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## 12.2.2 Mathematical Theory

The boundary element method  [Brebbia:83a], [Cruse:75a] has been used for many applications where it is necessary to solve a linear elliptic partial differential equation. Because of the linearity of the underlying differential equation, there is a Green's function expressing the solution throughout the three-dimensional domain in terms of the behavior at the boundaries, so that the problem may be transformed into an integral equation   on the boundary.

The discrete approximation to this integral equation results in the solution of a full set of simultaneous linear equations, one equation for each node of the boundary mesh; the conventional finite-difference  method would result in solving a sparse set of equations, one for each node of a mesh-filling space. Let us compare these methods in terms of efficiency and software cost.

To implement the finite-difference method, we would first make a mesh filling the domain of the problem (i.e., a three-dimensional mesh), then for each mesh point set up a linear equation relating its field value to that of its neighbors. We would then need to solve a set of sparse linear equations. In the case of an exterior problem such as ours, we would need to pay special attention to the farfield, making sure the mesh extends out far enough and that the proper approximation is made at this outer boundary.

With the boundary element method, we discretize only the surface of the domain, and again solve a set of linear equations, except that now they are no longer sparse. The far field is no longer a problem, since this is taken care of analytically.

If it is possible to make a regular grid surrounding the domain of interest, then the finite-difference method is probably more efficient, since multigrid methods or alternating direction methods will be faster than the solution of a full matrix.  It is with complex geometries, however, that the boundary element method can be faster and more efficient on sequential or distributed-memory machines. It is much easier to produce a mesh covering a curved two-dimensional manifold than a three-dimensional mesh filling the space exterior to the manifold. If the manifold is changing from step to step, the two-dimensional mesh need only be distorted, whereas a three-dimensional mesh must be completely remade, or at least strongly smoothed, to prevent tangling. If the three-dimensional mesh is not regular, the user faces the not inconsiderable challenge of explicit load balancing and communication at the processor boundaries.     Next: 12.2.3 Results Up: Simulation of the Previous: 12.2.1 Physical Model

Guy Robinson
Wed Mar 1 10:19:35 EST 1995