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 = 0.004809678530125144
CG iteration 2, residual = 0.003322319013244249
CG iteration 3, residual = 0.0033115883177540404
CG iteration 4, residual = 0.002746757977924567
CG iteration 5, residual = 0.0014658765270595345
CG iteration 6, residual = 0.001216702499924432
CG iteration 7, residual = 0.0008096582309573263
CG iteration 8, residual = 0.0006570076838868008
CG iteration 9, residual = 0.00047508691834428733
CG iteration 10, residual = 0.00036224214074475914
CG iteration 11, residual = 0.0002544183424853315
CG iteration 12, residual = 0.00016194989094436935
CG iteration 13, residual = 0.00011358302702051965
CG iteration 14, residual = 8.950382818006133e-05
CG iteration 15, residual = 5.3941268122197165e-05
CG iteration 16, residual = 3.9160417841646826e-05
CG iteration 17, residual = 2.7082316720700467e-05
CG iteration 18, residual = 1.824957433700401e-05
CG iteration 19, residual = 1.3584581218418906e-05
CG iteration 20, residual = 9.87185568991092e-06
CG iteration 21, residual = 1.1747756135746651e-05
CG iteration 22, residual = 5.689604510090456e-06
CG iteration 23, residual = 3.5938636600788913e-06
CG iteration 24, residual = 2.5589089530977755e-06
CG iteration 25, residual = 1.9779478028751224e-06
CG iteration 26, residual = 1.2770934681836885e-06
CG iteration 27, residual = 8.740832403670703e-07
CG iteration 28, residual = 5.927228746857567e-07
CG iteration 29, residual = 4.0089017034467763e-07
CG iteration 30, residual = 2.856458196958205e-07
CG iteration 31, residual = 2.0292685846592206e-07
CG iteration 32, residual = 1.319963698729181e-07
CG iteration 33, residual = 9.850315544718852e-08
CG iteration 34, residual = 6.734297590254276e-08
CG iteration 35, residual = 4.756060986925794e-08
CG iteration 36, residual = 2.920225772474484e-08
CG iteration 37, residual = 2.7058789207734893e-08
CG iteration 38, residual = 2.3013665851657412e-08
CG iteration 39, residual = 1.3023707222350178e-08
CG iteration 40, residual = 8.3954751113175e-09
CG iteration 41, residual = 5.711978991725656e-09
CG iteration 42, residual = 4.181049841253561e-09
CG iteration 43, residual = 2.794949339477854e-09
CG iteration 44, residual = 1.8595331197253478e-09
CG iteration 45, residual = 1.2654495203794215e-09
CG iteration 46, residual = 8.583836389038076e-10
CG iteration 47, residual = 5.766587359293559e-10
CG iteration 48, residual = 4.2177701588874824e-10
CG iteration 49, residual = 2.761187784286927e-10
CG iteration 50, residual = 2.1705299876293148e-10
CG iteration 51, residual = 1.2859647851652642e-10
CG iteration 52, residual = 8.660405808130159e-11
CG iteration 53, residual = 7.871837743851414e-11
CG iteration 54, residual = 6.745733853689017e-11
CG iteration 55, residual = 3.49901815082063e-11
CG iteration 56, residual = 2.5358315209408285e-11
CG iteration 57, residual = 2.0884643134965627e-11
CG iteration 58, residual = 1.2878454803393964e-11
CG iteration 59, residual = 8.49637420342712e-12
CG iteration 60, residual = 5.923327987902687e-12
CG iteration 61, residual = 3.942608600663536e-12
CG iteration 62, residual = 2.6956164650590013e-12
CG iteration 63, residual = 1.738707830332622e-12
CG iteration 64, residual = 1.2104137130027736e-12
CG iteration 65, residual = 7.623361117375037e-13
CG iteration 66, residual = 4.941739489198503e-13
CG iteration 67, residual = 3.2083699561915177e-13
CG iteration 68, residual = 2.2293268555797602e-13
CG iteration 69, residual = 3.0727405957359694e-13
CG iteration 70, residual = 1.3672832840905744e-13
CG iteration 71, residual = 9.423545566295174e-14
CG iteration 72, residual = 6.131450384121317e-14
CG iteration 73, residual = 4.207963333436131e-14
CG iteration 74, residual = 2.7667293558865323e-14
CG iteration 75, residual = 1.790333132317225e-14
CG iteration 76, residual = 1.3155846219212124e-14
CG iteration 77, residual = 1.0016368904650376e-14
CG iteration 78, residual = 6.483087011511075e-15
CG iteration 79, residual = 3.9723319228138424e-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);

[ ]:



[ ]: