This page was generated from unit-2.4-Maxwell/Maxwell.ipynb.

2.4 Maxwell’s Equations

[Peter Monk: "Finite Elements for Maxwell’s Equations"]

Magnetostatic field generated by a permanent magnet

magnetic flux \(B\), magnetic field \(H\), given magnetization \(M\):

\[\DeclareMathOperator{\Grad}{grad} \DeclareMathOperator{\Curl}{curl} \DeclareMathOperator{\Div}{div} B = \mu (H + M), \quad \Div B = 0, \quad \Curl H = 0\]

Introducing a vector-potential \(A\) such that \(B = \Curl A\), and putting equations together we get

\[\Curl \mu^{-1} \Curl A = \Curl M\]

In weak form: Find \(A \in H(\Curl)\) such that

\[\int \mu^{-1} \Curl A \Curl v = \int M \Curl v \qquad \forall \, v \in H(\Curl)\]

Usually, the permeability \(\mu\) is given as \(\mu = \mu_r \mu_0\), with \(\mu_0 = 4 \pi 10^{-7}\) the permeability of vacuum.

[1]:

from ngsolve import *
from ngsolve.webgui import Draw
from netgen.occ import *

Geometric model and meshing of a bar magnet:

[2]:

# box = OrthoBrick(Pnt(-3,-3,-3),Pnt(3,3,3)).bc("outer")
# magnet = Cylinder(Pnt(-1,0,0),Pnt(1,0,0), 0.3) * OrthoBrick(Pnt(-1,-3,-3),Pnt(1,3,3))
# air = box - magnet
box = Box( (-3,-3,-3), (3,3,3))
box.faces.name = "outer"

magnet = Cylinder((-1,0,0),X, r=0.3, h=2)
magnet.mat("magnet")
magnet.faces.col = (1,0,0)

air = box-magnet
air.mat("air")
shape = Glue([air,magnet])
geo = OCCGeometry(shape)

Draw (shape, clipping={ "z" : -1, "function":True})

mesh = Mesh(geo.GenerateMesh(maxh=2, curvaturesafety=1))
mesh.Curve(3);

[3]:

mesh.GetMaterials(), mesh.GetBoundaries()

[3]:

(('air', 'magnet'),
 ('outer',
  'outer',
  'outer',
  'outer',
  'outer',
  'outer',
  'default',
  'default',
  'default'))

Define space, forms and preconditioner.

  • To obtain a regular system matrix, we regularize by adding a very small \(L_2\) term.

  • We solve magnetostatics, so we can gauge by adding and arbitrary gradient field. A cheap possibility is to delete all basis-functions which are gradients (flag 'nograds')

[4]:

fes = HCurl(mesh, order=3, dirichlet="outer", nograds=True)
print ("ndof =", fes.ndof)
u,v = fes.TnT()

from math import pi
mu0 = 4*pi*1e-7
mur = mesh.MaterialCF({"magnet" : 1000}, default=1)

a = BilinearForm(fes)
a += 1/(mu0*mur)*curl(u)*curl(v)*dx + 1e-8/(mu0*mur)*u*v*dx
c = Preconditioner(a, "bddc")

f = LinearForm(fes)
mag = mesh.MaterialCF({"magnet" : (1,0,0)}, default=(0,0,0))
f += mag*curl(v) * dx("magnet")

ndof = 32867

Assemble system and setup preconditioner using task-parallelization:

[5]:

with TaskManager():
    a.Assemble()
    f.Assemble()

Finally, declare GridFunction and solve by preconditioned CG iteration:

[6]:

gfu = GridFunction(fes)
with TaskManager():
    solvers.CG(sol=gfu.vec, rhs=f.vec, mat=a.mat, pre=c.mat, printrates=True)

CG iteration 1, residual = 0.004814181258911797
CG iteration 2, residual = 0.0033328051014519145
CG iteration 3, residual = 0.0033139002762755548
CG iteration 4, residual = 0.002757325239273902
CG iteration 5, residual = 0.001469986689294459
CG iteration 6, residual = 0.001219507301161758
CG iteration 7, residual = 0.0008104084245895238
CG iteration 8, residual = 0.0006520529125240901
CG iteration 9, residual = 0.0004666815820542922
CG iteration 10, residual = 0.00035507600340398387
CG iteration 11, residual = 0.0002571940102212773
CG iteration 12, residual = 0.00016286203318134057
CG iteration 13, residual = 0.00011424246776661224
CG iteration 14, residual = 8.973472770213239e-05
CG iteration 15, residual = 5.4162280790692985e-05
CG iteration 16, residual = 3.9245861895959295e-05
CG iteration 17, residual = 2.71752157196757e-05
CG iteration 18, residual = 1.813043559668647e-05
CG iteration 19, residual = 1.362710592386056e-05
CG iteration 20, residual = 1.0393583309578536e-05
CG iteration 21, residual = 1.2402795995164653e-05
CG iteration 22, residual = 5.516520597266753e-06
CG iteration 23, residual = 3.5879040915616046e-06
CG iteration 24, residual = 2.6133743546602437e-06
CG iteration 25, residual = 1.9878789662783013e-06
CG iteration 26, residual = 1.2676324790731277e-06
CG iteration 27, residual = 8.694609350024391e-07
CG iteration 28, residual = 6.013535442844186e-07
CG iteration 29, residual = 4.067634925341953e-07
CG iteration 30, residual = 2.9509352359354325e-07
CG iteration 31, residual = 2.117872395226892e-07
CG iteration 32, residual = 1.3216362909857353e-07
CG iteration 33, residual = 9.609369386109396e-08
CG iteration 34, residual = 6.597519053357834e-08
CG iteration 35, residual = 4.711733350444784e-08
CG iteration 36, residual = 2.9499060912291516e-08
CG iteration 37, residual = 3.555528054850609e-08
CG iteration 38, residual = 1.969399400749539e-08
CG iteration 39, residual = 1.3063856005788996e-08
CG iteration 40, residual = 8.520335107233993e-09
CG iteration 41, residual = 5.591493702942125e-09
CG iteration 42, residual = 3.808185403153706e-09
CG iteration 43, residual = 2.673626060527726e-09
CG iteration 44, residual = 1.871588316256048e-09
CG iteration 45, residual = 1.2905890748323585e-09
CG iteration 46, residual = 8.426050333693052e-10
CG iteration 47, residual = 5.391829901534644e-10
CG iteration 48, residual = 4.2036006988892354e-10
CG iteration 49, residual = 2.8107382994241623e-10
CG iteration 50, residual = 2.0086402054313228e-10
CG iteration 51, residual = 1.265664593157528e-10
CG iteration 52, residual = 8.667920666907893e-11
CG iteration 53, residual = 1.0155129148189875e-10
CG iteration 54, residual = 5.83974865213093e-11
CG iteration 55, residual = 3.481626029683064e-11
CG iteration 56, residual = 2.3434816281565803e-11
CG iteration 57, residual = 1.6564035268910128e-11
CG iteration 58, residual = 1.3336640847389208e-11
CG iteration 59, residual = 8.775235104139566e-12
CG iteration 60, residual = 5.903999614689763e-12
CG iteration 61, residual = 3.996487893360913e-12
CG iteration 62, residual = 2.6697966589873063e-12
CG iteration 63, residual = 1.7455484858413465e-12
CG iteration 64, residual = 1.2238739730766962e-12
CG iteration 65, residual = 7.550562639215013e-13
CG iteration 66, residual = 4.924837908129914e-13
CG iteration 67, residual = 3.2576806910638517e-13
CG iteration 68, residual = 2.345667408224882e-13
CG iteration 69, residual = 3.0920022968999797e-13
CG iteration 70, residual = 1.294628135021101e-13
CG iteration 71, residual = 8.64394074406997e-14
CG iteration 72, residual = 5.6021414778463436e-14
CG iteration 73, residual = 3.631755136375615e-14
CG iteration 74, residual = 2.47530875081316e-14
CG iteration 75, residual = 1.9050239099919977e-14
CG iteration 76, residual = 1.3487857608407042e-14
CG iteration 77, residual = 8.959988885666436e-15
CG iteration 78, residual = 6.34464953859546e-15
CG iteration 79, residual = 4.0946899284085405e-15

[7]:

Draw (curl(gfu), mesh, "B-field", draw_surf=False, \
      clipping = { "z" : -1, "function":True}, \
      vectors = { "grid_size":50}, min=0, max=2e-5);

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