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 = 0.004809678530125142
CG iteration 2, residual = 0.003322319013244244
CG iteration 3, residual = 0.003311588317754003
CG iteration 4, residual = 0.002746757977924583
CG iteration 5, residual = 0.0014658765270595328
CG iteration 6, residual = 0.00121670249992443
CG iteration 7, residual = 0.0008096582309573267
CG iteration 8, residual = 0.0006570076838868
CG iteration 9, residual = 0.0004750869183442863
CG iteration 10, residual = 0.0003622421407447587
CG iteration 11, residual = 0.0002544183424853307
CG iteration 12, residual = 0.00016194989094436884
CG iteration 13, residual = 0.00011358302702052049
CG iteration 14, residual = 8.950382818008935e-05
CG iteration 15, residual = 5.3941268123261675e-05
CG iteration 16, residual = 3.9160417899363725e-05
CG iteration 17, residual = 2.7082318800643046e-05
CG iteration 18, residual = 1.8249671506442466e-05
CG iteration 19, residual = 1.3588505747413478e-05
CG iteration 20, residual = 9.970047689801577e-06
CG iteration 21, residual = 1.2106617809981544e-05
CG iteration 22, residual = 5.633813118947844e-06
CG iteration 23, residual = 3.5935786326750384e-06
CG iteration 24, residual = 2.5589062210833012e-06
CG iteration 25, residual = 1.977947752132881e-06
CG iteration 26, residual = 1.2770934675151967e-06
CG iteration 27, residual = 8.740832403608536e-07
CG iteration 28, residual = 5.927228746856991e-07
CG iteration 29, residual = 4.008901703446766e-07
CG iteration 30, residual = 2.856458196958167e-07
CG iteration 31, residual = 2.0292685846579955e-07
CG iteration 32, residual = 1.319963698672179e-07
CG iteration 33, residual = 9.850315517521054e-08
CG iteration 34, residual = 6.734296630042151e-08
CG iteration 35, residual = 4.75601911888085e-08
CG iteration 36, residual = 2.9178929710790255e-08
CG iteration 37, residual = 2.58875371683341e-08
CG iteration 38, residual = 2.3724458663687883e-08
CG iteration 39, residual = 1.3041846032263621e-08
CG iteration 40, residual = 8.395601321726048e-09
CG iteration 41, residual = 5.7119798498803875e-09
CG iteration 42, residual = 4.181049850445758e-09
CG iteration 43, residual = 2.794949339566155e-09
CG iteration 44, residual = 1.8595331196597855e-09
CG iteration 45, residual = 1.2654495200555894e-09
CG iteration 46, residual = 8.583836371050536e-10
CG iteration 47, residual = 5.766587266630985e-10
CG iteration 48, residual = 4.2177698559796915e-10
CG iteration 49, residual = 2.761186700145379e-10
CG iteration 50, residual = 2.1705276009099897e-10
CG iteration 51, residual = 1.285944322084197e-10
CG iteration 52, residual = 8.645700712372103e-11
CG iteration 53, residual = 7.24708312477361e-11
CG iteration 54, residual = 7.194642170588019e-11
CG iteration 55, residual = 3.495870942865291e-11
CG iteration 56, residual = 2.4740104485438625e-11
CG iteration 57, residual = 2.0598430093788943e-11
CG iteration 58, residual = 1.2990174075233084e-11
CG iteration 59, residual = 8.505986850556846e-12
CG iteration 60, residual = 5.924273145540524e-12
CG iteration 61, residual = 3.94268820735523e-12
CG iteration 62, residual = 2.69562396736522e-12
CG iteration 63, residual = 1.7387082684879425e-12
CG iteration 64, residual = 1.210413747650964e-12
CG iteration 65, residual = 7.623361091526621e-13
CG iteration 66, residual = 4.941735434764237e-13
CG iteration 67, residual = 3.207990843261383e-13
CG iteration 68, residual = 2.1983976644027027e-13
CG iteration 69, residual = 2.998595114031235e-13
CG iteration 70, residual = 1.381159660752287e-13
CG iteration 71, residual = 9.424809273032807e-14
CG iteration 72, residual = 6.131190917352415e-14
CG iteration 73, residual = 4.206999042757946e-14
CG iteration 74, residual = 2.762927393286732e-14
CG iteration 75, residual = 1.7673840380086824e-14
CG iteration 76, residual = 1.1714152955258731e-14
CG iteration 77, residual = 8.76683687393098e-15
CG iteration 78, residual = 6.761949188900661e-15
CG iteration 79, residual = 4.0421023355802414e-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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