# 2D channel with time-dependent boundary conditions¶

Here we extend the 2D channel example by adding constant and time dependent boundary conditions.

We begin by defining the domain and solver as before:

from thetis import *

lx = 40e3
ly = 2e3
nx = 25
ny = 2
mesh2d = RectangleMesh(nx, ny, lx, ly)

P1_2d = FunctionSpace(mesh2d, 'CG', 1)
bathymetry_2d = Function(P1_2d, name='Bathymetry')
depth = 20.0
bathymetry_2d.assign(depth)

# total duration in seconds
t_end = 12 * 3600
# export interval in seconds
t_export = 300.0

solver_obj = solver2d.FlowSolver2d(mesh2d, bathymetry_2d)
options = solver_obj.options
options.simulation_export_time = t_export
options.simulation_end_time = t_end
options.timestepper_type = 'CrankNicolson'
options.timestep = 50.0


We will force the model with a constant volume flux at the right boundary (x=40 km) and impose a tidal volume flux on the left boundary (x=0 km). Note that we have increased t_end and t_export to better illustrate tidal dynamics.

Boundary condtitions are defined for each external boundary using their ID. In this example we are using a RectangleMesh() which assigns IDs 1, 2, 3, and 4 for the four sides of the rectangle:

left_bnd_id = 1
right_bnd_id = 2


At each boundary we need to define the external value of the prognostic variables, i.e. in this case the water elevation and velocity. The value should be either a Firedrake Constant or Function (in case the boundary condition is not uniform in space).

We store the boundary conditions in a dictionary:

swe_bnd = {}
in_flux = 1e3
swe_bnd[right_bnd_id] = {'elev': Constant(0.0),
'flux': Constant(-in_flux)}


Above we set the water elevation to zero and prescribe a constant volume flux. The volume flux is defined as outward normal flux, i.e. a negative value stands for flow into the domain. Alternatively we could also prescribe the normal velocity (with key 'un') or the 2D velocity vector ('uv'). For all supported boundary conditions, see module shallowwater_eq.

In order to set time-dependent boundary conditions we first define a python function that evaluates the time dependent variable:

def timedep_flux(simulation_time):
"""Time-dependent flux function"""
tide_amp = -2e3
tide_t = 12 * 3600.
flux = tide_amp*sin(2 * pi * simulation_time / tide_t) + in_flux
return flux


We then create a Constant object with the initial value, and assign it to the left boundary:

tide_flux_const = Constant(timedep_flux(0))
swe_bnd[left_bnd_id] = {'flux': tide_flux_const}


Boundary conditions are now complete, and we assign them to the solver object:

solver_obj.bnd_functions['shallow_water'] = swe_bnd


Note that if boundary conditions are not assigned for some boundaries (the lateral boundaries 3 and 4 in this case), Thetis assumes impermeable land conditions.

The only missing piece is to add a mechanism that re-evaluates the boundary condition as the simulation progresses. For this purpose we use the optional update_forcings argument of the iterate() method. update_forcings is a python function that updates all time dependent Constants or Functions used to force the model. In this case we only need to update tide_flux_const:

def update_forcings(t_new):
"""Callback function that updates all time dependent forcing fields"""
tide_flux_const.assign(timedep_flux(t_new))


and finally pass this callback to the time iterator:

solver_obj.iterate(update_forcings=update_forcings)


This tutorial can be dowloaded as a Python script here.