Diagram showing local pressures inside a water rocket engine maintained at the test bench and without taking into account the height of the water column. The aim here is to show how, in application of the Bernoulli principle, the local static pressure decreases on the wall at the approach of the nozzle. It is this very localized reduction of the pressure on the wall which explains that the thrust of such an engine is 2PStuy and not simply PStuy.
<a href="//commons.wikimedia.org/wiki/User:Bernard_de_Go_Mars" title="User:Bernard de Go Mars">Bernard de Go Mars</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>