Use of centrifugal systems for investigating water flow processes in unsaturated soils

Centrifugal modelling, both physical and numerical, has been used for studying groundwater flow and transport processes in the past. However, there was disagreement in previous studies whether numerical models can be used in simulating centrifugal systems under unsaturated flow condition. In the present study, a numerical model based on Richards' equation was developed to predict one-dimensional unsaturated flow in centrifugal systems. The validity of the model was tested using data from physical models in four published benchmark problems. The ability of the numerical model to close mass balance was also tested. It was shown that the newly developed numerical model was able to recreate the four benchmark problems quite successfully, indicating that using such a model under unsaturated flow condition is feasible. The mass conservation result shows that the model is more sensitive to spatial grid resolution than to specified temporal step. Therefore, fine spatial discretization is suggested to ensure the simulation quality. Additionally, adaptive temporal time stepping method can be used to improve the computational efficiency. It was found that the dimensionless factors used for scaling physical dimensions by 1/N, seepage velocity by N, and temporal dimension by 1/N² were useful parameters for scaling centrifugal systems.