The ramifications of plastic pollution on the environment are becoming increasingly serious in various forms throughout the world. In this context, rivers are the most important suppliers of plastics entering the marine environment. However, rivers that contain high loads of plastic waste also directly harm the livelihoods of people living near these rivers. An example is the flooding of urban areas in Indonesia due to clogging of the hydraulic and drainage systems, which is caused by the blockage of local hydraulic structures by plastic debris.
At this moment, there is a lack of knowledge on this accumulation process and its underlying dynamics, since observational and experimental research is lacking. Numerical modeling has proven to be a great tool for expanding experimental research. However, no suitable numerical method has been identified yet to model the plastic accumulation process, since traditional mesh-based CFD numerical methods are expected to be not a viable option, due to their inability to model the individual interaction between plastic particles, critical during this process. A possible solution could be the SPH-DEM method, which is a two-way coupled numerical approach that simulates fluid and debris as discrete particles and elements.
The objective of this report was to find out if SPH-DEM could be a suitable numerical method to model the dynamic processes of the plastic debris accumulation against hydraulic structures. To accomplish this, the first goal was to realistically model a turbulent open-channel flow and the buoyancy of individual plastic debris, which would be validated by experimental research. The second goal was to investigate which are the most important (numerical) parameters affecting the mentioned plastic debris accumulation.
In this report, experimental research was carried out in the form of buoyancy tests and flume tests, and numerical research was carried out in the form of the design of numerical simulations. In the buoyancy tests, the rising velocities of four plastic fragments that differed in size and density were measured, which were released multiple times in a graduated cylinder filled with water. In the flume tests, first the water elevation was measured along the the flume, after which the passing ratio’s and carpet lengths were measured for the four different released fragments for three different gate configurations. Two types of numerical models were designed that represented both types of experimental tests, for which several design choices had to be made to compensate for several physical phenomena, which can’t be directly represented in the model design.
The numerical buoyancy test was validated with the rising velocities obtained from the experimental equivalent. It was discovered, that for relatively low resolution modeled fragments, the rising velocity is heavily influenced by numerical diffusion. The smoothing length was identified as an important numerical parameter, which can compensate this effect. Furthermore, it was discovered that the degree of numerical diffusion is dependent on the depth of the fragment in the water. The numerical flume test was validated with the water elevation obtained from the experimental equivalent. For uniform flows, by adjusting the boundary viscosity coefficient, smooth turbulent velocity profiles could be simulated throughout the flume corresponding to theoretical values. However, no single value of was found in which the velocity profiles of the uniform flow and the validated water elevation of the gradually varied flows were both in agreement with their theoretical values. After validation, fragments were added to numerical flume model. Per fragment type and gate configuration,
four different numerical scenarios were executed, where each scenario was defined by a combination of a certain density ½s and restitution coefficient e. Finally, the best corresponding scenarios were used to simulate mixed fragments released in the flow. It was found that the gate opening height , density and restitution of the plastic fragments have the largest influence on the passing ratio’s , carpet length , carpet shape and carpet stability. Furthermore, it was confirmed that individual fragment interactions play a crucial role in the accumulation process. However, the model is mainly limited by its low resolution and the absence of suitable turbulence models. This means that many forms of fragment behavior seen in the experimental research such of buoyancy, trajectory and individual interactions, which are heavily influenced by turbulence, cannot be sufficiently represented in the numerical model. However, it is shown that by adjusting the density the buoyancy behavior can be partly replicated and by adjusting the restitution coefficient the turbulent individual interactions can be partly replicated. In conclusion it can be stated that SPH-DEM is an interesting option to model the dynamic processes of the accumulation of plastic debris against a sluice gate; however, further improvements in computational power and turbulence models are needed to be more widely applied.