A robust high-resolution split-type compact FD scheme for spatial direct numerical simulation of boundary-layer transition

In this paper an efficient split-type Finite-Difference (FD) scheme with high modal resolution - most important for the streamwise convection terms that cause wave transport and interaction - is derived for a mixed Fourier-spectral/FD method that is designed for the spatial direct numerical simulation (DNS) of boundary-layer transition and turbulence. Using a relatively simple but thorough and instructive modal analysis we discuss some principal trouble sources of the related FD discretization. The new scheme is based on a 6th-order compact FD discretization in streamwise and wall-normal direction and the classical 4th-order Runge-Kutta time-integration scheme with symmetrical final corrector step. Exemplary results of a fundamental-(K-) type breakdown simulation of a strongly decelerated Falkner-Skan boundary layer (Hartree parameter beta(H) = -0.18) using 70 mega grid points in space are presented up to the early turbulent regime (Re-theta,Re-turb approximate to 820). The adverse pressure gradient gives rise to local separation zones during the breakdown stage and intensifies final breakdown by strong amplification of (background) disturbances thus enabling rapid transition at moderate Reynolds number. The appearance and dynamics of small scale vortical structures in early turbulence basically similar to the large-scale structures at transition can be observed corroborating Kachanov's hypothesis on the importance of the K-regime of breakdown for coherent structures in turbulence.