Torricelli's Curtain: Morphology of Horizontal Laminar Jets Under Gravity

TL;DR

This study investigates the morphology of horizontal laminar jets under gravity, combining experiments, theory, and simulations to understand the complex structure of the 'Torricelli's curtain' phenomenon.

Contribution

It introduces a theoretical model for the curtain's shape based on particle trajectories combining axial flow and gravity, supported by numerical simulations.

Findings

The model accurately predicts the primary jet trajectory.

Surface tension effects may explain secondary jet discrepancies.

Recirculating vortices are observed in the primary jet.

Abstract

Viscous fluid exiting a long horizontal circular pipe develops a complex structure comprising a primary jet above and a smaller secondary jet below with a thin fluid curtain connecting them. We present here a combined experimental, theoretical and numerical study of this 'Torricelli's curtain' phenomenon, focusing on the factors that control its morphology. We propose a theoretical model for the curtain in which particle trajectories result from the composition of two motions: a horizontal component corresponding to the evolving axial velocity profile of an axisymmetric viscous jet, and a vertical component due to free fall under gravity. The model predicts well the trajectory of the primary jet, but somewhat less well that of the secondary jet. We suggest that the remaining discrepancy may be explained by surface tension-driven (Taylor-Culick) retraction of the secondary jet. Finally, direct numerical simulation reveals recirculating 'Dean' vortices in vertical sections of the primary jet, placing Torricelli's curtain firmly within the context of flow in curved pipes.