Systems and methods for numerically simulating inflation of an airbag configured with more than one connected flexible-boundary volumes (i.e., primary and secondary pouches) are disclosed. A finite element analysis model of such airbag is defined in the airbag's folded configuration. Numerical simulation of inflating the primary pouch is based on corpuscular particle theory by interacting simulated corpuscular or gas particles with one another and with the shell elements representing the primary pouch. The simulated corpuscular particles are created by flow characteristics generated by an explosive blast. Numerical simulation of inflating the secondary pouch is based on control volume theory by converting kinetic energy of those of the simulated corpuscular particles having flowed through the interconnected opening from the primary to the secondary pouch to a uniform pressure. The uniform pressure is then applied onto the shell elements representing the secondary pouch.