# How do you set up a subdomain for a source that grows in size?

 Subdomains do not normally change size but it is possible to set a subdomain to be the whole domain and use a CEL expression to define the source volume. For example the following partial CEL computes a volume inside a cylinder of radius 0.3m and height 0.3m. The CEL variable insideSource will be 1 inside the source region and zero outside it. It is a simple matter to modify this expression to be a function of time.LIBRARY: CEL: EXPRESSIONS: z0 = 0.0 [m] x0 = 0.5 [m] y0 = 0.5 [m] sourceRadius = 0.3 [m] sourceHeight = 0.3 [m] unitL = 1 [m] rSource = sqrt((x-x0)^2 + (y-y0)^2) hSource = z-z0 insideSource = step((sourceRadius-rSource)/unitL)*step((sourceHeight-hSource)unitL) source = insideSource * 100 [W m^-3] END ENDEND>updateFLOW: DOMAIN: Domain 1 SUBDOMAIN: Subdomain 1 SOURCES: EQUATION SOURCE: energy Option = Source Source = source END END END ENDEND>updateIt is also possible to compute the actual volume of the source region. This may be important if a total source is required and/or the source volume includes non fluid regions such as cut outsolids. Note that you cannot use the 'Total Source' option in CFX-Pre because that will divide the source value by the volume of the subdomain which is now the whole domain. Instead, a variable can be defined to be equal to the insideSource variable and a CEL expression can be used to compute the volume integral of this variable. It is a good idea to set an appropriate minimum volume to prevent a division by zero error:LIBRARY: ADDITIONAL VARIABLE: SourceFlag Option = Definition Tensor Type = SCALAR Units = [ ] Variable Type = Unspecified END CEL: EXPRESSIONS: volSource = volumeInt(SourceFlag)@Subdomain 1 minVolume = 0.001 [m^3] source = insideSource * 100[W] / max(volSource, minVolume) END ENDENDFLOW: DOMAIN: Domain 1 FLUID MODELS: ADDITIONAL VARIABLE: SourceFlag Additional Variable Value = insideSource Option = Algebraic Equation END END ENDEND>update