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Math Expressions

The math expression parser allows to use simple mathematical expressions in combination with if statements inside of the xml files of openCFS.

Defining loads

Math expressions can be used to define a synthetic load. For the following example a transient acoustic simulation (Singlefield -> AcousticPDE) is considered. The load is impressed onto a certain node, and defined as a sinus function

    <pressure name="excite" value="2*sin(2*pi*100*t)"/>

In this example a sinus load with 100 Hz and an amplitude of 2 is defined. t denotes the control variable. A different possibility for defining such a synthetic load is by using one of the custom functions of openCFS:

  • spike(duration, timeval)
  • fadeIn(duration, mode, timeval)
  • sinBurst(freq, nperiods, nfadein, nfadeout, timeval)
  • squareBurst(freq, nperiods, bipolar, pwidth, rtime, timeval)
  • gauss(mue, sigma, normval, timeval)
  • if(condition, trueval, elseval)

Following we will show two different examples for such custom functions:

sinBurst :

    <pressure name="excite" value="2*sinBurst(4400,5,2,2,t)")"/>

if statement:

    <pressure name="excite" value="(t lt 1)? (cos(2*pi*(t-0.5))+1)*sin(2*pi*t): 0")"/>

Furthermore, it is possible to superpose multiple different custom functions, with e.g. if statements. In the following example we superpose a time depending if statement with a spatial if statement

    <pressure name="exciteSurface" value="((t lt 1)? (cos(2*pi*(t-0.5))+1)*sin(2*pi*t): 0"))*((x lt 1)? (1 : 0))"/>

Hint: With multiple if statements it is especially important to look after the positioning of braces.

Defining fields

Math expressions can also be used for defining e.g flow, or temperature fields.

  <flow name="backward_Z"> 
   <comp dof="x" value="0"/>
   <comp dof="y" value="120000/60*2*pi*z"/> 
   <comp dof="z" value="-120000/60*2*pi*y"/>

This can also be used for defining blending functions, which e.g. necessary for the usage of non-conforming interfaces in combination with aeroacoustic source terms. For more details look into Singlefield -> AcousticPDE

Usable functions

  • exp(-1^t)
  • sin(2pi100*t)
  • cos(2pi100*t)