The gravitational air grain Rayleigh Taylor (RT) flow instability in a Hele-Shaw cell was studied using a parallel three-dimensional discrete particle model (DPM). The onset of flow instability and the development of fingering flow structures were well captured by the model. Power spectra analysis of solid volume fraction field indicated the non-linear coarsening process of the fingering flow structures. The sensitivity of the flow patterns to the initial porosity, the Atwood number, and the ratio of particle size to the Hele-Shaw cell width was also demonstrated. The excellent agreement of DPM simulation results with the reported experimental observations proved the robustness and reliability of the numerical approach to model complex multiphase flows such as granular RI instability. (C) 2015 Elsevier Ltd. All rights reserved.