Large-scale computations of flow around a circular cylinder

A series of Direct Numerical Simulations of three-dimensional, incompressible flow around a circular cylinder in the lower subcritical range has been performed on the NEC SX-8 of HLRS in Stuttgart. The Navier-Stokes solver, that is employed in the simulations, is vectorizable and parallellized using the standard Message Passing Interface (MPI) protocol. Compared to the total number of operations, the percentage of vectorizable operations exceeds 99.5% in all simulations. In the spanwise direction Fourier transforms are used to reduce the originally three-dimensional pressure solver into two-dimensional pressure solvers for the parallel (x, y) planes. Because of this reduction in size of the problem, also the vectors lengths are reduced which was found to lead to a reduction in performance on the NEC. Apart from the performance of the code as a function of the average vector-lenght, also the performance of the code as a function of the number of processors is assessed. An increase in the number of processors by a factor of 5 is found to lead to a decrease in performance by approximately 22%. Snapshots of the instantaneous flow field immediately behind the cylinder show that free shear-layers from as the boundary layers along the top an bottom surface of the cylinder separate. Alternatingly, the most upstream part of the shear layers rolls up, becomes three-dimensional and undergoes rapid transition to turbulence. The rolls of rotating turbulent flow are subsequently convected downstream to form a von Karman vortex street.