  We present a multi-block finite-difference solver for massively parallel
Direct Numerical Simulations (DNS) of incompressible flows. The algorithm
combines the versatility of a multi-block solver with the method of
eigenfunctions expansions, to speedup the solution of the pressure Poisson
equation. This is achieved by employing FFT-based transforms along one
homogeneous direction, which effectively reduce the problem complexity at a low
cost. These FFT-based expansions are implemented in a framework that unifies
all valid combinations of boundary conditions for this type of method.
Subsequently, a geometric multigrid solver is employed to solve the reduced
Poisson equation in a multi-block geometry. Particular care was taken here, to
guarantee the parallel performance of the multigrid solver when solving the
reduced linear systems equations. We have validated the overall numerical
algorithm and assessed its performance in a massively parallel setting. The
results show that 2- to 8-fold reductions in computational cost may be easily
achieved when exploiting FFT acceleration for the solution of the Poisson
equation. The solver, SNaC, has been made freely available and open-source
under the terms of an MIT license.
