In this tutorial we will explore Nutils' main building blocks by solving a simple 1D Laplace problem. The tutorial assumes knowledge of the Python programming language, as well as familiarity with the third party modules Numpy and Matplotlib. It also assumes knowledge of advanced calculus, weak formulations, and the Finite Element Method, and makes heavy use of Einstein notation.

Whetting your Appetite

The computation that we will work towards amounts to about 20 lines of Nutils code, including visualization. The entire script is presented below, in copy-pasteable form suitable for interactive exploration using for example ipython. In the sections that follow we will go over these lines ones by one and explain the relevant concepts involved.

from nutils import function, mesh, solver
from nutils.expression_v2 import Namespace
import numpy
from matplotlib import pyplot as plt

topo, geom = mesh.rectilinear([numpy.linspace(0, 1, 5)])

ns = Namespace()
ns.x = geom
ns.define_for('x', gradient='∇', normal='n', jacobians=('dV', 'dS'))
ns.basis = topo.basis('spline', degree=1)
ns.u = function.dotarg('lhs', ns.basis)

sqr = topo.boundary['left'].integral('u^2 dS' @ ns, degree=2)
cons = solver.optimize('lhs', sqr, droptol=1e-15)
# optimize > constrained 1/5 dofs
# optimize > optimum value 0.00e+00

res = topo.integral('∇_i(basis_n) ∇_i(u) dV' @ ns, degree=0)
res -= topo.boundary['right'].integral('basis_n dS' @ ns, degree=0)
lhs = solver.solve_linear('lhs', residual=res, constrain=cons)
# solve > solving 4 dof system to machine precision using arnoldi solver
# solve > solver returned with residual 9e-16±1e-15

bezier = topo.sample('bezier', 32)
nanjoin = lambda array, tri: numpy.insert(array.take(tri.flat, 0).astype(float),
    slice(tri.shape[1], tri.size, tri.shape[1]), numpy.nan, axis=0)
sampled_x = nanjoin(bezier.eval('x_0' @ ns), bezier.tri)
def plot_line(func, **arguments):
    plt.plot(sampled_x, nanjoin(bezier.eval(func, **arguments), bezier.tri))
    plt.xticks(numpy.linspace(0, 1, 5))

plot_line(ns.u, lhs=lhs)


You are encouraged to execute this code at least once before reading on, as the code snippets that follow may assume certain products to be present in the namespace. In particular the plot_line function is used heavily in the ensuing sections.