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Modelling of three common entrainment mechanisms in fluid flow, namely plunging jet, return wave and rising jet, that generating defects during casting were conducted and validated by A356 alloy. Previous research highlighted surface turbulence in liquid metals could result in the fold-in of surface films into bulk liquid and leave cracks or porosity in the solidified component. An algorithm applied face normals of free surface and their interaction to capture the entrainment of surface films was used in study. The model integrated the algorithm into CFD package FLOW-3D and was used to track the entrainment defects formation and distribution involved in different entrainment behaviours in casting process. The defects density in local volume plotted by the model was then employed as quantitative criteria to predict the reliability of castings. Directly observation of transient flows in moulds by real-time X-ray radiography showed good correlation between real filling scenario and simulation results. Fracture strength achieved by four-point bend tests on the samples from corresponding locations with models in castings showed some extent relationship between defects density and reliability of castings quantitatively. Studies also indicated the plunging jet mechanism usually leaded to high variation of mechanical properties due to intense turbulence and entrainment.