Numerical techniques involving linearizations of nonlinear functions require the repeated solution of systems of linear equations whose coefficient matrix is the Jacobian of that nonlinear function. If the Jacobian is large and sparse, iterative methods offer the advantage that they involve the Jacobian solely in the form of matrix-vector products. Techniques of automatic differentiation are capable of evaluating these Jacobian-vector products efficiently and accurately in a matrix-free fashion. So, the numerical technique does not need to store the Jacobian explicitly. When the solution of the linear system is preconditioned, however, there is currently a considerable gap between automatic differentiation and preconditioning because the latter typically requires to explicitly store the Jacobian in a sparse data format. In an attempt to bridge this gap, we introduce an approach based on block diagonal preconditioning that brings together known computational building blocks in a novel way. The crucial methodological ingredient to that approach is the formulation and solution of a partial coloring problem in which colors are assigned to only a subset of the vertices of the underlying graph. Numerical experiments are reported that demonstrate the feasibility of this approach.
Implicit Time Integration for FlowThe last decades have witnessed a steady increase in airline passenger traffic. This enormous air traffic and the resulting airport congestion has stimulated growing interest in the design of high-capacity aircrafts in which the number of passengers in a transport aircraft is large. Thus, it is no surprise that, in science and industry, numerical simulations that accurately and reliably predict the flow field around a high-capacity aircraft continue to be important.At RWTH Aachen University, Germany, an interdisciplinary team of researchers from mathematics, computer science, and engineering disciplines is developing the software package QUADFLOW [3,4,6]. This * Chair for Advanced Computing, Institute for Computer Science, Friedrich Schiller University Jena, Germany † Michael Stifel Center Jena for Data-driven and Simulation Science, 07737 Jena, Germany ‡ Düsseldorf, Germany adaptive finite volume flow solver represents the underlying physics of stationary flow by the Euler and Navier-Stokes equations. It employs two-and threedimensional meshes in multiblocks; see Fig. 1 for an example of such a mesh in two space dimensions. Summarizing the discussion in [6], let V denote any control volume and let u represent the vector of unknown conserved quantities. Then, spatial discretization results in the system of differential equations V