# Restart a problem¶

This demo demonstrates the functionality of restarting a problem. This could be useful for example if one wants to run the simulation for longer than anticipated on initial solve, if one wants parts of the solution to have a different temporal resolution or if one wants to completely change the boundary conditions of the problem at a certain point.

It is based on the Beltrami problem, a 3D problem with a known analytical solution.

The source code for this demo can be found in Restart.py.

We start by importing the problem,

from os import path
import sys
# Add Beltrami problem as example problem
sys.path.insert(0, path.join(path.dirname(path.realpath(__file__)), '../../../demo/undocumented/Beltrami'))
from Beltrami import Beltrami


and cbcflow:

from cbcflow import *


## Play¶

We define the normal solve routine, by first initiating a problem and scheme:

def play():
problem = Beltrami(dict(dt=1e-2, T=1.0))
scheme = IPCS()


Note that we here use the IPCS scheme.

We then creates some fields to save for this solve:

fields = [
Velocity(dict(save=True, stride_timestep=5)),
Pressure(dict(save=True, stride_timestep=10)),
L2norm("Velocity", dict(save=True, stride_timestep=2))
]


Note that we must save velocity and pressure for restart to work.

We then add the fields to a NSPostProcessor instance,

postprocessor = NSPostProcessor(dict(casedir='results'))


and solves the equation:

solver = NSSolver(problem, scheme, postprocessor)
solver.solve()


## Restart¶

When we restart the problem, we wish to reuse most of the parameters of the original solve. These are save by the postprocessor to the case directory, and can be unpickled:

def restart():
import pickle


If we don’t change any of the parameters, we would basically be solving the exact same problem. Thus, we change the end time and time step of the problem:

problem = Beltrami(params.problem)
problem.params.T = 2.0
problem.params.dt = 5e-3


We are also free to change the scheme, so we change the scheme to IPCS_Stable:

scheme = IPCS_Stable(params.scheme)


On the restart, we also set up a different set of fields

# Set up postprocessor with new fields
fields = [
Velocity(dict(save=True)),
Pressure(dict(save=True)),
WSS(dict(save=True)),
]


and a new NSPostProcessor instance:

postprocessor = NSPostProcessor(dict(casedir='results'))


We then need to define our new solver, and set some restart-specific parameters:

solver = NSSolver(problem, scheme, postprocessor)
solver.params["restart"] = True
solver.params["restart_time"] = 0.5


The solver will try to search for a solution in the postprocessors case directory at time 0.5, and replace the the method initial_conditions() in the NSProblem instance to reflect the solution at t=0.5.

Our call to solve will then restart this problem from the specified parameters, and solve the problem:

solver.solve()