Unit systems in RADIOSS - Usage of /BEGIN and /UNIT
Altair Employee
The units set at the two last lines of /BEGIN can have several roles :
1. Unit conversion of the input parameters
If the two lines are different (not matching), Radioss will consider that the input data (the input model) are defined using the unit system set at the first line and will perform a unit conversion to the unit system set at the second line (working unit system). Then, the converted data will be used to run the simulation and output results.
If the two lines are identical, the data is processed without unit conversion/modification, except if one or more /UNIT keywords are set in the model.
It is possible to define one or several other unit systems with /UNIT and to refer to it for any Radioss keyword, in setting the unit_id (for example with a material type 2: /MAT/ LAW2/mat_ID/unit_ID) to specify that this particular keyword is defined using this unit system and that Radioss must convert the input parameters of this keyword in the working unit system which is the one defined at the last line of /BEGIN.
By using //SUBMODEL with the reference to a /UNIT id, it is even possible to define a whole part of the model input in a different unit system.
These unit conversion mechanisms can be very useful but also tricky. It is easy to be lost in the unit systems at some point. Thus, it is maybe easier and safer to handle manually the conversion of the input parameters in the working unit system before setting them in the Radioss model.
(Note: Some preprocessing softwares such as HyperWorks are also offering unit conversion capabilities using the information available in /BEGIN.)
2. Unit conversion of Radioss embedded data
Except for the conversion mechanisms detailed above and for some special features/keywords for which the behaviour is specified in the help, the vast majority of Radioss commands are not doing anything with the unit systems defined in /BEGIN and it is up to the user to pay attention to make the parameters given in input consistent in one unit system and it is always up to the user to determine what are the units of the results, based on the units chosen as input. The Radioss documentation indicates only the dimensions of parameters (i.e. length, time, mass or any combination of the three) by providing an example of units in the column "SI Unit Example" in the parameter definition tables of most keywords. There is only a very small number of keywords which have to take into account the units indicated in /BEGIN, such as keywords that use embedded experimental data/tables or default physical parameters. In these cases, it is indicated in the documentation. The keyword /LOAD/PBLAST is such an example.
3. Saving/providing the information about which unit system is used in the model
To end this list with probably the first common usage: indicating units in /BEGIN is also a good practice to allow another person to know which unit system is used for the model or to remember it even when you are the creator of the model. You can also prefer to set all the units at 1 in /BEGIN and indicate the unit system used in the model title or as a comment at the beginning of the input deck. But it is important that this information is somewhere to avoid having to guess it with the parameter values. It is mandatory to have this information to be able to modify correctly the model but also to just be able to analyse the results !