The calculation of a nonsteady flow discharging into the atmosphere, or into a large reservoir, is generally based on the assumption that the effective exit pressure is the same as if the flow were steady. In reality, however, the steady-flow boundary conditions are asymptotically approached after a disturbance produced by an incident wave, and recently published investigations provide a better approximation to these transient boundary conditions. Utilizing these results, one can compute the rate of discharge and compare it with the rate obtained in the conventional manner. The difference between the results of the two calculations is used to define a lag error in the conventional calculations. Examples for discharges through an open end and through a sharp-edged orifice indicate that the actual transient flow rate may deviate considerably from that computed on the basis of steady-flow boundary conditions.

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