Influence of model drag coefficient on LES prediction accuracy of vegetated open-channel flows

Satoshi Yokojima, Yoshihisa Kawahara

Thursday 2 july 2015

15:05 - 15:20h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 12B. Engineering - River

The influence of the model drag coefficient on prediction accuracy of turbulent open-channel flows past emergent vegetation canopies by a classical drag-force model has been examined. Two types of numerical simulations, i.e., preliminary analysis based on a two-dimensional immersed boundary method (IBM) and main analysis employing a three-dimensional large-eddy simulation (LES) together with the drag-force model are carried out. The preliminary analysis focuses on an estimation of an appropriate drag coefficient distribution for the target problem by fully resolving the fluid motions around each element of the canopies in a domain of a geometrical minimal unit. The estimated drag coefficient is then introduced into the main analysis where a wider computational domain is covered. The prediction accuracy of the main analysis is critically evaluated against the corresponding experimental data and another set of LES results based on a uniform drag coefficient within the canopy. The present LES with the sophisticated drag coefficient shows an improved prediction especially on stagnation phenomena at the leading edge of the vegetation patches. The improvement, however, is found mainly at the time-averaged flow level only. This is because the drag coefficient is obtained from the time-averaged flow field and does not reflect the instantaneous flow features directly.