#! /usr/bin/env python
#
def resid_poiseuille ( nu, rho, n, x, y, t ):

#*****************************************************************************80
#
## RESID_POISEUILLE returns Poiseuille residualss.
#
#  Location:
#
#    http://people.sc.fsu.edu/~jburkardt/py_src/navier_stokes_2d_exact/resid_poiseuille.py
#
#  Licensing:
#
#    This code is distributed under the GNU LGPL license.
#
#  Modified:
#
#    22 July 2015
#
#  Author:
#
#    John Burkardt
#
#  Parameters:
#
#    Input, real NU, the kinematic viscosity.
#
#    Input, real RHO, the density.
#
#    Input, integer N, the number of points at which the solution is to
#    be evaluated.
#
#    Input, real X(N), Y(N), the coordinates of the points.
#
#    Input, real T(N), the time coordinate or coordinates.
#
#    Output, real UR(N), VR(N), PR(N), the residuals in the U, V and P equations.
#
  import numpy as np
  from rhs_poiseuille import rhs_poiseuille

  ur = np.zeros ( n )
  vr = np.zeros ( n )
  pr = np.zeros ( n )
#
#  Get the right hand side functions.
#
  f, g, h = rhs_poiseuille ( nu, rho, n, x, y, t );
#
#  Form the functions and derivatives for the left hand side.
#
  u = 1.0 - y ** 2
  dudt = 0.0
  dudx = 0.0
  dudxx = 0.0
  dudy = - 2.0 * y
  dudyy = - 2.0

  v = 0.0
  dvdt = 0.0
  dvdx = 0.0
  dvdxx = 0.0
  dvdy = 0.0
  dvdyy = 0.0

  p = - 2.0 * nu * rho * x
  dpdx = - 2.0 * nu * rho
  dpdy = 0.0
#
#  Evaluate the residuals.
#
  ur = dudt - nu * ( dudxx + dudyy ) \
    + u * dudx + v * dudy + dpdx / rho - f

  vr = dvdt - nu * ( dvdxx + dvdyy ) \
    + u * dvdx + v * dvdy + dpdy / rho - g

  pr = dudx + dvdy - h

  return ur, vr, pr

def resid_poiseuille_test ( ):

#*****************************************************************************80
#
## RESID_POISEUILLE_TEST samples the Poiseuille residual.
#
#  Licensing:
#
#    This code is distributed under the GNU LGPL license.
#
#  Modified:
#
#    30 January 2015
#
#  Author:
#
#    John Burkardt
#
  import numpy as np
  import platform
  from r8vec_uniform_ab import r8vec_uniform_ab

  nu = 1.0
  rho = 1.0

  print ( '' )
  print ( 'RESID_POISEUILLE_TEST' )
  print ( '  Python version: %s' % ( platform.python_version ( ) ) )
  print ( '  Poiseuille Flow:' )
  print ( '  Sample the Navier-Stokes residuals' )
  print ( '  at the initial time T = 0, over a channel region.' )
  print ( '  Kinematic viscosity NU = %g' % ( nu ) )
  print ( '  Fluid density RHO = %g' % ( rho ) )

  n = 1000
  x_lo = 0.0
  x_hi = 6.0
  y_lo = -1.0
  y_hi = +1.0
  seed = 123456789
  x, seed = r8vec_uniform_ab ( n, x_lo, x_hi, seed )
  y, seed = r8vec_uniform_ab ( n, y_lo, y_hi, seed )
  t = 0.0

  ur, vr, pr = resid_poiseuille ( nu, rho, n, x, y, t )

  print ( '' )
  print ( '           Minimum       Maximum' )
  print ( '' )
  print ( '  Ur:  %14.6g  %14.6g' % ( np.min ( np.abs ( ur ) ), np.max ( np.abs ( ur ) ) ) )
  print ( '  Vr:  %14.6g  %14.6g' % ( np.min ( np.abs ( vr ) ), np.max ( np.abs ( vr ) ) ) )
  print ( '  Pr:  %14.6g  %14.6g' % ( np.min ( np.abs ( pr ) ), np.max ( np.abs ( pr ) ) ) )
#
#  Terminate.
#
  print ( '' )
  print ( 'RESID_POISEUILLE_TEST:' )
  print ( '  Normal end of execution.' )
  return

if ( __name__ == '__main__' ):
  from timestamp import timestamp
  timestamp ( )
  resid_poiseuille_test ( )
  timestamp ( )