Evaluation of a multigrid method for the reduction of the computational time of SWAN

Evaluation of a multigrid method for the reduction of the computational time of SWAN

Van der Westhuysen, A.J.
Van Vledder, G.P.

SBW

2007-12-01

The spectral wind wave model SWAN plays a key role in the estimation of the Hydraulic Boundary Conditions (HBC) for the primary sea defences of the Netherlands. Since some uncertainty remains with respect to the reliability of SWAN for application to the geographically complex area of the Wadden Sea, a number of activities have been initiated under project H4918 ‘Uitvoering Plan van Aanpak SBW-RVW Waddenzee’ (Plan of Action on the Boundary Conditions for the Wadden Sea) to devise a strategy for the improvement of the model. In this context, hindcast and sensitivity studies carried out with SWAN for the Amelader Zeegat in the Wadden Sea have shown that significant computational times are required to achieve results with the desired levels of numerical accuracy. This finding has led to a drive towards exploring ways to reduce the computational time required by SWAN. The present study investigates the application of a multigrid method to SWAN, which would improve the initial guess used in the iterative solution procedure. The aim of the present study is to investigate the application of this multigrid method to stationary SWAN simulations of typical storm conditions in the Wadden Sea. It is firstly aimed to assess whether this method leads to a reduction in computational time in these hindcasts. A second, equally important aim is to determine whether the application of the multigrid method has any negative impacts on model accuracy. The application of a multigrid technique to SWAN was considered in two stages. During the first stage, multigrid operation is enabled by sequentially running two separate SWAN simulations - the first on a coarse computational grid and the second on the final, detailed grid resolution. The second stage, prompted by the positive results of the first stage, was to incorporate the multigrid into the SWAN source code. It was found that a reduction in geographical space appears to be the most promising candidate to use in the initial guess run. The evaluation of the present implementation revealed that simulation time can be reduced by up to 23%, without a significant loss in accuracy (measured in terms of the convergence error). It was also found that in some cases simulation times are not significantly reduced, but that the accuracy of the model result was strongly improved

swan
Amelander zeegat
computational grid

TLJ300200

http://resolver.tudelft.nl/uuid:1fa83c4b-a7db-48fd-b0b9-3521266d3456

Deltares (WL)

H4918 38 for rijkswaterstaat

Hydraulic Engineering Reports

report

(c) 2007 Deltares