LAUSR

It was recently claimed by Bhagat et al. J. Fluid Mech. vol. 851 (2018), R5 that the scientific literature on the circular hydraulic jump in a thin liquid layer is flawed by improper treatment and severe underestimation of the influence of surface tension. In stead of working directly with the Navier-Stokes equations, Bhagat et al. use an energy equation containing a new surface energy term, and conclude that the location of the hydraulic jump is determined by surface tension alone. We show that this approach is wrong and derive a corrected energy equation based on the Laplace pressure coming from the curvature of the surface, which is the only effect of surface tension. We show that this reduces the influence of surface tension strongly, invalidating the conclusion of Bhagat et al.. The proper treatment of surface tension in thin film flows is of general interest beyond hydraulic jumps, and we include an introduction to the standard approach and important theorems. We finally discuss the influence of viscosity, which plays a large role in thin film flows.