Author(s): Jim Ly; Colin D. Rennie
Linked Author(s): Colin Rennie
Keywords: Constriction; Numerical modelling; River engineering; RANS; Turbulence
Abstract: River constrictions are frequently encountered by engineers, and improper understanding can lead to designs that do not adequately protect against flooding. This paper studies river constrictions using the three dimensional Reynolds Averaged Navier Stokes (RANS) model SSIIM. The model was validated using an experimental laboratory constriction test case (Duc and Rodi 2008) by comparing measured and predicted velocity fields. Subsequently, four different degrees of channel constriction were simulated, and energy losses were estimated based on predicted water levels. In order to explain the physics of the energy losses that occur in a constriction, flow patterns near the constriction were examined, focusing on the eddy reattachment length as well as the distribution of turbulent kinetic energy. As expected, as the constriction ratio increases, both energy losses and the turbulent kinetic energy within the flow field increase. Using the results of the 3D model combined with an analytical expression (Henderson 1966), the contraction head loss coefficient (K) is calculated at various levels of constriction. It is demonstrated that K increases with constriction ratio, despite the fact that constriction ratio is considered in the analytical expression. These results can be used to calibrate 1D models such as HECRAS that are frequently used in industry. Erroneous K values can lead to unsuitable designs with consequences such as flooding causing loss of life and property.