# Euler number (physics)

The Euler number (Eu) is a dimensionless number used in fluid flow calculations. It expresses the relationship between a local pressure drop e.g. over a restriction and the kinetic energy per volume, and is used to characterize losses in the flow, where a perfect frictionless flow corresponds to an Euler number of 1. The inverse of the Euler number is referred to as the Ruark Number with the symbol Ru.

It is defined as

$\mathrm{Eu}=\frac{p_\mathrm{upstream} - p_\mathrm{downstream}}{\rho V^2}$

where

• $\rho$ is the density of the fluid.
• $p_{\mathrm{upstream}}$ is the upstream pressure.
• $p_{\mathrm{downstream}}$ is the downstream pressure.
• $V$ is a characteristic velocity of the flow.

The cavitation number has a similar structure, but a different meaning and use:

The Cavitation number (Ca) is a dimensionless number used in flow calculations. It expresses the relationship between the difference of a local absolute pressure from the vapor pressure and the kinetic energy per volume, and is used to characterize the potential of the flow to cavitate.

It is defined as

$\mathrm{Ca}=\frac{p - p_\mathrm{v}}{\frac{1}{2}\rho V^2}$

where

• $\rho$ is the density of the fluid.
• $p$ is the local pressure.
• $p_\mathrm{v}$ is the vapor pressure of the fluid.
• $V$ is a characteristic velocity of the flow.