anchor.diff:     nu = u%TowerMotion%NNodes * 9            & ! 3 Translation Displacements + 3 orientations + 3 Translation velocities at each node
         + u%hubMotion%NNodes   * 9            & ! 3 Translation Displacements + 3 orientations + 3 Rotation velocities at each node
         + size( u%InflowOnBlade)              &
  -      + size( u%InflowOnTower)
  +      + size( u%InflowOnTower)              &
  +      + size( u%UserProp)
   
      do i=1,p%NumBlades
         nu = nu + u%BladeMotion(i)%NNodes * 9 & ! 3 Translation Displacements + 3 orientations + 3 Translation velocities at each node
  
positive.code:       end do !j      
     end do !i
     
     
        !......................................
        ! default perturbations, p%du:
        !......................................
  
negative.code:       
     CASE (22) !Module/Mesh/Field: u%InflowOnTower(:,:)   = 22;
        u%InflowOnTower(fieldIndx,node) = u%InflowOnTower(fieldIndx,node) + du * perturb_sign
     CASE (23) !Module/Mesh/Field: u%UserProp(:,1)   = 23; 
        u%UserProp(node,1) = u%UserProp(node,1) + du * perturb_sign
     CASE (24) !Module/Mesh/Field: u%UserProp(:,2)   = 23; 
        u%UserProp(node,2) = u%UserProp(node,2) + du * perturb_sign
     CASE (25) !Module/Mesh/Field: u%UserProp(:,3)   = 23; 
        u%UserProp(node,3) = u%UserProp(node,3) + du * perturb_sign
     END SELECT
        
  END SUBROUTINE Perturb_u
  