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userfunctions:coulomb [Elmer/Ice Wiki]

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userfunctions:coulomb [2012/11/15 02:26]
gag [General Description]
userfunctions:coulomb [2015/12/02 12:24]
gag [General Description]
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 ==== General Informations ==== ==== General Informations ====
-  * **USF Fortran File:** //USF_Sliding.f90// +  * **USF Fortran File:** ''USF_Sliding.f90'' 
-  * **USF Name:** //Friction_Coulomb// +  * **USF Name:** ''Friction_Coulomb'' 
-  * **Required Input Variable(s):** A //Flow Solution// in //Flow Solution Name////Normal Vector////Stress//+  * **Required Input Variable(s):** A ''Flow Solution'' in ''Flow Solution Name''''Normal Vector''''Stress'' or the ''Effective Pressure'' variable.
  
  
 ==== General Description ==== ==== General Description ====
-The file //USF_Sliding.f90// contains two user functions to apply non-linear friction at the base of glacier.  +The file ''USF_Sliding.f90'' contains three user functions to apply non-linear friction at the base of glacier.  
  
-The first user function (//Sliding_Weertman//) is a non-linear Weertman-type friction law and is described [[:userfunctions:weertman|here]]. The second user function (//Friction_Coulomb//) is a non-linear water pressure dependant friction law, as proposed by Schoof (2005) and Gagliardini et al. (2007), and is presented in this page.+The first user function (''Sliding_Weertman'') is a non-linear Weertman-type friction law and is described [[:userfunctions:weertman|here]]. The second user function (''Friction_Coulomb'') is a non-linear water pressure dependant friction law, as proposed by Schoof (2005) and Gagliardini et al. (2007), and is presented in this page. The third user function (''Sliding_Budd'') is described [[:userfunctions:budd|here]] and is from Budd et al 1984 (Annals of Glaciology 5, page 29-36)
      
  
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 <m>  chi = {u_b}/{C^n N^n A_s}  </m>\\ <m>  chi = {u_b}/{C^n N^n A_s}  </m>\\
  
-The +The //Slip Coefficient// in Elmer is then given as\\ 
 +<m>  C.N {[{ {chi . {u_b}^{-n} }/ {(1 + a . chi^q)} }]}^{1/n}  </m>\\ 
 +When  <m>u_b < u_{t0}</m>, <m>u_b</m> in the previous equation is replaced by <m>u_{t0}</m>
  
 The parameters to be given are:\\ The parameters to be given are:\\
-  * Friction Law Sliding Coefficient      -> <m>A_s</m> +  * ''Friction Law Sliding Coefficient''      -> <m>A_s</m> 
-  * Friction Law Post-Peak Exponent       -> <m>q >= 1</m> +  * ''Friction Law Post-Peak Exponent''       -> <m>q >= 1</m> 
-  * Friction Law Maximum Value            -> <m>C</m> ~ max bed slope +  * ''Friction Law Maximum Value''            -> <m>C</m> ~ max bed slope 
-  * Friction Law Exponent        -> m = (n Glen's law) +  * ''Friction Law Exponent''       -> m = (n Glen's law) 
-  * Friction Law Linear Velocity         +  * ''Friction Law Linear Velocity'' -> <m>u_{t0}</m>   
 + 
 +The effective pressure is defined as <m>N = -sigma_{nn} -p_w</m>,  where <m>sigma_{nn}</m> is the normal Cauchy stress and <m>p_w</m> the water pressure. If a variable ''Effective Pressure'' exists, it is used to evaluate directly <m>N</m>. Else, the normal Cauchy stress is estimated from the stress computed at previous timestep. The water pressure is prescribed as an ''External Pressure'' (Negative - Compressive convention, and therefore 'External Pressure' should be equal to the opposite of the water pressure in the sif).  
 +      
    
  
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   Normal-Tangential Velocity = Logical True   Normal-Tangential Velocity = Logical True
   Flow Force BC = Logical True   Flow Force BC = Logical True
 +   
 +  !! Water pressure given through the Stokes 'External Pressure' parameter  
 +  !! (Negative = Compressive) 
 +  External Pressure = Equals Water Pressure 
 +   
   Velocity 1 = Real 0.0   Velocity 1 = Real 0.0
      
   Slip Coefficient 2 =  Variable Coordinate 1   Slip Coefficient 2 =  Variable Coordinate 1
-    Real Procedure "./USF_Sliding" "Friction_Coulomb"+    Real Procedure "ElmerIceUSF" "Friction_Coulomb"
   Slip Coefficient 3 =  Variable Coordinate 1   Slip Coefficient 3 =  Variable Coordinate 1
-    Real Procedure "./USF_Sliding" "Friction_Coulomb"+    Real Procedure "ElmerIceUSF" "Friction_Coulomb"
          
   !! Parameters needed for the Coulomb Friction Law   !! Parameters needed for the Coulomb Friction Law
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   Friction Law Maximum Value = Real 1.0            !(C=1)   Friction Law Maximum Value = Real 1.0            !(C=1)
   Friction Law PowerLaw Exponent = Real 3.0        !(m = n = 3 Glen's law)    Friction Law PowerLaw Exponent = Real 3.0        !(m = n = 3 Glen's law) 
-  Friction Law Linear Velocity = Real 0.01         !(=1m/a dans un premier temps)+  Friction Law Linear Velocity = Real 0.01         
 End End
 </code> </code>
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 ==== Examples ==== ==== Examples ====
-An example of the usage of the user function //Friction_Weertman// TODO +The Coulomb friction law is tested in ''[ELMER_TRUNK]/elmerice/Tests/Friction_Coulomb'' with a direct input of the effective pressure and ''[ELMER_TRUNK]/elmerice/Tests/Friction_Coulomb_Pw'' with the effective pressure computed from the stress and a prescribed water pressure.  
  
  
userfunctions/coulomb.txt · Last modified: 2015/12/02 12:24 by gag
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