Numerical investigations on the function of flush waves in a reservoir sewer

Abstract

The intention of this thesis was to apply the approach of three-dimensional numerical modeling to investigate the hydrodynamic principles of flush waves in a reservoir sewer in a high spatial and temporal resolution. First, to give an overview on the topic of sewer sediments, the basics of sedimentation, content of deposits and remobilisation due to erosion are explained. In the following the basics of numerical modelling are given by the introduction of the three-dimensional Reynolds averaged Navier-Stokes equations and the turbulence modelling using the $k-\epsilon$ turbulence model to have a better understanding of the model StarCD, which was used for the following investigations. The discretisation in space and time with the Finite Volume Method is displayed as well as the determination of the free water surface with the Volume of Fluid method. The wall shear stress is the major parameter for the evaluation of the cleaning success of a flush wave. Therefore definition and calculation of the shear stress are explained in detail. The analytical determination of the dam-break wave is derived from the three-dimensional Reynolds averaged Navier-Stokes equations over to the two-dimensional depth-averaged classical shallow water equations and then to the one-dimensional Saint-Venant equations. Two analytical solutions of Ritter and Dressler and their hydraulic basics are presented. An overview on convenient one-, two- and three-dimensional simulation models to calculate flush waves is given followed by literature reviews on flushing devices and major investigations on flush waves and flush cleaning in sewers. The investigated reservoir sewer August-Bebel Ring in Offenbach is described in its geometry and boundary conditions together with the location of the six ultrasonic probes and the used measurement approach. These probes recorded the data of six flush tests with different storage levels at the gate which was then used for the calibration and validation of the numerical model. The investigations of the behaviour, development and effect of flush waves, carried out with the numerical model StarCD, were split up in eight different topics. First the initial phase of the flushing wave was analyzed using the analytical Ritter and Dressler solutions and then compared to the results of a refined numerical model. The analysis of the bottom shear stress was the main investigation in this thesis. The effective flushing length for the dry-weather channel and the slopes against the storage level at the gate and the flushing volume were calculated and simple functions were developed. The influence of the flushing gate, the storage volume and the time step size on the bottom shear stress and the effective flushing distance were the next investigations which were carried out. A very detailed calculation of the bottom shear stresses determined the viscous boundary layer for the initial phase and for the longer running flush waves. The results of the modelling with very fine bottom cells were then compared with the common approach used in the before mentioned investigations. The inclusion of the sunk wave into the cleaning approach of a sewer channel is a very important issue in practical applications. Therefore the development of the sunk wave and its cleaning efficiency were calculated in a refined numerical model. The last investigation regarded the exceeding duration of the critical shear stress along the sewer channel depending on the initial storage level and the flushing volume.