Intermediate length scale of water jets under gravity: An experimental result

Abstract

The profile of water jets along its length before breakup is indirectly inferred from the observation of surface waves created by the jet as it plunges into a reservoir of pure and contaminated water. The inference is drawn based on the measurement of surface wavelengths as a function of jet length before it touches the reservoir surface. It is observed that the nature of variation of wavelength changes abruptly at a certain jet length $L_0$. It is found that the dimensionless $L_0$ depends only on the local velocity and diameter of the jet at L $=$ $L_0$ apart from the local acceleration due to gravity but is not a function of the surface tension of the reservoir; i.e., $L_0$ is a function of the Froude number Fr at L $=$ $L_0$. Our analysis suggests the existence of an intermediate length scale $L_0$ that demarcates the inertia-dominated region ($L < L_0$) of the jet from the gravity-dominated region ($L > L_0$).