Multiscale modelling of physical processes along water-soil interface
Final Report Abstract
While it is not easy to perform laboratory or model tests with the ability to “look inside” the soil structure and investigate what happens at particle-scale, numerical tools can be an adequate tool. Recently, due to increasing computational power, methods that map the motion of grains on particle-scale level have become more and more important. The interest in multi-scale methods, where we can investigate the micro-mechanical processes of particles subjected to fluid flow, increases continuously. One of these methods is unresolved coupled CFD-DEM. Unresolved coupled CFD-DEM is a combination of the Computational Fluid Dynamics (CFD) with the Discrete Element Method (DEM). CFD solves the fluid flow while DEM tracks the soil grain motion. However, before being able to use numerical methods like this for an in-depth analysis of problems at particle-scale, the tool needs to be verified if it captures the physics accurately. While considerable work has been carried out in the field of chemical engineering with unresolved coupled CFD-DEM, in geotechnical engineering, research only begins. Therefore, it is essential to verify the applicability of unresolved CFD-DEM in geotechnical engineering against experimental data for each investigated problem. For understanding macroscopic phenomena at the air-water interface within soils and their effect onto them, it is necessary to consider the effects at a refined level. Unresolved CFD-DEM is suitable for doing so, and one of the main achievements of this project was a better applicability to large scale cases: an improvement of the neighbor list algorithm lead to a significant speedup of polydisperse simulations and the implementation of an insertion algorithm for dense packings helps to reduce the time required for the initial case setups. In general, suffusion has been widely studied at a macroscopic scale. These efforts improved our understanding of the macroscopic effects of suffusion, but the mechanisms at microscopic scale associated with the initiation and evolution of suffusion remain unclear. Numerical studies can, therefore, give valuable insight into the leading mechanisms of suffusion and its consequences on the soil matrix. Therefore, in the framework of unresolved CFD-DEM, especially the drag force, which is the driving force in fluid-particle interactions, needs to be verified. As an analytical solution of the drag force in particle assemblies does not yet exists, different models are available in the literature. To analyse the influence of the drag force on the erosion of fines in a suffusive particle assembly, the drag force model of Schiller und Naumann (1935) was extended with a voidage function. The voidage function enabled the variation of the drag force. The extensive investigation of the drag force on the erosion of fine particles in suffusion reveal a significant impact and showed how the drag force alters the erosion behaviour of fine particles in a coarse particle matrix. Although a validation and calibration of the drag force model could not be carried out, the significance of the chose drag force model is demonstrated. Thus, increasing the drag force promotes clogging of the fine particles in the coarse particle matrix with an increased tendency towards sudden blowouts. Reducing the drag force yields a continuous erosion of fines. Furthermore, a first step towards modelling of unsaturated media with unresolved coupled CFD-DEM was carried out. The developed capillary models allow for the depiction of the impact of liquid bridge forces between the particles at the fluid-air interface, which is normally not captured with unresolved CFD-DEM. All these improvements made it possible to simulate the presented application examples. Due to the collaboration between TUHH and DCS it was possible the validate many of the simulation results against experiments. The hydraulic failure next to a sheetpile could be modelled with agreement to the experimental results. This illustrates the wide range of possibilities that unresolved coupled CFD-DEM offers us in the modelling of geotechnical problems. The fluidisation of the soil next to a quay wall can be modelled and thus contribute to the estimation of the risk of hydraulic failure near quay walls in the future. In addition, the phreatic line in a dam could be simulated using unresolved coupled CFD-DEM and matched the analytical formulation. This is particularly important in connection with dike stability, as CFD-DEM makes it possible to estimate soil failure due to the rise in water level on the seaward side. These investigations enable to estimate at what water level and in what way a dike will fail. The different backgrounds of the two project partners were beneficial throughout the whole project and lead to a mutual increase of expertise. The presented project enabled a fruitful collaboration between both applicants TUHH and DCS. Thanks to this partnership the project members at TUHH could extend their expertise by specialized knowledge on numerical modelling with coupled CFD-DEM and improve to work on new problems independently. The research carried out within the scope of the project furthermore lead to a deeper understanding of the tools required to simulate soil and the interaction between soil and fluids and the development and implementation of such models.
Publications
- (2017): Numerical investigations of the extraction of submerged foundations by coupled CFD-DEM. In: Proceedings of ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering. Trondheim, Norway
Kanitz, M., Grabe, J., Hager, A., Goniva, C. und C. Kloss
(See online at https://doi.org/10.1115/OMAE2017-61299) - (2018): Experimental Study of the Inuence of the Pore Water Pressure Evolution and the Shear Band Formation on the Extraction Resistance of Submerged Anchor Plates. In: Proceedings of ASME 2018 37th International Conference on Ocean, Offshore and Arctic Engineering. Madrid, Spain
Kanitz, M.; Grabe, J.
(See online at https://doi.org/10.1115/OMAE2018-78306) - (2018): Multiscale investigations on the failure mechanisms of submarine sand slopes with coupled CFD-DEM. In: Proceedings of 9th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE). Porto, Portugal. CRC Press, Taylor & Francis Group, p. 1485-1492
Kanitz, M.; Grabe, J.
- (2019): Influence of Suction Dredging on the Failure Mechanism of Sandy Submarine Slopes: Revisited With a Coupled Numerical Approach. In: Proceedings of ASME 2018 39th International Conference on Ocean, Offshore and Arctic Engineering. Glasgow, Scotland
Kanitz, M.; Grabe, J.
(See online at https://doi.org/10.1115/OMAE2019-95151) - (2019): Multiscale investigation of suffusion with coupled CFD-DEM. In: Proceedings of 2nd International Conference on the Material Point Method for Soil-Water-Structure Interaction. Cambridge, UK, p. 122-128
Kanitz, M.; Grabe, J.
- (2019): Numerical and experimental analysis of the extraction mechanism of an anchor plate embedded in saturated sand. In: Computers and Geotechnics 111, p. 191-201
Kanitz, M.; Hager, A.; Grabe, J.; Goniva, C.
(See online at https://doi.org/10.1016/j.compgeo.2019.03.014) - (2020): The influence of the void fraction on the particle migration: a coupled computational fluid dynamics-discrete element method study about drag force correlations. In: International Journal for Numerical and Analytical Methods in Geomechanics
Kanitz, M.; Grabe, J.
(See online at https://doi.org/10.1002/nag.3131)
