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After an overview of the multiphase flow equations and models, I will go over some of the shortcomings that these models currently present.Particularly, I will highlight how the multiphase flows with particles modeled using an EulerEuler formulation often neglect the stress comingfrom the particlefluidparticle (PFP) interactions. Consider an array of particles moving in otherwise quiescent inviscid fluid. For a potential uniform flow over a single sphere, the forces are zero due to symmetry, implying that the presence of the fluid has no effect on theparticles. In reality, the PFP interactions will break this symmetry which leads to a nonzero force. This force is represented by a stressgradient term in the momentum equation which to our best knowledge has never been quantified and modeled. In this presentation, we discuss thedefinition and calculation of this stress. For finite Reynolds numbers, the stress can be approximately calculated using the PairwiseInteraction Point Particle (PIEP) model (Akiki et al. JFM 2017) valid up to a moderate volume fractions of 0.2. To calculate the stress, weneed to compute the average forces acting on particles conditional on the nearest particle location and then integrate over the relative pairlocations over the space. The use of quantities conditional on the nearest particles ensures the convergence of the integral.Collaborators: Duan Zhong Zhang, Rick M. Rauenzahn, Marianne M. Francois Host: Arvind T. Mohan 