The aim of the present research project is to develop generic knowledge on the numerical computation of flow about hydraulic structures. Important questions include: how can the flow best be simulated given a certain geometry, which simulation model is appropriate and which properties should this model have, how should the simulation results be interpreted, what is the accuracy of the computations, and what are the shortcomings of the simulation?
The local flow near hydraulic structures usually has strong three-dimensional properties; standard flow simulation models with depth-averaging or a hydrostatic pressure assumption cannot be applied. Variables such as the lay-out and geometry of the structure, the surrounding geometry, the hydraulic roughness of walls and other flow boundaries, and the properties of water (viscosity, density) strongly affect the local flow about the structure. Important hydrodynamic aspects are: the development of the free water surface, the development of boundary layers on walls and bottom, the formation of flow separation points on structures, the formation of free boundary layers in the fluid, vortex generation, spreading of turbulence and damping, expansion of the flow and eddy formation, effects of turbulence on velocity profiles and stream patterns, energy losses, and effects caused by density differences. In the year 2003 an exploratory study was conducted into the present status of the simulation of flow around structures. Attention has also been paid to computational grids and their properties. In the year 2004 flow velocity measurements were taken in a scale model of a section of a groyne (in a related research project). The flow was also simulated using the CFX code and the Delft3D code. These simulations were aimed to check the numerical results on aspects such as the effect of the turbulence model on flow pattern and flow velocity profiles, the effect of wall roughness modelling, the effect of water surface modelling (in CFX). Two basic problems were encountered in the CFX simulations: the construction of a suited grid, and the modelling of very rough walls. The present CFX models still have a moderate convergence and do not yet fully reproduce the actual flow over the groyne. Also the results of the Delft3D simulation with Z-layers are not yet sufficiently satisfactory. |
