Applicability of DualSPHysics model to derivation of drag coefficient in vegetation

Tomohiro Suzuki, Haruhi Oyaizu, Corrado Altomare, Alex Crespo, Jose Dominguez

Monday 29 june 2015

16:30 - 16:45h at Central America (level 0)

Themes: (T) Special session, (ST) Smoothed particle hydrodynamics and other meshfree methods

Parallel session: 3H. Special session: Smoothed Particle Hydrodynamics and other meshfree methods

Vegetation in coastal regions works as natural wave attenuator. Even though many efforts have been done in the last decades, determination of wave attenuation by vegetation is still remained as a challenging task. One of the reasons is that it is often difficult to choose an appropriate drag coefficient to estimate wave attenuation by vegetation. In practice many authors (e.g. Iimura et al., 2007) assume the bulk drag coefficient of vertical rigid cylinders to be 1.0 in the case of a simple cylindrical array for subcritical Reynolds numbers. This value is decided based on flow conditions and is applied for wave conditions. This choice though practical lacks background theories. For instance, the drag coefficient of a single cylinder in planar oscillations varies from 0.5 to 2.5 according to Sarpkaya (1976) and it is sensitive to hydraulic parameters such as the Reynolds number and the Keulegan–Carpenter number. Not only that the value can be also changed in different spacing when a multiple cylinder case is considered. In this study a SPH model is tested and applied to estimate the drag coefficient in different flow and vegetation conditions. By using the SPH model, it is easy to deal with an arbitrary shape (e.g. cylinders) due to its grid free feature while grid based models need extra treatment for the boundary condition on the surface. Another benefit of using the SPH model is that an object can be moved easily inside the domain. This feature makes a new approach possible: obtaining the drag coefficient by moving the cylinder instead of putting the cylinder in a flow. This approach will be especially useful for multiple cylinder case. Furthermore, the rigid cylinder can be extended to a flexible one. This is important since most of actual vegetation are flexible. In this work some validations are conducted based on measured values in literatures and the sensitivity for different numerical and physical parameters in the model are shown. Promising results have been obtained and the applicability of DualSPHysics model to derivation of drag coefficient in vegetation is confirmed.