/*====================================================================================
! Purpose(s): 
!
! Contain(s): CSetFricVel
!
! Author(s):  Javier Lopez Lara (jav.lopez@unican.es)
!             Gabriel Barajas   (barajasg@unican.es)
!====================================================================================*/

#include "CSetFricVel.h"
#include "CVars.h"


void CSetFricVel(double &AUx, double &AUy, double &AVx, double &AVy, double AFricVel, double ADist, TVector const &AObsSrfce)
/*!=======================================================================
! Purpose(s): This function modifies the velocity gradient tensor in the vicinity of a surface.
!             It rotates the tensor to a coordinate system where surface is horizontal and the surface vector is (0,1).
!             Then it substitutes the du'/dy' tensor component by the value predicted by the logarithmic law profile
!             (FricVel/(Kappa*y)) and rotates the tensor back to the original coordinate system.
!
! Author(s):  Javier Lopez Lara (jav.lopez@unican.es)
!             Gabriel Barajas   (barajasg@unican.es)
!=======================================================================*/
{
  
  // Components of the unitary tangent vector.
  double tx= AObsSrfce.y;
  double ty=-AObsSrfce.x;

  double tx2 = tx*tx;
  double ty2 = ty*ty;

  double txty = tx*ty;

  double tx2ty2 = tx2*ty2;

  double tx3ty = tx2*txty;
  double txty3 = ty2*txty;

  double Uyp = AFricVel/(Kappa*ADist);

  double newUx=-txty3*(AUy+AVx)+tx2ty2*(AVy-AVx)+txty*(AUy-Uyp)+AUx;
  double newUy=tx2ty2*(AUy+AVx)+AUy*ty2+tx3ty*(AVx-AVy)+Uyp*tx2;
  double newVx=tx2ty2*(AUy+AVx)-Uyp*ty2+tx3ty*(AVx-AVy)+txty*(AVy-AVx)+AVx*tx2;
  double newVy=-tx3ty*(AUy+AVx)+txty*(Uyp+AVx)+tx2*tx2*(AVy-AVx)+tx2*(AVx-AVy)+AVy;

  AUx=newUx;
  AUy=newUy;
  AVx=newVx;
  AVy=newVy;
}