Initialization of Auxiliary Dependent Variables
In addition to user-defined variables that can be inserted into a model using the Auxiliary Dependent Variable feature, many physics features and physics interface settings automatically define built-in auxiliary dependent variables for quantities such as particle temperature, mass, and number of collisions. The particle release features include options to control the way in which these auxiliary dependent variables are initialized when particles are released into the model.
Controlling the Order in which Variables are Initialized
When assigning initial values to particle degrees of freedom, the components of the particle position vector are always assigned first. The initial values of the remaining degrees of freedom, which may include velocity or momentum as well as auxiliary dependent variables, may depend on the initial particle position. In addition, the initial value of any variable may be defined in terms of other variables that are initialized before it.
By default, user-defined auxiliary dependent variables are initialized after all other particle variables. However, in the Initial Value of Auxiliary Dependent Variables section of the Settings window for each release feature, the text field for the initial value of each auxiliary dependent variable is accompanied by a check box, Initialize before particle momentum. When this check box is selected, the corresponding variable is initialized immediately after the particle position vector components and before any other degrees of freedom. If multiple user-defined auxiliary dependent variables are present, the order in which they are initialized is determined by their relative position in the model tree; variables that appear earlier in the model tree are evaluated before variables that appear below them.
The particle degrees of freedom are then initialized in the following order:
1
Position vector components.
2
User-defined auxiliary dependent variables for which the Initialize before particle momentum check box has been selected. These variables are initialized in the order in which the Auxiliary Dependent Variable nodes appear in the model tree.
3
Particle mass or diameter (Particle Tracing for Fluid Flow interface only).
4
Particle temperature (Particle Tracing for Fluid Flow interface only).
5
Particle velocity or momentum.
6
Other built-in auxiliary dependent variables, including the out-of-plane degrees of freedom in a 2D and 2D axisymmetric geometry. This also includes collision counters created by the Collisions node and the various collision types that can be added to it (Charged Particle Tracing interface only).
7
User-defined auxiliary dependent variables for which the Initialize before particle momentum check box is cleared.
It is possible to express the initial values of auxiliary dependent variables in terms of other particle degrees of freedom, including position, velocity, and other auxiliary dependent variables. However, particle degrees of freedom can only be defined in terms of other degrees of freedom that are initialized before them; that is, quantities that appear earlier in the above list.
Releasing Distributions of Auxiliary Dependent Variables
It is possible to assign a single initial value to an auxiliary dependent variable or to sample multiple values from a distribution. The type of distribution can be selected using the Distribution function setting in the Initial Value of Auxiliary Dependent Variables section of all release features. Select from the following built-in distributions:
•
None: Enter a single value or expression.
•
Normal, Lognormal, or Uniform: Sample a number of values from the chosen distribution type. Enter the Mean (default 0) and Standard deviation (default 1).
For example, it is possible to create a set of uniformly distributed numbers between 0 and 1 by selecting the Uniform distribution and entering a Mean of 0.5 and a Standard deviation of sqrt(1/12).
•
List of values: Type a list of numeric values directly. The valued do not need to be at regular intervals and do not need to be entered in any particular order.