Solve a circular distance function equation and then advect it.

This example first imposes a circular distance function: The variable is advected with, The scheme used in the FirstOrderAdvectionTerm preserves the var as a distance function. The solution to this problem will be demonstrated in the following script. Firstly, setup the parameters.

>>> from fipy import CellVariable, Grid2D, DistanceVariable, TransientTerm, FirstOrderAdvectionTerm, AdvectionTerm, Viewer
>>> from fipy.tools import numerix

>>> L = 1.
>>> N = 25
>>> velocity = 1.
>>> cfl = 0.1
>>> velocity = 1.
>>> distanceToTravel = L / 10.
>>> radius = L / 4.
>>> dL = L / N
>>> timeStepDuration = cfl * dL / velocity
>>> steps = int(distanceToTravel / dL / cfl)


Construct the mesh.

>>> mesh = Grid2D(dx=dL, dy=dL, nx=N, ny=N)


Construct a distanceVariable object.

>>> var = DistanceVariable(
...     name = 'level set variable',
...     mesh = mesh,
...     value = 1.,
...     hasOld = 1)


Initialize the distanceVariable to be a circular distance function.

>>> x, y = mesh.cellCenters
>>> initialArray = numerix.sqrt((x - L / 2.)**2 + (y - L / 2.)**2) - radius
>>> var.setValue(initialArray)


The advection equation is constructed.

>>> advEqn = TransientTerm() + FirstOrderAdvectionTerm(velocity)


The problem can then be solved by executing a serious of time steps.

>>> from builtins import range
>>> if __name__ == '__main__':
...     viewer.plot()
...     for step in range(steps):
...         var.updateOld()
...         viewer.plot()


The result can be tested with the following commands.

>>> from builtins import range
>>> for step in range(steps):
...     var.updateOld()
>>> x = numerix.array(mesh.cellCenters)
>>> distanceTravelled = timeStepDuration * steps * velocity
>>> answer = initialArray - distanceTravelled
>>> solution = numerix.where(answer < 0., -1001., numerix.array(var))
>>> numerix.allclose(answer, solution, atol=4.7e-3)
1


If the advection equation is built with the AdvectionTerm() the result is more accurate,

>>> var.setValue(initialArray)