As a strategy of autonomously organising nanoparticles into patterns or structures, colloidal self-assembly has attracted significant interests in both fundamental research and applied science. Discrete element method (DEM) coupled with a simplified fluid flow model is applied to investigate convective colloidal self-assembly. The model developed takes into account the interparticle interactions, i.e. the electrostatic repulsion, van der Waals attraction, Brownian motions, and the hydrodynamic effect. Therefore, a detailed insight of the combined influences of fluid flow field, geometrical confinement, and the interparticle interactions on the self-assembly process can be obtained. In this study, we simulated different self-assembled structures and various transition areas where a growing crystal transits from n to n + 1 layer as a function of varied 3 phase contact angle, which is represented by a wedge geometry, and the velocity and direction of fluid flow. The crystal defects and the formation mechanism of different defects are theoretically studied through numerical simulation.