In physics and mathematics, the Helmholtz decomposition theorem or the fundamental theorem of vector calculus states that any sufficiently smooth, rapidly decaying vector field in three dimensions can be resolved into the sum of an irrotational (curl-free) vector field and a solenoidal (divergence-free) vector field. It is named after Hermann von Helmholtz.
For a vector field
F\inC1(V,Rn)
V\subseteqRn
G\inC1(V,Rn)
R\inC1(V,Rn)
\Phi\inC2(V,R)
\nabla\Phi
\nabla ⋅ R
R
G
R
The Helmholtz decomposition in three dimensions was first described in 1849 by George Gabriel Stokes for a theory of diffraction. Hermann von Helmholtz published his paper on some hydrodynamic basic equations in 1858, which was part of his research on the Helmholtz's theorems describing the motion of fluid in the vicinity of vortex lines. Their derivation required the vector fields to decay sufficiently fast at infinity. Later, this condition could be relaxed, and the Helmholtz decomposition could be extended to higher dimensions. For Riemannian manifolds, the Helmholtz-Hodge decomposition using differential geometry and tensor calculus was derived.
The decomposition has become an important tool for many problems in theoretical physics, but has also found applications in animation, computer vision as well as robotics.
A
R=\nabla x A
Let
F
V\subseteqR3
V
S
V
F
where
and
\nabla'
r'
r
If
V=\R3
F
1/r
r\toinfty
F
R3
If
(\Phi1,{A1})
F
(\Phi2,{A2})
\Phi1-\Phi2=λ
A1-A2={A}λ+\nabla\varphi,
where
λ
{A}λ
\nabla x {A}λ=\nablaλ,
\varphi
Proof: Set
λ=\Phi2-\Phi1
{B=A2-A1}
-\nablaλ+\nabla x B=0
Taking the divergence of each member of this equation yields
\nabla2λ=0
λ
Conversely, given any harmonic function
λ
\nablaλ
\nabla ⋅ (\nablaλ)=\nabla2λ=0.
Thus, according to the above section, there exists a vector field
{A}λ
\nablaλ=\nabla x {A}λ
If
{A'}λ
C={A}λ-{A'}λ
\nabla x {C}=0
C=\nabla\varphi
\varphi
The term "Helmholtz theorem" can also refer to the following. Let be a solenoidal vector field and d a scalar field on which are sufficiently smooth and which vanish faster than at infinity. Then there exists a vector field such that
if additionally the vector field vanishes as, then is unique.
In other words, a vector field can be constructed with both a specified divergence and a specified curl, and if it also vanishes at infinity, it is uniquely specified by its divergence and curl. This theorem is of great importance in electrostatics, since Maxwell's equations for the electric and magnetic fields in the static case are of exactly this type. The proof is by a construction generalizing the one given above: we set
where
l{G}
The Helmholtz decomposition can be generalized by reducing the regularity assumptions (the need for the existence of strong derivatives). Suppose is a bounded, simply-connected, Lipschitz domain. Every square-integrable vector field has an orthogonal decomposition:
where is in the Sobolev space of square-integrable functions on whose partial derivatives defined in the distribution sense are square integrable, and, the Sobolev space of vector fields consisting of square integrable vector fields with square integrable curl.
For a slightly smoother vector field, a similar decomposition holds:
where .
Note that in the theorem stated here, we have imposed the condition that if
F
F
1/r
F
G
The Fourier transform of a scalar field is a scalar field, and the Fourier transform of a vector field is a vector field of same dimension.
Now consider the following scalar and vector fields:
Hence
\hatF
F
Now we apply an inverse Fourier transform to each of these components. Using properties of Fourier transforms, we derive:
Since
\nabla x (\nabla\Phi)=0
\nabla ⋅ (\nabla x A)=0
we can get
so this is indeed the Helmholtz decomposition.
The generalization to
d
Let
F
V\subseteqRd
|r|-\delta
|r|\toinfty
\delta>2
The scalar potential is defined similar to the three dimensional case as:where as the integration kernel
K(r,r')
Vd=
| ||||
\pi |
/\Gamma(\tfrac{d}{2}+1)
\Gamma(r)
For
d=3
Vd
4\pi | |
3 |
style\binom{d}{2}
A=[A1,A2,A3]=[A23,A31,A12]
style\binom{d}{2}=d
d=3
As in the three-dimensional case, the gradient field is defined asThe rotational field, on the other hand, is defined in the general case as the row divergence of the matrix:In three-dimensional space, this is equivalent to the rotation of the vector potential.
In a
d
d ≠ 3
\mu
Following the same steps as above, we can writewhere
\delta\mu\nu
\varepsilon
d\ge2
\alpha
(d-2)
We can therefore writewhereNote that the vector potential is replaced by a rank-
(d-2)
d
Because
G(r,r')
r-r'
\partial | |
\partialr\mu |
→ -
\partial | |
\partialr'\mu |
S=\partialV
V
For a further generalization to manifolds, see the discussion of Hodge decomposition below.
The Hodge decomposition is closely related to the Helmholtz decomposition, generalizing from vector fields on R3 to differential forms on a Riemannian manifold M. Most formulations of the Hodge decomposition require M to be compact. Since this is not true of R3, the Hodge decomposition theorem is not strictly a generalization of the Helmholtz theorem. However, the compactness restriction in the usual formulation of the Hodge decomposition can be replaced by suitable decay assumptions at infinity on the differential forms involved, giving a proper generalization of the Helmholtz theorem.
Most textbooks only deal with vector fields decaying faster than
|r|-\delta
\delta>1
|r|-\delta
\delta>0
K(r,r')
K'(r,r')=K(r,r')-K(0,r')
For all analytic vector fields that need not go to zero even at infinity, methods based on partial integration and the Cauchy formula for repeated integration can be used to compute closed-form solutions of the rotation and scalar potentials, as in the case of multivariate polynomial, sine, cosine, and exponential functions.
H(r)
\DeltaH(r)=0
H(r)
\Phi(r)
For vector fields
F
H(r)=0
This uniqueness does not apply to the potentials: In the three-dimensional case, the scalar and vector potential jointly have four components, whereas the vector field has only three. The vector field is invariant to gauge transformations and the choice of appropriate potentials known as gauge fixing is the subject of gauge theory. Important examples from physics are the Lorenz gauge condition and the Coulomb gauge. An alternative is to use the poloidal–toroidal decomposition.
The Helmholtz theorem is of particular interest in electrodynamics, since it can be used to write Maxwell's equations in the potential image and solve them more easily. The Helmholtz decomposition can be used to prove that, given electric current density and charge density, the electric field and the magnetic flux density can be determined. They are unique if the densities vanish at infinity and one assumes the same for the potentials.
In fluid dynamics, the Helmholtz projection plays an important role, especially for the solvability theory of the Navier-Stokes equations. If the Helmholtz projection is applied to the linearized incompressible Navier-Stokes equations, the Stokes equation is obtained. This depends only on the velocity of the particles in the flow, but no longer on the static pressure, allowing the equation to be reduced to one unknown. However, both equations, the Stokes and linearized equations, are equivalent. The operator
P\Delta
In the theory of dynamical systems, Helmholtz decomposition can be used to determine "quasipotentials" as well as to compute Lyapunov functions in some cases.
For some dynamical systems such as the Lorenz system (Edward N. Lorenz, 1963), a simplified model for atmospheric convection, a closed-form expression of the Helmholtz decomposition can be obtained:The Helmholtz decomposition of
F(r)
\Phi(r)=\tfrac{a}{2}
2 | |
r | |
1 |
+\tfrac{1}{2}
2 | |
r | |
2 |
+\tfrac{c}{2}
2 | |
r | |
3 |
The quadratic scalar potential provides motion in the direction of the coordinate origin, which is responsible for the stable fix point for some parameter range. For other parameters, the rotation field ensures that a strange attractor is created, causing the model to exhibit a butterfly effect.
In magnetic resonance elastography, a variant of MR imaging where mechanical waves are used to probe the viscoelasticity of organs, the Helmholtz decomposition is sometimes used to separate the measured displacement fields into its shear component (divergence-free) and its compression component (curl-free). In this way, the complex shear modulus can be calculated without contributions from compression waves.
The Helmholtz decomposition is also used in the field of computer engineering. This includes robotics, image reconstruction but also computer animation, where the decomposition is used for realistic visualization of fluids or vector fields.
\nabla x A