3.6. Scheme Macros

The text interface of Ansys Fluent executes a Scheme interpreter, which allows you to define your own variables that can be stored in Ansys Fluent and accessed via a UDF. This capability can be very useful, for example, if you want to alter certain parameters in your case, and you do not want to recompile your UDF each time. Suppose you want to apply a UDF to multiple zones in a mesh. You can do this manually by accessing a particular Zone ID in the graphical user interface, hard-coding the integer ID in your UDF, and then recompiling the UDF. This can be a tedious process if you want to apply the UDF to a number of zones. By defining your own Scheme variable, if you want to alter the variable later, then you can do it from the text interface using a Scheme command.

Macros that are used to define and access user-specified Scheme variables from the text interface are identified by the prefix rp, (for example, rp-var-define). Macros that are used to access user-defined Scheme variables in an Ansys Fluent solver, are identified by the prefix RP (for example, RP_Get_Real). These macros are executed within UDFs.

For more information, see the following sections:

 

3.6.1. Defining a Scheme Variable in the Text Interface

To define a Scheme variable named user/pres_av/thread-id in the text interface, you can use the following Scheme command:

   (rp-var-define ’user/pres_av/thread-id 2 ’integer #f)

This will create the Scheme variable as an integer type and assign it the initial value 2. Your Scheme variable and its value will be saved and restored with the case file. Other types besides 'integer include 'real, 'boolean, and 'string.

Typically, you should avoid redefining an existing Scheme variable. The following Scheme command will only define a new Scheme variable if there is not an existing Scheme variable with the same name, therefore it is preferred over the (rp-var-define '…) command:

   (make-new-rpvar 'user/pres_av/thread-id 2 'integer)

This command first checks that the variable user/pres_av/thread-id is not already defined, and then sets it up as an integer with an initial value of 2. If a Scheme variable with this name is already defined, its value will not be changed/overwritten by this command.

Note that the string ’/’ is allowed in Scheme variable names (as in user/pres_av/thread-id), and is a useful way to organize variables so that they do not interfere with each other.

 

3.6.2. Accessing a Scheme Variable in the Text Interface

After you define a Scheme variable in the text interface, you can access the variable. For example, if you want to check the current value of the variable (for example, user/pres_av/thread-id) on the Scheme side, you can type the following command in the text window:

 (%rpgetvar ’user/pres_av/thread-id)

Important:  It is recommended that you use %rpgetvar when you are retrieving an Ansys Fluent variable using a Scheme command. The % symbol forces Ansys Fluent to check for the stored value of the variable instead of the buffered value. Access to buffered values using rpgetvar (without the % symbol) is faster, but when the RP variable is changed from the C side (a UDF or Ansys Fluent), the buffered value might be outdated. Only in instances where the variable is not changed from the C side, can you neglect the % symbol and use the buffered value with confidence.

 

3.6.3. Changing a Scheme Variable to Another Value in the Text Interface

Alternatively, if you want to change the value of the variable you have defined (user/pres_av/thread-id) to say, 7, then you will need to use rpsetvar and issue the following command in the text window:

 (rpsetvar ’user/pres_av/thread-id 7)
 

3.6.4. Accessing a Scheme Variable in a UDF

After a new variable is defined on the Scheme side (using a text command), you will need to bring it over to the solver side to be able to use it in your UDF. ‘ RP’ macros are used to access Scheme variables in UDFs, and are listed below.

RP_Variable_Exists_P("variable-name")

Returns true if the variable exists

RP_Get_Real("variable-name")

Returns the double value of variable-name

RP_Get_Integer("variable-name")

Returns the integer value of variable-name

RP_Get_String("variable-name")

Returns the const char* value of variable-name

RP_Get_Boolean("variable-name")

Returns the Boolean value of variable-name

Get_Input_Parameter("variable-name")

Returns the input parameter value of variable-name

For example, to access the user-defined Scheme variable user/pres_av/thread-id in your UDF C function, you will use RP_Get_Integer. You can then assign the variable returned to a local variable you have declared in your UDF (for example, surface_thread_id) as demonstrated below:

  surface_thread_id = RP_Get_Integer("user/pres_av/thread-id");

You can also change a Scheme variable in your UDF using the following RP macros:

RP_Set_Real("variable-name", ...)

Sets the double value of variable-name

RP_Set_Integer("variable-name", ...)

Sets the integer value of variable-name

RP_Set_String("variable-name", ...)

Sets the const char* value of variable-name

RP_Set_Boolean("variable-name", ...)

Sets the Boolean value of variable-name

Important:  When you change the value of a Scheme variable in a UDF using RP_Set_..., it must be done on every process (host and nodes) that will access the value with the RP_Get_... macro. Ensure that the value is synchronized between the node(s) and host processes before using RP_Set_....

To enable the consistent use of the changed value on the Scheme side, RP_Set_... must be called on the host process and the Scheme side must use (%rpgetvar '…) (refer to Accessing a Scheme Variable in the Text Interface for more information).