Mass flow takes place when all the material in the hopper is moving during discharge. This means that it is sliding on all the wall surfaces, which are usually quite steep. In a funnel flow hopper the contents remain static on the walls and the pressure is not affected by discharge, but determined by the filling process and calculated according to the Janssen equation.

The main feaure of mass flow wall pressure is at the transition, where the stress regime changes from an active to a passive state and gives rise to a 'kick' pressure that is well documented.

I suggest you see Prof. Michael J. Rotters book on the 'The economic guide to the design of circular metal silos'.

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