"""Deformation of a square droplet. (15 minutes)


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Initially, two two fluids of the same density are distinguished by a
color index assigned to them and allowed to settle under the effects
of surface tension. It is expected that the surface tension at the
interface between the two fluids deforms the initially square droplet
into a cirular droplet to minimize the interface area/length.

The references for this problem are

 - J. Morris "Simulating surface tension with smoothed particle
   hydrodynamics", 2000, IJNMF, 33, pp 333--353 [JM00]

 - S. Adami, X.Y. Hu, N.A. Adams "A new surface tension formulation
   for multi-phase SPH using a reproducing divergence approximation",
   2010, JCP, 229, pp 5011--5021 [AHA10]

 - M. S. Shadloo, M. Yildiz "Numerical modelling of Kelvin-Helmholtz
   instability using smoothed particle hydrodynamics", IJNME, 2011,
   87, pp 988--1006 [SY11]

The surface-tension model used currently is the CSF model based on
interface curvature and normals computed from the color function.

"""
import numpy

# Particle generator
from pysph.base.utils import get_particle_array
from pysph.base.kernels import CubicSpline, WendlandQuintic, Gaussian, QuinticSpline

# SPH Equations and Group
from pysph.sph.equation import Group

from pysph.sph.wc.viscosity import ClearyArtificialViscosity

from pysph.sph.wc.transport_velocity import SummationDensity, MomentumEquationPressureGradient,\
    SolidWallPressureBC, SolidWallNoSlipBC, StateEquation,\
    MomentumEquationArtificialStress, MomentumEquationViscosity

from pysph.sph.surface_tension import InterfaceCurvatureFromNumberDensity, \
    ShadlooYildizSurfaceTensionForce, CSFSurfaceTensionForce, \
    SmoothedColor, AdamiColorGradient, MorrisColorGradient, SY11ColorGradient, \
    SY11DiracDelta, AdamiReproducingDivergence

from pysph.sph.gas_dynamics.basic import ScaleSmoothingLength

# PySPH solver and application
from pysph.solver.application import Application
from pysph.solver.solver import Solver

# Integrators and Steppers
from pysph.sph.integrator_step import TransportVelocityStep
from pysph.sph.integrator import PECIntegrator

# Domain manager for periodic domains
from pysph.base.nnps import DomainManager

# problem parameters
dim = 2
domain_width = 1.0
domain_height = 1.0

# numerical constants
sigma = 1.0

# set factor1 to [0.5 ~ 1.0] to simulate a thick or thin
# interface. Larger values result in a thick interface.
factor1 = 0.8
factor2 = 1./factor1

# discretization parameters
dx = dy = 0.0125
dxb2 = dyb2 = 0.5 * dx
volume = dx*dx
hdx = 1.3
h0 = hdx * dx
rho0 = 1000.0
c0 = 20.0
p0 = c0*c0*rho0
nu = 1.0/rho0

# correction factor for Morris's Method I. Set with_morris_correction
# to True when using this correction.
epsilon = 0.01/h0

# time steps
tf = 1.0
dt_cfl = 0.25 * h0/( 1.1*c0 )
dt_viscous = 0.125 * h0**2/nu
dt_force = 1.0

dt = 0.9 * min(dt_cfl, dt_viscous, dt_force)

class SquareDroplet(Application):
    def add_user_options(self, group):
        choices = ['adami', 'morris', 'shadloo']
        group.add_option(
            "--scheme", action="store", dest="scheme", default='morris',
            type="choice", choices=choices,
            help="Specify scheme to use among %s"%choices
        )

    def create_particles(self):
        x, y = numpy.mgrid[ dxb2:domain_width:dx, dyb2:domain_height:dy ]
        x = x.ravel(); y = y.ravel()

        m = numpy.ones_like(x) * volume * rho0
        rho = numpy.ones_like(x) * rho0
        h = numpy.ones_like(x) * h0
        cs = numpy.ones_like(x) * c0

        # additional properties required for the fluid.
        additional_props = [
            # volume inverse or number density
            'V',

            # color and gradients
            'color', 'scolor', 'cx', 'cy', 'cz', 'cx2', 'cy2', 'cz2',

            # discretized interface normals and dirac delta
            'nx', 'ny', 'nz', 'ddelta',

            # interface curvature
            'kappa',

            # transport velocities
            'uhat', 'vhat', 'what', 'auhat', 'avhat', 'awhat',

            # imposed accelerations on the solid wall
            'ax', 'ay', 'az', 'wij',

            # velocity of magnitude squared
            'vmag2',

            # variable to indicate reliable normals and normalizing
            # constant
            'N', 'wij_sum',

            ]

        # get the fluid particle array
        fluid = get_particle_array(
            name='fluid', x=x, y=y, h=h, m=m, rho=rho, cs=cs,
            additional_props=additional_props)

        # set the color of the inner square
        for i in range(x.size):
            if ( (fluid.x[i] > 0.35) and (fluid.x[i] < 0.65) ):
                if ( (fluid.y[i] > 0.35) and (fluid.y[i] < 0.65) ):
                    fluid.color[i] = 1.0

        # particle volume
        fluid.V[:] = 1./volume

        # set additional output arrays for the fluid
        fluid.add_output_arrays(['V', 'color', 'cx', 'cy', 'nx', 'ny', 'ddelta',
                                 'kappa', 'N', 'scolor', 'p'])

        print("2D Square droplet deformation with %d fluid particles"%(
                fluid.get_number_of_particles()))

        return [fluid,]

    def create_domain(self):
        return DomainManager(
            xmin=0, xmax=domain_width, ymin=0, ymax=domain_height,
            periodic_in_x=True, periodic_in_y=True)

    def create_solver(self):
        kernel = QuinticSpline(dim=2)
        integrator = PECIntegrator( fluid=TransportVelocityStep() )
        solver = Solver(
            kernel=kernel, dim=dim, integrator=integrator,
            dt=dt, tf=tf, adaptive_timestep=False)
        return solver

    def create_equations(self):
        sy11_equations = [
            # We first compute the mass and number density of the fluid
            # phase. This is used in all force computations henceforth. The
            # number density (1/volume) is explicitly set for the solid phase
            # and this isn't modified for the simulation.
            Group(equations=[
                    SummationDensity( dest='fluid', sources=['fluid'] )
                    ] ),

            # Given the updated number density for the fluid, we can update
            # the fluid pressure. Additionally, we can compute the Shepard
            # Filtered velocity required for the no-penetration boundary
            # condition. Also compute the gradient of the color function to
            # compute the normal at the interface.
            Group(equations=[
                    StateEquation(dest='fluid', sources=None, rho0=rho0, p0=p0),
                    SY11ColorGradient(dest='fluid', sources=['fluid'])
                    ] ),

            #################################################################
            # Begin Surface tension formulation
            #################################################################
            # Scale the smoothing lengths to determine the interface
            # quantities.
            Group(equations=[
                    ScaleSmoothingLength(dest='fluid', sources=None, factor=factor1)
                    ], update_nnps=False ),

            # Compute the discretized dirac delta with respect to the new
            # smoothing length.
            Group(equations=[
                    SY11DiracDelta(dest='fluid', sources=['fluid'])
                    ],
                  ),

            # Compute the interface curvature using the modified smoothing
            # length and interface normals computed in the previous Group.
            Group(equations=[
                    InterfaceCurvatureFromNumberDensity(dest='fluid', sources=['fluid'],
                                                        with_morris_correction=True),
                    ], ),

            # Now rescale the smoothing length to the original value for the
            # rest of the computations.
            Group(equations=[
                    ScaleSmoothingLength(dest='fluid', sources=None, factor=factor2)
                    ], update_nnps=False,
                  ),
            #################################################################
            # End Surface tension formulation
            #################################################################

            # The main acceleration block
            Group(
                equations=[

                    # Gradient of pressure for the fluid phase using the
                    # number density formulation. No penetration boundary
                    # condition using Adami et al's generalized wall boundary
                    # condition. The extrapolated pressure and density on the
                    # wall particles is used in the gradient of pressure to
                    # simulate a repulsive force.
                    MomentumEquationPressureGradient(
                        dest='fluid', sources=['fluid'], pb=p0),

                    # Artificial viscosity for the fluid phase.
                    MomentumEquationViscosity(
                        dest='fluid', sources=['fluid'], nu=nu),

                    # Surface tension force for the SY11 formulation
                    ShadlooYildizSurfaceTensionForce(dest='fluid', sources=None, sigma=sigma),

                    # Artificial stress for the fluid phase
                    MomentumEquationArtificialStress(dest='fluid', sources=['fluid']),

                    ], )
            ]

        morris_equations = [

            # We first compute the mass and number density of the fluid
            # phase. This is used in all force computations henceforth. The
            # number density (1/volume) is explicitly set for the solid phase
            # and this isn't modified for the simulation.
            Group(equations=[
                    SummationDensity( dest='fluid', sources=['fluid'] )
                    ] ),

            # Given the updated number density for the fluid, we can update
            # the fluid pressure. Additionally, we can compute the Shepard
            # Filtered velocity required for the no-penetration boundary
            # condition. Also compute the smoothed color based on the color
            # index for a particle.
            Group(equations=[
                    StateEquation(dest='fluid', sources=None, rho0=rho0, p0=p0),
                    SmoothedColor( dest='fluid', sources=['fluid'], smooth=True ),
                    ] ),

            #################################################################
            # Begin Surface tension formulation
            #################################################################
            # Compute the gradient of the smoothed color field. At the end of
            # this Group, we will have the interface normals and the
            # discretized dirac delta function for the fluid-fluid interface.
            Group(equations=[
                    MorrisColorGradient(dest='fluid', sources=['fluid'],
                                        epsilon=epsilon),
                    ],
                  ),

            # Compute the interface curvature computed in the previous Group.
            Group(equations=[
                    InterfaceCurvatureFromNumberDensity(dest='fluid', sources=['fluid'],
                                                        with_morris_correction=True),
                    ], ),
            #################################################################
            # End Surface tension formulation
            #################################################################

            # The main acceleration block
            Group(
                equations=[

                    # Gradient of pressure for the fluid phase
                    MomentumEquationPressureGradient(
                        dest='fluid', sources=['fluid'], pb=p0),

                    # Artificial viscosity for the fluid phase.
                    MomentumEquationViscosity(
                        dest='fluid', sources=['fluid'], nu=nu),

                    # Surface tension force for the Morris formulation
                    CSFSurfaceTensionForce(dest='fluid', sources=None, sigma=sigma),

                    # Artificial stress for the fluid phase
                    MomentumEquationArtificialStress(dest='fluid', sources=['fluid']),

                    ], )
            ]

        adami_equations = [

            # We first compute the mass and number density of the fluid
            # phase. This is used in all force computations henceforth. The
            # number density (1/volume) is explicitly set for the solid phase
            # and this isn't modified for the simulation.
            Group(equations=[
                    SummationDensity( dest='fluid', sources=['fluid'] )
                    ] ),

            # Given the updated number density for the fluid, we can update
            # the fluid pressure. Additionally, we can compute the Shepard
            # Filtered velocity required for the no-penetration boundary
            # condition.
            Group(equations=[
                    StateEquation(dest='fluid', sources=None, rho0=rho0, p0=p0),
                    ] ),

            #################################################################
            # Begin Surface tension formulation
            #################################################################
            # Compute the gradient of the color field.
            Group(equations=[
                    AdamiColorGradient(dest='fluid', sources=['fluid']),
                    ],
                  ),

            # Compute the interface curvature using the color gradients
            # computed in the previous Group.
            Group(equations=[
                    AdamiReproducingDivergence(dest='fluid', sources=['fluid'],
                                               dim=2),
                    ], ),
            #################################################################
            # End Surface tension formulation
            #################################################################

            # The main acceleration block
            Group(
                equations=[

                    # Gradient of pressure for the fluid phase
                    MomentumEquationPressureGradient(
                        dest='fluid', sources=['fluid'], pb=p0),

                    # Artificial viscosity for the fluid phase.
                    MomentumEquationViscosity(
                        dest='fluid', sources=['fluid'], nu=nu),

                    # Surface tension force for the CSF formulation
                    CSFSurfaceTensionForce(dest='fluid', sources=None, sigma=sigma),

                    # Artificial stress for the fluid phase
                    MomentumEquationArtificialStress(dest='fluid', sources=['fluid']),

                    ], )
            ]

        if self.options.scheme == 'morris':
            return morris_equations
        elif self.options.scheme == 'adami':
            return adami_equations
        else:
            return sy11_equations


if __name__ == '__main__':
    app = SquareDroplet()
    app.run()
