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~~NOTOC~~
===== User Function USF_Zs =====

==== General Informations ====
  * **USF Fortran File:** ''USF_Zs.f90''
  * **USF Name:** ''ZsIni'', ''ZsMZsIni'', ''ZsTopIni'', ''ZsTopMZsIni'',...  
  * **Required Input Variable(s):** ''Zs'' or ''Zs Top'' or ''Zs Bottom''


==== General Description ====
The aim of these user functions is to get the appropriate value of the mesh update from the surface displacement.  

The first type of user function (''~Ini'') is used to initialize the free surface elevation (surface elevation equals to the altitude of the nodes belonging on this surface). The second type (''~MZsIni'') calculate the mesh update variable as the new surface elevation minus the initial elevation. 

As presented [[tips:freesurface|here]], these user functions can be easily replaced by MATC functions (recalling that a MATC function consumes more cpu than a user function).  


==== SIF contents ====
The required keywords in the SIF file for this solver are:
<code>
Initial Condition 2
  Zs Top = Variable Coordinate 3
    Real Procedure "ElmerIceUSF" "ZsTopIni"
End

Initial Condition 3
  Zs Bottom = Variable Coordinate 3
    Real Procedure "ElmerIceUSF" "ZsBottomIni"
End

! Bottom boundary condition
Boundary Condition 1
  Target Boundaries = 1
!!! this BC is equal to body no. 3 !!!
  Body Id = 3
  
  Mesh Update 1 = Real 0.0
  Mesh Update 2 = Real 0.0
  Mesh Update 3 = Variable Zs Bottom
    Real Procedure "ElmerIceUSF" "ZsBottomMzsIni"
End

! Upper Surface
Boundary Condition 2
  Target Boundaries = 2
!!! this BC is equal to body no. 2 !!!
  Body Id = 2

  Mesh Update 1 = Real 0.0
  Mesh Update 2 = Real 0.0
  Mesh Update 3 = Variable Zs Top
    Real Procedure "ElmerIceUSF" "ZsTopMzsIni"
End
</code>


==== Examples ====
Examples of the usage of these user functions can be found in the [[:courses:courses|Elmer/Ice course material]] (ISMIP application for one upper free surface and  Tête Rousse application for an upper and lower free surfaces).