Hydrodynamic modelling of roughness discontinuities

Hydrodynamic modelling of roughness discontinuities

Reinders, M.

Stelling, G.S. (mentor)
Battjes, J.A. (mentor)
Stive, M.J.F. (mentor)
Massie, W.W. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2001-03-01

In this study the effect of a roughness discontinuity caused by a pipeline cover layer in the sea bed is investigated. As a result of this non-uniform flow configuration, the environmental forces acting on the pipeline cover layer change. To investigate the effect of roughness discontinuities, a numerical model of the flow is made; thereby neglecting the effect of waves. This is done by making use of the flow solvers Delft3D-FL0W and CFX. The most important difference between these solvers is the hydrostatic pressure assumption in Delft3D-FL0W, while CFX solves the full 3D Navier Stokes equations. In order to create reliable simulations, both hydrostatic and non-hydrostatic calculations are performed and compared with experiments and calculations found in literature. These calculations deal with the transition from hydraulically smooth to hydraulically rough surfaces and vice versa. The phenomena of overshooting and undershooting of the wall/bed shear stress near the discontinuity and the development of a sub or 'internal' boundary layer, concluded from previous investigations, are calculated with Delft3D-FL0W as CFX as well. Modelling abrupt roughness discontinuities presents considerable difficulties with respect to the convergence to a grid independent solution in both Delft3D-FL0W as CFX, particularly in the smooth to rough case. The (equilibrium) wall function concept, strictly speaking only valid for uniform flow conditions, could be an important explanation for this problem Based on these test case calculations, the flow near an abrupt roughness increment caused by a pipeline cover layer of 10 m length is modelled with CFX. The model is used to make qualitative predictions of the flow and loads on the pipeline cover layer, but is not able to predict the exact loads. The different calculations lead to the consideration that both the instantaneous bed shear stress and its standard deviation increase near the roughness discontinuity. It is therefore concluded that the stones are under heavier attack in comparison to uniform flow conditions. For realistic situations, when the influence of the bed forms resulting from sediment transport are taken into account, these variations are probably small. It is suggested that these findings are verified experimentally and extended with investigation to the effect of waves and the influence on the problem of sediment transport.

Delft 3D
pipeline cover
roughness

http://resolver.tudelft.nl/uuid:a5a8e082-f332-40ce-a198-1997c94b8703

Student theses

master thesis

(c) 2001 Reinders, M.