Understanding Choked Flow: How Pump Speed Affects Mass Flow and Velocity Changes

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foglght
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There is a long pipe connected to the inlet of a pump (air). As the speed/flow of the pump increases, according to the mass flow equation (pVA) something has to give. At what point does the velocity begin to increase? Ambient conditions have an infinite cross sectional area. Density has to change, but I keep wondering when it starts to change. Obviously the change occurs with velocity.

And then, mass flow is highest at choked conditions. If you continue to speed up the pump it appears to not flow any more mass. What happens?
 
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and to add, I notice pressure drops across the pipe to a certain point in a linear fashion as volumetric airflow rate increases to a point and then becomes non-linear. How close are you to choked flow when it becomes non-linear?