Shear Number as a Practical Tool for Flow Regime Evaluation and Education

Turbulence is a complex and challenging concept, often simplified for students through the use of the Reynolds Number (Re), used to indicate when flow transitions from laminar to turbulent occurs. However, the Re number focus on the ratio of inertial to viscous forces limits its ability to explain the physical mechanisms driving turbulence, particularly in cases involving different flow boundaries or two-phase stratified flows including flows with phase change. This article presents the development and reasoning of the Shear number (Sn) as a new turbulence indicator, based on the ratio of shear to viscous forces, which provides a more intuitive and accurate representation of turbulence initiation. By comparing Sn to Re, this study demonstrates how the Sn number enhances the understanding of turbulent flow and offers a more accessible tool for both teaching and practical applications in fluid dynamics.