When the flow transitions between different regimes (subsonic/sonic/supersonic), the predictability of the system goes down dramatically. If we stick a bb in the flow, I believe that the flow passes by the bb, it will experience conditions similar to a converging/diverging nozzle, which means that it is very likely that the bb is within the transonic regime for a large portion of its time within the barrel. The flow immediately after passing the bb will be supersonic, so as this supersonic flow collides with the sonic flow ahead of the bb, the low pressure supersonic flow combines with the lower pressure sonic flow, creating a vortex around the front upper half of the bb. In this case, we can see that a high initial pressure/low barrel length might be advantageous. Compared to the 2 transitions of the previous two cases (subsonic>sonic and sonic>supersonic>sonic), we see that it is possible to keep the flow supersonic the entire length of the barrel. Initially, the flow is supersonic immediately after leaving the air nozzle, is slowed as it passes by the bb, and possibly remains supersonic the entire time, we now achieve a single flow regime throughout the entire barrel. Similarly, I believe that "barrel pop" is a result of a sonic flow accelerating past the bb and hitting supersonic as the bb exits the barrel. |
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