Flow and Transport Modeling in Heterogeneous Sediments Using an Integral Approach.

An integral approach which can simultaneously model turbulent flow and transport at the sediment-water interface has been recently developed and validated for homogeneous sediment which was achieved by comparing numerical results to flume experiments on flow and transport over a rippled streambed and through the sediment for neutral, gaining, and losing conditions. In the present study, we validated the approach for heterogeneous conditions by comparing numerical simulations of flow and transport in heterogeneous sediment to analytical solutions as well as flume experiments on flow and transport through rippled streambed consisting of heterogeneous sediment. For this complex setup, simulation and experimental results agree well showing that flow and tracer transport prefer paths through areas with bigger grain diameters and higher porosities. The effect of flow redirections under losing and gaining conditions on hyporheic flow and residence times is discussed.

High-Resolution Integrated Transport Model for Studying Surface Water-Groundwater Interaction.

Transport processes that lead to exchange of mass between surface water and groundwater play a significant role for the ecological functioning of aquatic systems, for hydrological processes and for biogeochemical transformations. In this study, we present a novel integral modeling approach for flow and transport at the sediment-water interface. The model allows us to simultaneously simulate turbulent surface and subsurface flow and transport with the same conceptual approach. For this purpose, a conservative transport equation was implemented to an existing approach that uses an extended version of the Navier-Stokes equations. Based on previous flume studies which investigated the spreading of a dye tracer under neutral, losing and gaining flow conditions the new solver is validated. Tracer distributions of the experiments are in close agreement with the simulations. The simulated flow paths are significantly affected by in- and outflowing groundwater flow. The highest velocities within the sediment are found for losing condition, which leads to shorter residence times compared to neutral and gaining conditions. The largest extent of the hyporheic exchange flow is observed under neutral condition. The new solver can be used for further examinations of cases that are not suitable for the conventional coupled models, for example, if Reynolds numbers are larger than 10. Moreover, results gained with the integral solver provide high-resolution information on pressure and velocity distributions at the rippled streambed, which can be used to improve flow predictions. This includes the extent of hyporheic exchange under varying ambient groundwater flow conditions.

Using computational fluid dynamics to describe H2S mass transfer across the water-air interface in sewers.

For the past 70 years, researchers have dealt with the investigation of odour in sewer systems caused by hydrogen sulphide formations and the development of approaches to describe it. The state-of-the-art models are one-dimensional. At the same time, flow and transport phenomena in sewers can be three-dimensional, for example the air flow velocities in circular pipes or flow velocities of water and air in the reach of drop structures. Within the past years, increasing computational capabilities enabled the development of more complex models. This paper uses a three-dimensional two-phase computational fluid dynamics model to describe mass transfer phenomena between the two phases: water and air. The solver has been extended to be capable of accounting account for temperature dependency, the influence of pH value and a conversion to describe simulated air phase concentrations as partial pressure. Its capabilities are being explored in different application examples and its advantages compared to existing models are demonstrated in a highly complex three-dimensional test case. The resulting interH2SFoam solver is a significant step in the direction of describing and analysing H2S emissions in sewers.