Fringe region technique and the Fourier method used in the direct numerical simulation of spatially evolving viscous flows

To eliminate the problem with artificial boundary conditions and facilitate the use of Fourier methods, the fringe region (or filter, damping layer, absorbing layer, sponge layer) technique has been used in direct simulations of transitional and turbulent boundary layers. Despite the fact that good computational results have been obtained with this technique, it is not fully understood. The analysis in this paper indicates that the primary importance of the fringe region technique is to damp out the deviation associated with large scales in the direction normal to the wall. The lack of boundary conditions is compensated by the knowledge of an exact solution in the fringe region of the computational domain. The upstream influence from the fringe region is small. Numerical experiments verifying the theoretical predictions are presented.