# Effect of depression in the tank

**Dimensioning of an hydraulic system**

**for water bombing from an aircraft**

**Effect of depression in the tank**

It is possible to take into account a difference of pressure between the top and the bottom of the tank. Up to now we have considered that $P_{in}=P_{out}=P_{atm}$, but water dropping may induce a depression at the top of the tank. This situation is described as it follows.

During fluid discharge, **a vent at the top of the tank allows the volume of fluid lost to be replaced by air**. The vent is closed by a valve which induces a pressure drop at the restriction hence a negative pressure at the top of the tank. Such pressure difference can have an impact on the flow-rate.

It appears to be interesting to evaluate the influence of the vent diameter $D_v$ on the pressure difference. For that, the study should include a combination of the constants for air density, mean pressure, friction factor and the universal gas constant. The gas compression plays here an important role in the depression. Besides we have to determine the volume of air admitted in the tank that replaces the water dropped ($V_{air}$).