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Loops

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lukas edited this page May 19, 2017 · 1 revision

Table of Contents

This page is about creating loops with FDSgeogen.

Basics

A loop tag in FDSgeogen has the following basic structure:

<loop var="loop_variable_name" start="start_value" stop="stop_value" >
    
    <!-- loop body -->

</loop>

The above loop defines:

  • The name of the new variable (attribute var, here with the value loop_variable_name) which will be used to iterate the loop values.
  • The starting and ending values of the loop variable, here start_value and stop_value. The value of the loop variable starts with the start_value is incremented by 1 until it reaches the value stop_value.
  • The instructions inside the loop tag are evaluated for each iteration.

Basic Example

FDSgeogen allows to use loops via the loop tag. A simple loop tag defines a loop variable and the loop range (start and stop). The example file basic_loop.xml illustrates a simple loop:

<fds>

    <!-- file naming -->
    <info chid="'basic_loop'" outfile="'basic_loop.fds'"/>

    <!-- the following loop iterates the value of the variable 'count' from 1 to 10 -->
    <loop var="count" start="1" stop="10">

        <!-- print the following message for each loop iteration -->
        <dbg print="'value of count: %d'%count" />

    </loop>

</fds>

The above example defines a loop with following properties:

  • the loop variable, i.e. the variable which values will be changed from iteration to iteration, is named count
  • the starting value of the loop variable is 1, it is increased by 1 until it reaches the last value of 10
  • for each iteration, the loop body (here the tag dbg) is evaluated

The output of the above example is a dummy FDS input file (basic_loop.fds) and at the console the following lines:

[...]
value of count: 1
value of count: 2
value of count: 3
value of count: 4
value of count: 5
value of count: 6
value of count: 7
value of count: 8
value of count: 9
value of count: 10

Example with Obstructions

While the above example did just create some debug output, the following one shows the creation of obstacles via a loop. See obst_loop.xml.

<fds>

    <!-- file naming -->
    <info chid="'obst_loop'" outfile="'obst_loop.fds'"/>

    <!-- fixed input parameter -->
    <input text="TIME T_END=0.0" />

    <!-- create mesh -->
    <fds_mesh ijk="32, 32, 32" xb="0, 32, 0, 32, 0, 32" />


    <!-- create obstractions in a loop -->
    <!-- position and height as a function of the loop variable 'count' -->
    <loop var="count" start="1" stop="10">

        <dbg print="'value of count: %d'%count" />

        <var x="10+count" z="count * 2" />

        <fds_obst xb="x, x+1.0, 0, 32, 0, z"/>

    </loop>

</fds>

Explanation:

  • The part before the loop creates a basic FDS input file structure.
  • The loop iterates the count variable from 1 to 10.
  • The body of the loop contains
  • a simple dbg output
  • the computation of two new variables x and z which will determine the position in x-direction and the height of the obstructions to be created
  • the creation of a obstruction as a function of x and z

The resulting FDS input file is

&HEAD CHID='obst_loop', TITLE='title' /
&TIME T_END=0.0 /
&MESH ID='none', IJK=32, 32, 32, XB=0, 32, 0, 32, 0, 32/
&OBST ID='none', XB=11, 12.000000, 0, 32, 0, 2/
&OBST ID='none', XB=12, 13.000000, 0, 32, 0, 4/
&OBST ID='none', XB=13, 14.000000, 0, 32, 0, 6/
&OBST ID='none', XB=14, 15.000000, 0, 32, 0, 8/
&OBST ID='none', XB=15, 16.000000, 0, 32, 0, 10/
&OBST ID='none', XB=16, 17.000000, 0, 32, 0, 12/
&OBST ID='none', XB=17, 18.000000, 0, 32, 0, 14/
&OBST ID='none', XB=18, 19.000000, 0, 32, 0, 16/
&OBST ID='none', XB=19, 20.000000, 0, 32, 0, 18/
&OBST ID='none', XB=20, 21.000000, 0, 32, 0, 20/
&TAIL/

Example with devices

The following example extends the simple burner setup by a variable device placement. This illustrates how to define a series of devices as a function of prescribed values, here the position of a fire, in a loop. The relevant parts are:

<fds>

    <!-- variables for the position and diameter of the burner -->
    <var xpos = "0.6"  ypos = "1.2" diameter = "0.4"/>

    [...]

    <!--================================
    ========= fire definition ==========
    =================================-->

    <!-- definition of the reaction -->
    <fds_reac id="'METHANE'" fuel="'METHANE'" soot_yield="0.05"/> 

    <!-- burning surface -->
    <fds_surf id="'burner'" hrrpua="1000" />
    
    <!-- creating and positioning the burner -->
    <fds_obst xb="xpos-diameter/2., xpos+diameter/2., ypos-diameter/2., ypos+diameter/2., 0.0, 0.4" surf_ids="'burner', 'INERT', 'INERT'"/>
    
    <!--================================
    ======== output definition =========
    =================================-->    
    
    <!-- creates 10 temperature devices with vertical distance of 0.1 m above the center of the burner. The lowest device is in a height of 0.5m -->
    <loop var="z" start="0" stop="9">
        <devc id="'TEMP_z%i'%(z)" x="xpos" y="ypos" z="0.5+0.1*z" q="'TEMPERATURE'" />
    </loop>
	
    <fds_slcf pby="ypos" quantity="'VELOCITY'" vector="True" />

</fds>

Explanation:

  • As previously described, the position and extension of the fire in the ''simple burner'' example is defined by xpos, ypos and diameter.
  • The fire definition is done in the "fire definition" section.
  • The "output definition" section defines two outputs:
  • Ten temperature devices with the following properties are defined as a function of the loop variable z
  • The device id depends on the loop variable
  • The position in x- and y-direction depends on the fire placement
  • The height, i.e. the position in z-direction, is a function of z
  • A single velocity slice, which is placed according to the fire position
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