CONSTRUCTION OF A MODIFICATION OF THE TRAPEZOIDAL QUADRATURE FORMULA BASED ON THE THIRD-ORDER LOCAL INTERPOLATION SPLINE FUNCTION WITH A DEFECT EQUAL TO TWO

Two recently introduced quadrature schemes for weakly singular integrals are investigated in the context of boundary integral equations arising in the isogeometric formulation of Galerkin Boundary Element Method (BEM). In the first scheme, the regular part of the integrand is approximated by a suitable quasi–interpolation spline. In the second scheme the regular part is approximated by a product of two spline functions. The two schemes are tested and compared against other standard and novel methods available in literature to evaluate different types of integrals arising in the Galerkin formulation. Numerical tests reveal that under reasonable assumptions the second scheme convergences with the optimal order in the Galerkin method, when performing h-refinement, even with a small amount of quadrature nodes. The quadrature schemes are validated also in numerical examples to solve 2D Laplace problems with Dirichlet boundary conditions.