Neumann boundary condition
In mathematics, the Neumann (or second-type) boundary condition is a type of boundary condition, named after Carl Neumann. When imposed on an ordinary or a partial differential equation, it specifies the values that the derivative of a solution is to take on the boundary of the domain.
It is possible to describe the problem using other boundary conditions: a Dirichlet boundary condition specifies the values of the solution itself (as opposed to its derivative) on the boundary, whereas the Cauchy boundary condition, mixed boundary condition and Robin boundary condition are all different types of combinations of the Neumann and Dirichlet boundary conditions.
Ordinary differential equation
For an ordinary differential equation, for instance,
the Neumann boundary conditions on the interval take the form
where and are given numbers.
Partial differential equation
For a partial differential equation, for instance,
where denotes the Laplace operator, the Neumann boundary conditions on a domain take the form
The normal derivative, which shows up on the left side, is defined as
It becomes clear that the boundary must be sufficiently smooth such that the normal derivative can exist, since for example at corner points of the boundary the normal vector is not well defined.
The following engineering applications involve the use of Neumann boundary conditions:
- In thermodynamics, a prescribed heat flux from a surface would serve as boundary condition. For example, a perfect insulator would have a flux of zero, where as an electrical component may be dissipating a known power.
- Dirichlet boundary condition
- Mixed boundary condition
- Cauchy boundary condition
- Robin boundary condition