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ESARAD Geometry Conversion

erg-reader  module

Activation of the erg-reader  module

The ESARAD erg-reader  module of TASverter is activated from the command line using the option:


When the TASverter GUI is used, the "Model Type" of the "From"-file must be set to "ESARAD erg" (see figure below):

Activation of the erg-reader  module through the TASverter GUI

Figure 4: Activation of the erg-reader  module through the TASverter GUI

When this option is activated, TASverter expects an ESARAD erg formatted file as input. The contents of this file are read and stored in memory in the internal STEP-TAS  format (the so-called  Part 21).

The development of the erg-reader  module was based on the information found in the ESARAD Reference Manual. If any construct in the input file does not comply with the format specified in the manual, TASverter will stop and provide an error message to the user (this error message will be displayed in the output box when the TASverter GUI is used).

Conversion from ESARAD erg to STEP-TAS 

Length unit

The ESARAD erg language does not provide a means to specify the length unit to be used by the model. Therefore, the ESARAD erg-reader  module has to assume a unit. The chosen default unit is metre.

This default can be overridden by selecting a different unit using the command line option --source_length_unit

Cutting operations

Cutting or boolean operations are supported by the erg-reader  module, but with the following limitations:

The erg-reader  module stores the cutting operations in the STEP-TAS  dataset but does not evaluate them.

Note that any subsequent conversion of these cutting operations will only be "successful" if the destination tool format also supports cutting operations. If the destination tool does not support cutting operations the policy is to write out the "uncut" surface and comments that provide the definition of the surface that was supposed to be used as the cutting tool. The purpose of the comment is to supply as much information as possible so that the user can reconstruct the intended geometry in the destination tool using some other route if possible.

It is therefore recommended that users of tools that support cutting operations (such as ESARAD) should avoid the use of cutting operations if they know in advance that the model will need to be converted to a tool format that does not support cutting operations (such as THERMICA and TRASYS).

SHELL colours

The ESARAD colours specified for ESARAD SHELLs are converted to the STEP-TAS  colour definitions using RGB-values. 

ASSEMBLY variables

TASverter has been updated along with the STEP-TAS  protocol and libraries to support the Space Kinematic Model (SKM) module. The Esarad geometry reader and writer modules can now convert ASSEMBLY variables to and from the SKM representation. However, until the Space Mission Aspects (SMA) module has been implemented, the erg_writer  can only generate simple ASSEMBLY variables without mission-related orientation and pointing. TASverter does not handle any redefinition of the position and orientation of the ASSEMBLY components that may take place as a result of executing the ESARAD kernel after the ESARAD erg file was loaded.

In other words, the model that is created by TASverter will reflect the initial position and orientation of the ESARAD model when the ESARAD erg file is newly loaded into ESARAD, but before the analysis has been defined or run using the ESARAD kernel.

Note that converting an ESARAD model that contains an ASSEMBLY into a different output format will result in an non-kinematic model that will be equivalent to using a SHELL union to represent the ASSEMBLY.

Conversion of comments

The only comments which are stored in the STEP-TAS  dataset are those which precede the BEGIN_MODEL statement. These comments are stored as an information attribute of the STEP-TAS  geometrical model. All other comments are skipped.

The "label" attributes of the primitive SHELLs are the only descriptive elements that are transferred to the STEP-TAS dataset.

erg-writer  module

Activation of the erg-writer  module

The erg-writer  module of TASverter can be activated from the command line using the option:


When the TASverter GUI is used the "Model Type" of the "To" file must be set to "ESARAD erg" (see figure below):

Activation of erg-writer  module through the TASverter GUI

Figure 5: Activation of erg-writer  module through the TASverter GUI

Conversion from STEP-TAS  to ESARAD erg

Heading of the generated file

The generated ESARAD erg-file  starts with comments that describe:

When there is any descriptive information stored with the geometrical model, the erg-writer  module writes out this description just before the BEGIN_MODEL statement.

Length unit

The ESARAD erg language does not provide a means of specifying the length unit to be used by the model. TASverter assumes a default length unit of the metre for the length values in the generated ESARAD erg model. This default can be overridden using the --destination_length_unit  command line option to select a different unit.

If length unit conversion is applied, information about the conversion is included at the beginning of the generated model file in the form of a comment.

Variable names

The ESARAD modelling language uses variables. Examples of variable types are SHELL and OPTICAL. The erg-writer  obtains the variable names from the from the identifiers of the corresponding entities in the STEP-TAS  dataset. These STEP-TAS  identifiers may start with a digit, but ESARAD does not allow variable names which begin with a digit. If the STEP-TAS  identifier corresponding to an ESARAD OPTICAL variable starts with a digit or underscore, the erg-writer  prefixes the identifier with 'o_'. Similarly STEP-TAS  identifiers corresponding to ESARAD SHELL variables are prefixed with 's_'. If necessary, sequence numbers are appended to the generated variable names to avoid problems with duplicated names.

OPTICAL variables

The STEP-TAS  dataset contains information about optical properties using STEP-TAS  materials and material environments. If the STEP-TAS  dataset contains a single material environment, the erg-writer  converts the optical properties information into a set of ESARAD OPTICAL variables. If the STEP-TAS  dataset contains more than one material environments, the erg-writer  generates a series of IF-THEN-ELSE  constructs, with each branch representing one of the material environments and containing a different definition of the ESARAD OPTICAL variables.

The user can select which material environment to use during the ESARAD radiative analysis by entering a string variable in the Material Environment input field in the TASverterGUI. If no value is provided, the first material environment will be used by default.

Setting the Material Environment input field in the TASverterGUI is equivalent to specifying the TASverter option:


Note that once TASverter has created the ESARAD erg file, the user is free to edit the file to change the selected environment.

Generated primitives

ESARAD allows most geometrical primitives to be specified in two ways: by using points, or by using parameters (the so-called  SCS method).

The erg-writer  module generates primitives using the method that is closest to the STEP-TAS  representation of that primitive. This means that the following SHELL definition procedure calls are used for defining primitives in the target ESARAD erg file:

The positions and orientations of quadrilaterals, rectangles and triangles are defined by points using the SHELL_QUADRILATERAL, SHELL_RECTANGLE and SHELL_TRIANGLE primitives. The other primitives are defined by parameters and are then moved to the correct position and orientation using TRANSLATE and ROTATE calls.

Model hierarchy

In principle, ESARAD supports an infinite number of levels in the model hierarchy. This means that the complete hierarchy of the model in STEP-TAS  can be represented in the ESARAD geometry model.

Boolean cutting operations

All boolean cutting operations that are stored in the STEP-TAS  dataset are converted to the ESARAD erg language, except for cutting with an infinite cylinder. Currently no mapping is implemented for this cutting tool shape.

SHELL colours

ESARAD supports colour specification on both sides of a SHELL. Each ESARAD colour that the erg-writer  assigns is the one with the RGB-values  that most closely match the RGB-values  of the STEP-TAS  colour.

Non-matching  thermal node meshing

ESARAD SHELLs have associated parametric directions that are used when defining the thermal node mesh dimensions and the order in which the mesh elements are numbered. How these parametric directions are defined depends on the ESARAD SHELL types. The thermal node numbers for each side of a SHELL are defined using these parametric directions and mesh element order in combination with a thermal node number specification that defines the base node number and the node number increment.

It is possible that the thermal node mesh associated with a STEP-TAS  representation of a SHELL is incompatible with the ESARAD representation using base node number and node increment. In this case, the erg-writer  generates an equivalent set of single node sub-SHELLs. 

Radiation blocking surfaces

STEP-TAS  supports surfaces that act as pure radiation blockers. ESARAD does not have this kind of SHELL, and indeed ESARAD considers a SHELL where both sides are inactive to be "invisible" as far as the radiative analysis is concerned. In order to emulate the pure radiation blocking surfaces of STEP-TAS,  the erg-writer  converts them to SHELLs where both sides are active and have solar absorptance and infra-red  emissivity values equal to 1.0 so that they absorb all incident radiation. The thermal node number of the nodes meshed on the SHELL is set to the inactive node number.

The inactive node number can be specified using the Inactive Node input field in the TASverterGUI. This corresponds to the TASverter option:


The user must take care that this node is also declared as inactive, i.e. an X-node,  in the $NODES block of any ESATAN input file produced after the ESARAD radiative analysis of the generated erg file.

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