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Vector operator producing the scalar quantity of a flow at a point
The divergence of different vector fields. The divergence of vectors from point (x,y) equals the sum of the partial derivative-with-respect-to-x of the x-component and the partial derivative-with-respect-to-y of the y-component at that point:
As an example, consider air as it is heated or cooled. The velocity of the air at each point defines a vector field. While air is heated in a region, it expands in all directions, and thus the velocity field points outward from that region. The divergence of the velocity field in that region would thus have a positive value. While the air is cooled and thus contracting, the divergence of the velocity has a negative value.
The divergence at a point x is the limit of the ratio of the flux through Si(red arrows) to the volume for any sequence of closed regions V1, V2, V3... enclosing x that approaches zero volume.
The divergence of a vector field F(x) at a point x0 is defined as the limit of the ratio of the surface integral of F out of the surface of a closed volume V enclosing x0 to the volume of V, as V shrinks to zero
where || is the volume of V, S(V) is the boundary of V, and n? is the outward unit normal to that surface. It can be shown that the above limit always converges to the same value for any sequence of volumes that approaches zero. The result, div F, is a scalar function of x.
A vector field with zero divergence everywhere is called solenoidal - in which case any closed surface has no net flux across it.
Physical interpretation of divergence
In physical terms, the divergence of a vector field is the extent to which the vector field flux behaves like a source at a given point. It is a local measure of its "outgoingness" - the extent to which there is more of the field vectors exiting an infinitesimal region of space than entering it. A point of positive divergence, representing outgoing flux at a point, is often called a "source" of the field. A point of negative divergence, representing inward flux at a point, is often called a "sink".
Divergence is often explained using the example of the velocity field of a fluid, a liquid or gas. If the fluid is heated it will expand. This will cause a net motion of fluid particles outward in all directions. Any closed surface in the fluid will enclose fluid which is expanding, so there will be an outward flux of fluid particles through the surface. So the velocity field will have positive divergence everywhere. Similarly, if the fluid is cooled, it will contract. There will be more room for fluid particles in any volume, so the external pressure of the fluid will cause a net flow of fluid volume inward through any closed surface. Therefore the velocity field has negative divergence everywhere. In contrast in an unheated fluid with a constant density, the volume rate of fluid flowing into any closed surface must equal the volume rate flowing out, so the net flux of fluid through any surface is zero. Thus the fluid has zero divergence everywhere.
If the fluid is heated only at one point or small region, the fluid there will expand, pushing fluid particles around it outward in all directions. This will cause an outward velocity field throughout the fluid. Any closed surface enclosing the heated point will have a flux of fluid particles passing out of it, so there is positive divergence at that point. However any closed surface not enclosing the point will have a constant density of fluid inside, so just as many fluid particles are entering as leaving the volume, thus the net flux out of the volume is zero. Therefore the divergence at any other point is zero.
Although expressed in terms of coordinates, the result is invariant under rotations, as the physical interpretation suggests. This is because the trace of the Jacobian matrix of an N-dimensional vector field F in N-dimensional space is invariant under any invertible linear transformation.
The common notation for the divergence ? · F is a convenient mnemonic, where the dot denotes an operation reminiscent of the dot product: take the components of the ? operator (see del), apply them to the corresponding components of F, and sum the results. Because applying an operator is different from multiplying the components, this is considered an abuse of notation.
The divergence of a continuously differentiable second-order tensor field? is a first-order tensor field:
where ea is the unit vector in direction a, the divergence is
The use of local coordinates is vital for the validity of the expression. If we consider x the position vector and the functions , , and , which assign the corresponding global cylindrical coordinate to a vector, in general , , and . In particular, if we consider the identity function , we find that:
In spherical coordinates, with ? the angle with the z axis and ? the rotation around the z axis, and again written in local unit coordinates, the divergence is
Using Einstein notation we can consider the divergence in general coordinates, which we write as x1, ..., xi, ...,xn, where n is the number of dimensions of the domain. Here, the upper index refers to the number of the coordinate or component, so x2 refers to the second component, and not the quantity x squared. The index variable i is used to refer to an arbitrary element, such as xi. The divergence can then be written via the Voss- Weyl formula, as:
where is the local coefficient of the volume element and Fi are the components of F with respect to the local unnormalizedcovariant basis (sometimes written as ). The Einstein notation implies summation over i, since it appears as both an upper and lower index.
The volume coefficient is a function of position which depends on the coordinate system. In Cartesian, cylindrical and spherical coordinates, using the same conventions as before, we have , and , respectively. It can also be expressed as , where is the metric tensor. Since the determinant is a scalar quantity which doesn't depend on the indices, we can suppress them and simply write . Another expression comes from computing the determinant of the Jacobian for transforming from Cartesian coordinates, which for n = 3 gives
Some conventions expect all local basis elements to be normalized to unit length, as was done in the previous sections. If we write for the normalized basis, and for the components of F with respect to it, we have that
using one of the properties of the metric tensor. By dotting both sides of the last equality with the contravariant element , we can conclude that . After substituting, the formula becomes:
It can be shown that any stationary flux v(r) that is at least twice continuously differentiable in R3 and vanishes sufficiently fast for || -> ? can be decomposed into an irrotational partE(r) and a source-free partB(r). Moreover, these parts are explicitly determined by the respective source densities (see above) and circulation densities (see the article Curl):
For the irrotational part one has
The source-free part, B, can be similarly written: one only has to replace the scalar potential?(r) by a vector potentialA(r) and the terms - by +? × A, and the source density div v
by the circulation density ? × v.
This "decomposition theorem" is a by-product of the stationary case of electrodynamics. It is a special case of the more general Helmholtz decomposition which works in dimensions greater than three as well.
The following properties can all be derived from the ordinary differentiation rules of calculus. Most importantly, the divergence is a linear operator, i.e.,
The divergence of the curl of any vector field (in three dimensions) is equal to zero:
If a vector field F with zero divergence is defined on a ball in R3, then there exists some vector field G on the ball with F = curl G. For regions in R3 more topologically complicated than this, the latter statement might be false (see Poincaré lemma). The degree of failure of the truth of the statement, measured by the homology of the chain complex
serves as a nice quantification of the complicatedness of the underlying region U. These are the beginnings and main motivations of de Rham cohomology.
Relation with the exterior derivative
One can express the divergence as a particular case of the exterior derivative, which takes a 2-form to a 3-form in R3. Define the current two-form as
It measures the amount of "stuff" flowing through a surface per unit time in a "stuff fluid" of density ? = 1 dx ? dy ? dz moving with local velocity F. Its exterior derivativedj is then given by
Thus, the divergence of the vector field F can be expressed as:
Here the superscript ♭ is one of the two musical isomorphisms, and ? is the Hodge star operator. Working with the current two-form and the exterior derivative is usually easier than working with the vector field and divergence, because unlike the divergence, the exterior derivative commutes with a change of (curvilinear) coordinate system.
The divergence of a vector field can be defined in any number of dimensions. If
in a Euclidean coordinate system with coordinates x1, x2, ..., xn, define
In the case of one dimension, F reduces to a regular function, and the divergence reduces to the derivative.
For any n, the divergence is a linear operator, and it satisfies the "product rule"
for any scalar-valued function ?.
The divergence of a vector field extends naturally to any differentiable manifold of dimension n that has a volume form (or density) ?, e.g. a Riemannian or Lorentzian manifold. Generalising the construction of a two-form for a vector field on R3, on such a manifold a vector field X defines an (n - 1)-form j = iX ? obtained by contracting X with ?. The divergence is then the function defined by
Standard formulas for the Lie derivative allow us to reformulate this as
This means that the divergence measures the rate of expansion of a volume element as we let it flow with the vector field.
Equivalently, some authors define the divergence of a mixed tensor by using the musical isomorphism♯: if T is a (p, q)-tensor (p for the contravariant vector and q for the covariant one), then we define the divergence of T to be the (p, q - 1)-tensor
that is, we take the trace over the first two covariant indices of the covariant derivative[a]
^ The choice of "first" covariant index of a tensor is intrinsic and depends on the ordering of the terms of the Cartesian product of vector spaces on which the tensor is given as a multilinear map V × V × ... × V -> R. But equally well defined choices for the divergence could be made by using other indices. Consequently, it is more natural to specify the divergence of T with respect to a specified index. There are however two important special cases where this choice is essentially irrelevant: with a totally symmetric contravariant tensor, when every choice is equivalent, and with a totally antisymmetric contravariant tensor (a.k.a. a k-vector), when the choice affects only the sign.
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