Some critical discussions are made on the structures of governing equations which are applied to one fluid model, two fluid model and fluid-solid particle model. Special concerns are paid on the interaction force in the conservation equations for moment and energy. It is shown that there are two conflicting concepts in evaluating the interaction force between liquid and solid phases; one is that the force should not be introduced positively in governing equations, as a form like body force, but as in stress tensors, and the other emphasizes the force is reasonable in present formulas.
