Abstract
This paper presents theoretical and experimental work on the effect of interstitial air pressure on the flow of granular materials from hoppers. In the first part of the paper, a theory is developed for the case of low Reynolds number flow in a conical hopper. In subsequent sections, the theory is extended to cylindrical bunkers, to higher Reynolds numbers, (where inertia effects cannot be neglected) and to cases where the compressibility of the gas is important. In all cases, the theory agrees well with experiment.