Inhomogeneous Helmholtz equation: Difference between revisions
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The '''inhomogeneous Helmholtz equation''' is an important [[elliptic partial differential equation]] arising in [[acoustics]] and [[electromagnetism]]. It models time-harmonic [[wave]] propagation in free space due to a localized source. | The '''inhomogeneous Helmholtz equation''' is an important [[elliptic partial differential equation]] arising in [[acoustics]] and [[electromagnetism]]. It models time-harmonic [[wave]] propagation in [[Free space (electromagnetism)|free space]] due to a localized source. | ||
More specifically, the inhomogeneous Helmholtz equation is the equation | More specifically, the inhomogeneous Helmholtz equation is the equation |
Revision as of 23:44, 30 November 2010
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The inhomogeneous Helmholtz equation is an important elliptic partial differential equation arising in acoustics and electromagnetism. It models time-harmonic wave propagation in free space due to a localized source.
More specifically, the inhomogeneous Helmholtz equation is the equation
where is the Laplace operator, is a constant, called the wavenumber, is the unknown solution, is a given function with compact support, and (theoretically, can be any positive integer, but since stands for the dimension of the space in which the waves propagate, only the cases with are physical).
Derivation from the wave equation
Wave propagation in free space due to a source is modeled by the wave equation
where and are real-valued functions of spatial variables, and one time variable, is given, the source of waves, and is the unknown wave function.
By taking the Fourier transform of this equation in the time variable, or equivalently, by looking for time-harmonic solutions of the form
with
(where and is a real number), the wave equation is reduced to the inhomogeneous Helmholtz equation with
Solution of the inhomogeneous Helmholtz equation
In order to solve the inhomogeneous Helmholtz equation uniquely, one needs to specify a boundary condition at infinity, which is typically the Sommerfeld radiation condition
uniformly in with , where the vertical bars denote the Euclidean norm. Physically, this states that energy travels from the source away to infinity, and not the other way around.
With this condition, the solution to the inhomogeneous Helmholtz equation is the convolution
(notice this integral is actually over a finite region, since has compact support). Here, is the Green's function of this equation, that is, the solution to the inhomogeneous Helmholtz equation with equaling the Dirac delta function, so satisfies
The expression for the Green's function depends on the dimension of the space. One has
for
for , where is a Hankel function, and
for
References
- Howe, M. S. (1998). Acoustics of fluid-structure interactions. Cambridge; New York: Cambridge University Press. ISBN 0-521-63320-6.
- A. Sommerfeld, Partial Differential Equations in Physics, Academic Press, New York, New York, 1949.