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- ItemDevelopment of a calibration method for coupled CFD-DEM(Stellenbosch : Stellenbosch University, 2023-03) Wasserfall, Jacob Gabriel; Coetzee, Corne; Meyer, Chris; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
Show more ENGLISH SUMMARY: A fully coupled Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) model was developed for a granular material submerged in water. The granular material was divided into three distinct particle size distributions (PSD). The particle size and shape were measured and used to create multi-sphere (composite) particles in the model. The bulk density was measured and used to calibrate the particle density, which was also measured directly. The remaining input parameters were calibrated using a large-scale Draw Down Test (DDT) rig under dry, wet and submerged saturation conditions. Due to geometric constraints and the relatively large size of the particles (4 mm to 32 mm), momentum source smoothing had to be used to allow for the large particle-to-cell size ratios. This technique was implemented using the software Simcenter StarCCM+. The effects of the smoothing were investigated and it was found that while the use of smoothing improved model stability, the use of large smoothing lengths resulted in instability. Additionally, a drag coefficient modifier was implemented to improve fluid-particle interactions which were reduced due to the momentum source smoothing method. First, the granular material was calibrated for the dry and wet conditions and the input values for the particle-particle coefficient of friction and the coefficient of restitution determined. For the larger PSD, it was found that the change in saturation condition did not affect the results of the DDT which meant the same calibration parameters were found for both. For the smaller PSD, some change was seen with the increase in saturation but similar input parameters were found for the two conditions. The smallest PSD was influenced the most by the change in saturation which resulted in two distinct sets of input parameters being found for the two conditions. Secondly, the model was calibrated for submerged conditions using the Haider- Levenspiel drag model. The input parameter values were initially set equal to those calibrated for the dry/wet conditions. Under submerged conditions, results showed that the particle-particle coefficient of friction and the drag modifier had a significant influence on the results. Therefore these two parameters were calibrated. It was found that the drag modifier had to be calibrated, while the particle-particle coefficient of friction, calibrated under dry conditions, could be used for the submerged conditions. Thus, the particle-particle coefficient of friction calibrated under dry conditions, could be used for the wet and submerged conditions for the larger PSDs. Finally, a vertical suction pipe validation experiment was conducted. The suction pipe had a constant diameter, but the fluid velocity and the distance the pipe opening was held from the granular bed were varied. The amount of mass (particles) removed as well as the size of the cavity that formed in the material bed was measured and compared to model predictions. The results showed that using the parameter values calibrated in the DDT, too much material was removed (error of 30%). Removing the drag modifier (setting it equal to unity) significantly improved the results (error of 6%). It is concluded that due to the difference in flow mechanism (particle-induced in the DDT versus fluid-induced in the suction pipe), the DDT is not a suitable experiment to calibrate the input parameter values for a suction pipe. It is hypothesized that the flow mechanism and dynamics of the granular material and the fluid in the calibration experiment should be similar to that of the final application being investigated.Show more