Multi-resolution isogeometric analysis - efficient adaptivity utilizing the multi-patch structure

Isogeometric Analysis (IgA) is a spline-based approach to the numerical solution of partial differential equations. The concept of IgA was designed to address two major issues. The first issue is the exact representation of domains generated from Computer-Aided Design (CAD) software. In practice, this can be realized only with multi-patch IgA, often in combination with trimming or similar techniques. The second issue is the realization of high-order discretizations (by increasing the spline degree) with a number of degrees of freedom comparable to low-order methods. High- order methods can deliver their full potential only if the solution to be approximated is sufficiently smooth; otherwise, adaptive methods are required. A zoo of local refinement strategies for splines has been developed in the last decades. Such approaches impede the utilization of recent advances that rely on tensor-product splines, e.g., matrix assembly and preconditioning. We propose a strategy for adaptive IgA that utilizes well-known approaches from the multi-patch IgA toolbox: using tensor-product splines locally, but allow for unstructured patch configurations globally. Our approach moderately increases the number of patches and utilizes different grid sizes for each patch. This allows reusing the existing code bases, recovers the convergence rates of other adaptive approaches, and increases the number of degrees of freedom only marginally. We provide an algorithm for the computation of a global basis and show that it works in any case. Additionally, we give approximation error estimates. Numerical experiments illustrate our results.