Helmholtz decomposition

Summary

In physics and mathematics, in the area of vector calculus, Helmholtz's theorem,[1][2] also known as the fundamental theorem of vector calculus,[3][4][5][6][7][8][9] 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; this is known as the Helmholtz decomposition or Helmholtz representation. It is named after Hermann von Helmholtz.

Definition edit

For a vector field   defined on a domain  , a Helmholtz decomposition is a pair of vector fields   and   such that:

 
Here,   is a scalar potential,   is its gradient, and   is the divergence of the vector field  . The irrotational vector field   is called a gradient field and   is called a solenoidal field or rotation field. This decomposition does not exist for all vector fields and is not unique.[10]

History edit

The Helmholtz decomposition in three dimensions was first described in 1849[11] by George Gabriel Stokes for a theory of diffraction. Hermann von Helmholtz published his paper on some hydrodynamic basic equations in 1858,[12][13] which was part of his research on the Helmholtz's theorems describing the motion of fluid in the vicinity of vortex lines.[13] 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.[10][14][15] For riemannian manifolds, the Helmholtz-Hodge decomposition using differential geometry and tensor calculus was derived.[10][13][16][17]

The decomposition has become an important tool for many problems in theoretical physics,[13][16] but has also found applications in animation, computer vision as well as robotics.[17]

Three-dimensional space edit

Many physics textbooks restrict the Helmholtz decomposition to the three-dimensional space and limit its application to vector fields that decay sufficiently fast at infinity or to bump function that are defined on a bounded domain. Then, a vector potential   can be defined, such that the rotation field is given by  , using the curl of a vector field.[18]

Let   be a vector field on a bounded domain  , which is twice continuously differentiable inside  , and let   be the surface that encloses the domain  . Then   can be decomposed into a curl-free component and a divergence-free component as follows:[19]

 
where
 

and   is the nabla operator with respect to  , not  .

If   and is therefore unbounded, and   vanishes at least as fast as   as  , then one has[20]

 
This holds in particular if   is twice continuously differentiable in   and of bounded support.

Derivation edit

Proof

Suppose we have a vector function   of which we know the curl,  , and the divergence,  , in the domain and the fields on the boundary. Writing the function using delta function in the form

 
where   is the Laplace operator, we have

 

where we have used the definition of the vector Laplacian:

 

differentiation/integration with respect to  by   and in the last line, linearity of function arguments:

 

Then using the vectorial identities

 

we get

 

Thanks to the divergence theorem the equation can be rewritten as

 

with outward surface normal  .

Defining

 
 

we finally obtain

 

Solution space edit

If   is a Helmholtz decomposition of  , then   is another decomposition if, and only if,

  and  
where
  •   is a harmonic scalar field,
  •   is a vector field which fulfills  
  •   is a scalar field.

Proof: Set   and  . According to the definition of the Helmholtz decomposition, the condition is equivalent to

 .

Taking the divergence of each member of this equation yields  , hence   is harmonic.

Conversely, given any harmonic function  ,   is solenoidal since

 

Thus, according to the above section, there exists a vector field   such that  .

If   is another such vector field, then   fulfills  , hence   for some scalar field  .

Fields with prescribed divergence and curl edit

The term "Helmholtz theorem" can also refer to the following. Let C be a solenoidal vector field and d a scalar field on R3 which are sufficiently smooth and which vanish faster than 1/r2 at infinity. Then there exists a vector field F such that

 

if additionally the vector field F vanishes as r → ∞, then F is unique.[20]

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.[20] The proof is by a construction generalizing the one given above: we set

 

where   represents the Newtonian potential operator. (When acting on a vector field, such as ∇ × F, it is defined to act on each component.)

Weak formulation edit

The Helmholtz decomposition can be generalized by reducing the regularity assumptions (the need for the existence of strong derivatives). Suppose Ω is a bounded, simply-connected, Lipschitz domain. Every square-integrable vector field u ∈ (L2(Ω))3 has an orthogonal decomposition:[21][22][23]

 

where φ is in the Sobolev space H1(Ω) of square-integrable functions on Ω whose partial derivatives defined in the distribution sense are square integrable, and AH(curl, Ω), the Sobolev space of vector fields consisting of square integrable vector fields with square integrable curl.

For a slightly smoother vector field uH(curl, Ω), a similar decomposition holds:

 

where φH1(Ω), v ∈ (H1(Ω))d.

Derivation from the Fourier transform edit

Note that in the theorem stated here, we have imposed the condition that if   is not defined on a bounded domain, then   shall decay faster than  . Thus, the Fourier transform of  , denoted as  , is guaranteed to exist. We apply the convention

 

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

 

Longitudinal and transverse fields edit

A terminology often used in physics refers to the curl-free component of a vector field as the longitudinal component and the divergence-free component as the transverse component.[24] This terminology comes from the following construction: Compute the three-dimensional Fourier transform   of the vector field  . Then decompose this field, at each point k, into two components, one of which points longitudinally, i.e. parallel to k, the other of which points in the transverse direction, i.e. perpendicular to k. So far, we have

 
 
 

Now we apply an inverse Fourier transform to each of these components. Using properties of Fourier transforms, we derive:

 
 
 

Since   and  ,

we can get

 
 

so this is indeed the Helmholtz decomposition.[25]

Generalization to higher dimensions edit

Matrix approach edit

The generalization to   dimensions cannot be done with a vector potential, since the rotation operator and the cross product are defined (as vectors) only in three dimensions.

Let   be a vector field on a bounded domain   which decays faster than   for   and  .

The scalar potential is defined similar to the three dimensional case as:

 
where as the integration kernel   is again the fundamental solution of Laplace's equation, but in d-dimensional space:
 
with   the volume of the d-dimensional unit balls and   the gamma function.

For  ,   is just equal to  , yielding the same prefactor as above. The rotational potential is an antisymmetric matrix with the elements:

 
Above the diagonal are   entries which occur again mirrored at the diagonal, but with a negative sign. In the three-dimensional case, the matrix elements just correspond to the components of the vector potential  . However, such a matrix potential can be written as a vector only in the three-dimensional case, because   is valid only for  .

As in the three-dimensional case, the gradient field is defined as

 
The 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.[10][26]

Tensor approach edit

In a  -dimensional vector space with  ,   can be replaced by the appropriate Green's function for the Laplacian, defined by

 
where Einstein summation convention is used for the index  . For example,   in 2D.

Following the same steps as above, we can write

 
where   is the Kronecker delta (and the summation convention is again used). In place of the definition of the vector Laplacian used above, we now make use of an identity for the Levi-Civita symbol  ,
 
which is valid in   dimensions, where   is a  -component multi-index. This gives
 

We can therefore write

 
where
 
Note that the vector potential is replaced by a rank-  tensor in   dimensions.

For a further generalization to manifolds, see the discussion of Hodge decomposition below.

Differential forms edit

The Hodge decomposition is closely related to the Helmholtz decomposition,[27] 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.[28] 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.

Extensions to fields not decaying at infinity edit

Most textbooks only deal with vector fields decaying faster than   with   at infinity.[18][15][29] However, Otto Blumenthal showed in 1905 that an adapted integration kernel can be used to integrate fields decaying faster than   with  , which is substantially less strict. To achieve this, the kernel   in the convolution integrals has to be replaced by  .[30] With even more complex integration kernels, solutions can be found even for divergent functions that need not grow faster than polynomial.[14][15][26][31]

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[32] 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.[10]

Uniqueness of the solution edit

In general, the Helmholtz decomposition is not uniquely defined. A harmonic function   is a function that satisfies  . By adding   to the scalar potential  , a different Helmholtz decomposition can be obtained:

 

For vector fields  , decaying at infinity, it is a plausible choice that scalar and rotation potentials also decay at infinity. Because   is the only harmonic function with this property, which follows from Liouville's theorem, this guarantees the uniqueness of the gradient and rotation fields.[33]

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.

Applications edit

Electrodynamics edit

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.[18]

Fluid dynamics edit

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   is called the Stokes operator.[34]

Dynamical systems theory edit

In the theory of dynamical systems, Helmholtz decomposition can be used to determine "quasipotentials" as well as to compute Lyapunov functions in some cases.[35][36][37]

For some dynamical systems such as the Lorenz system (Edward N. Lorenz, 1963[38]), a simplified model for atmospheric convection, a closed-form expression of the Helmholtz decomposition can be obtained:

 
The Helmholtz decomposition of  , with the scalar potential   is given as:

 
 

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.[10][39]

Computer animation and robotics edit

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.[17][40]

See also edit

Notes edit

  1. ^ Jean Bladel: On Helmholtz's Theorem in Finite Regions. Midwestern Universities Research Association, 1958.
  2. ^ Leo Koenigsberger: Hermann von Helmholtz. Clarendon Press, 1906, p. 357.
  3. ^ Daniel Alexander Murray: An Elementary Course in the Integral Calculus. American Book Company, 1898. p. 8.
  4. ^ J. W. Gibbs, Edwin Bidwell Wilson: Vector Analysis. 1901, p. 237, link from Internet Archive.
  5. ^ Oliver Heaviside: Electromagnetic theory. Volume 1, "The Electrician" printing and publishing company, limited, 1893.
  6. ^ Wesley Stoker Barker Woolhouse: Elements of the differential calculus. Weale, 1854.
  7. ^ William Woolsey Johnson: An Elementary Treatise on the Integral Calculus: Founded on the Method of Rates Or Fluxions. John Wiley & Sons, 1881.
    See also: Method of Fluxions.
  8. ^ James Byrnie Shaw: Vector Calculus: With Applications to Physics. D. Van Nostrand, 1922, p. 205.
    See also: Green's Theorem.
  9. ^ Joseph Edwards: A Treatise on the Integral Calculus. Volume 2. Chelsea Publishing Company, 1922.
  10. ^ a b c d e f Erhard Glötzl, Oliver Richters: Helmholtz decomposition and potential functions for n-dimensional analytic vector fields. In: Journal of Mathematical Analysis and Applications 525(2), 127138, 2023, doi:10.1016/j.jmaa.2023.127138, arXiv:2102.09556v3. Mathematica worksheet at doi:10.5281/zenodo.7512798.
  11. ^ George Gabriel Stokes: On the Dynamical Theory of Diffraction. In: Transactions of the Cambridge Philosophical Society 9, 1849, pp. 1–62. doi:10.1017/cbo9780511702259.015, see pp. 9–10.
  12. ^ Hermann von Helmholtz: Über Integrale der hydrodynamischen Gleichungen, welche den Wirbelbewegungen entsprechen. In: Journal für die reine und angewandte Mathematik 55, 1858, pp. 25–55, doi:10.1515/crll.1858.55.25 (sub.uni-goettingen.de, digizeitschriften.de). On page 38, the components of the fluid's velocity (uvw) are expressed in terms of the gradient of a scalar potential P and the curl of a vector potential (LMN).
  13. ^ a b c d Alp Kustepeli: On the Helmholtz Theorem and Its Generalization for Multi-Layers. In: Electromagnetics 36.3, 2016, pp. 135–148, doi:10.1080/02726343.2016.1149755.
  14. ^ a b Ton Tran-Cong: On Helmholtz’s Decomposition Theorem and Poissons’s Equation with an Infinite Domain. In: Quarterly of Applied Mathematics 51.1, 1993, pp. 23–35, JSTOR 43637902.
  15. ^ a b c D. Petrascheck, R. Folk: Helmholtz decomposition theorem and Blumenthal’s extension by regularization. In: Condensed Matter Physics 20(1), 13002, 2017, doi:10.5488/CMP.20.13002.
  16. ^ a b Wolfgang Sprössig: On Helmholtz decompositions and their generalizations – An overview. In: Mathematical Methods in the Applied Sciences 33.4, 2009, pp. 374–383, doi:10.1002/mma.1212.
  17. ^ a b c Harsh Bhatia, Gregory Norgard, Valerio Pascucci, Peer-Timo Bremer: The Helmholtz-Hodge Decomposition – A Survey. In: IEEE Transactions on Visualization and Computer Graphics 19.8, 2013, pp. 1386–1404, doi:10.1109/tvcg.2012.316.
  18. ^ a b c Dietmar Petrascheck: The Helmholtz decomposition revisited. In: European Journal of Physics 37.1, 2015, Artikel 015201, doi:10.1088/0143-0807/37/1/015201.
  19. ^ "Helmholtz' Theorem" (PDF). University of Vermont. Archived from the original (PDF) on 2012-08-13. Retrieved 2011-03-11.
  20. ^ a b c David J. Griffiths: Introduction to Electrodynamics. Prentice-Hall, 1999, p. 556.
  21. ^ Cherif Amrouche, Christine Bernardi, Monique Dauge, Vivette Girault: Vector potentials in three dimensional non-smooth domains. In: Mathematical Methods in the Applied Sciences 21(9), 1998, pp. 823–864, doi:10.1002/(sici)1099-1476(199806)21:9<823::aid-mma976>3.0.co;2-b, Bibcode:/abstract 1998MMAS...21..823A .
  22. ^ R. Dautray and J.-L. Lions. Spectral Theory and Applications, volume 3 of Mathematical Analysis and Numerical Methods for Science and Technology. Springer-Verlag, 1990.
  23. ^ V. Girault, P.A. Raviart: Finite Element Methods for Navier–Stokes Equations: Theory and Algorithms. Springer Series in Computational Mathematics. Springer-Verlag, 1986.
  24. ^ A. M. Stewart: Longitudinal and transverse components of a vector field. In: Sri Lankan Journal of Physics 12, pp. 33–42, 2011, doi:10.4038/sljp.v12i0.3504 arXiv:0801.0335
  25. ^ Robert Littlejohn: The Classical Electromagnetic Field Hamiltonian. Online lecture notes, berkeley.edu.
  26. ^ a b Erhard Glötzl, Oliver Richters: Helmholtz Decomposition and Rotation Potentials in n-dimensional Cartesian Coordinates. 2020, arXiv:2012.13157.
  27. ^ Frank W. Warner: The Hodge Theorem. In: Foundations of Differentiable Manifolds and Lie Groups. (= Graduate Texts in Mathematics 94). Springer, New York 1983, doi:10.1007/978-1-4757-1799-0_6.
  28. ^ Cantarella, Jason; DeTurck, Dennis; Gluck, Herman (2002). "Vector Calculus and the Topology of Domains in 3-Space". The American Mathematical Monthly. 109 (5): 409–442. doi:10.2307/2695643. JSTOR 2695643.
  29. ^ R. Douglas Gregory: Helmholtz's Theorem when the domain is Infinite and when the field has singular points. In: The Quarterly Journal of Mechanics and Applied Mathematics 49.3, 1996, pp. 439–450, doi:10.1093/qjmam/49.3.439.
  30. ^ Otto Blumenthal: Über die Zerlegung unendlicher Vektorfelder. In: Mathematische Annalen 61.2, 1905, pp. 235–250, doi:10.1007/BF01457564.
  31. ^ Morton E. Gurtin: On Helmholtz’s theorem and the completeness of the Papkovich-Neuber stress functions for infinite domains. In: Archive for Rational Mechanics and Analysis 9.1, 1962, pp. 225–233, doi:10.1007/BF00253346.
  32. ^ Cauchy, Augustin-Louis (1823). "Trente-Cinquième Leçon". Résumé des leçons données à l’École royale polytechnique sur le calcul infinitésimal (in French). Paris: Imprimerie Royale. pp. 133–140.
  33. ^ Sheldon Axler, Paul Bourdon, Wade Ramey: Bounded Harmonic Functions. In: Harmonic Function Theory (= Graduate Texts in Mathematics 137). Springer, New York 1992, pp. 31–44, doi:10.1007/0-387-21527-1_2.
  34. ^ Alexandre J. Chorin, Jerrold E. Marsden: A Mathematical Introduction to Fluid Mechanics (= Texts in Applied Mathematics 4). Springer US, New York 1990, doi:10.1007/978-1-4684-0364-0.
  35. ^ Tomoharu Suda: Construction of Lyapunov functions using Helmholtz–Hodge decomposition. In: Discrete & Continuous Dynamical Systems – A 39.5, 2019, pp. 2437–2454, doi:10.3934/dcds.2019103.
  36. ^ Tomoharu Suda: Application of Helmholtz–Hodge decomposition to the study of certain vector fields. In: Journal of Physics A: Mathematical and Theoretical 53.37, 2020, pp. 375703. doi:10.1088/1751-8121/aba657.
  37. ^ Joseph Xu Zhou, M. D. S. Aliyu, Erik Aurell, Sui Huang: Quasi-potential landscape in complex multi-stable systems. In: Journal of the Royal Society Interface 9.77, 2012, pp. 3539–3553, doi:10.1098/rsif.2012.0434.
  38. ^ Edward N. Lorenz: Deterministic Nonperiodic Flow. In: Journal of the Atmospheric Sciences 20.2, 1963, pp. 130–141, doi:10.1175/1520-0469(1963)020<0130:DNF>2.0.CO;2.
  39. ^ Heinz-Otto Peitgen, Hartmut Jürgens, Dietmar Saupe: Strange Attractors: The Locus of Chaos. In: Chaos and Fractals. Springer, New York, pp. 655–768. doi:10.1007/978-1-4757-4740-9_13.
  40. ^ Hersh Bhatia, Valerio Pascucci, Peer-Timo Bremer: The Natural Helmholtz-Hodge Decomposition for Open-Boundary Flow Analysis. In: IEEE Transactions on Visualization and Computer Graphics 20.11, Nov. 2014, pp. 1566–1578, Nov. 2014, doi:10.1109/TVCG.2014.2312012.

References edit

  • George B. Arfken and Hans J. Weber, Mathematical Methods for Physicists, 4th edition, Academic Press: San Diego (1995) pp. 92–93
  • George B. Arfken and Hans J. Weber, Mathematical Methods for Physicists – International Edition, 6th edition, Academic Press: San Diego (2005) pp. 95–101
  • Rutherford Aris, Vectors, tensors, and the basic equations of fluid mechanics, Prentice-Hall (1962), OCLC 299650765, pp. 70–72