_BODY_VIEW_FACTORS REUSE  NOCALC
PARAM RESTART RADIATIVE_CONDUCTANCES DELETE CALC
PARAM RESTART ORBITAL_VIEW_FACTORS DELETE CALC
PARAM RESTART HEAT_FLUX_VIEW_FACTORS DELETE CALC
PARAM RESTART ORBIT_HEAT_FLUXES DELETE CALC
$
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  2. Save the thermal input file and add to the run directory.
  3. Edit the solution, select the Thermal Include File check box, on the General page, and browse to the created include file.
  4. Solve the solution and review the results.

Restart process

The following chart shows the restart process:

Guidelines for the restart process

Follow these guidelines:
  • Fix the reverse side and Oppenheim elements through the offset to prevent label overlaps during the restart of black body view factors or radiative conductances.
  • Avoid using black body view factor calculations for enclosures containing space elements to prevent labeling conflicts.
  • Ensure thermal items from the previous run are deactivated using the radiation deactivation set in the restart to avoid double-counting thermal conditions.
  • Preserve the MODLF and VUFF files in a dedicated directory to prevent overwriting by the thermal solver.
  • Use a consistent file format for MODLF and VUFF files, either ASCII or binary, for both the original run and the restart to prevent crashes.
  • Do not restart radiative conductances or solar heat fluxes if thermo-optical properties have changed. Emissivities and absorptivities are part of radiative conductances and solar heat flux values.
  • Perform solar heating and view factor calculations on the same elements before using the restart option.