In the simulation of physical processes and phenomena overall there is a problem of numerical solutions of differential problems in partial derivatives. One of the methods for numerical solutions of mathematical physics equations is the approach, based on the finite difference approximation. However, the major drawback of this method is the need to use orthogonal hexahedral mesh. While solving problems in practice, sometimes a complex geometric configuration of the computational domain (for example, see Figure 1) is used, and therefore, the drawback, mentioned above, is enough critical.
Picture 1 – The example of a complex surface as a part of the computational domain
Due to the aforesaid, there is quite a natural problem of the geometrical configuration approximation of computational domain by cell faces of the given orthogonal hexahedral mesh.
Let us turn to a more detailed problem statement. Let the computational mesh in 3D space be orthogonal and hexahedral. Namely, let 
be a set of mesh divisions along 
axis arranged in ascending order, similar to it let us introduce the ordered sets of mesh divisions along 
and 
respectively. According to designations, the following set of points is the set nodes of computational mesh 
. Geometric configuration that requires approximation by mesh faces, is defined by triangulated surface. Let 
be a set of triangles. This set is a part of surface triangulation, and its quantity is 
.With the numerical solution of mathematical physics problem, besides geometric approximation of surface by faces of mesh cells there is a problem of adequate transfer of triangulated surface area on cells faces, involved in its approximation. Thus, by means of set of triangles it is necessary to determine the set of faces of mesh cells, that approximate the triangulated surface, and match the transferred area with each of such face.
Let us give the algorithm, which solves the set above problem
1) Let us consider the mesh, which is dual to the initial one, i.e. such a mesh the nodes of which are cells centers of the initial computational mesh. Let us store a 3D array 
of real numbers, elements of which correspond to the cells of dual mesh. First of all we initialize elements of array by zeros.