The radial location and flow discontinuity of the hydraulic jump in a
sink are discussed. The importance of viscosity in determining the loc
ation of the jump is emphasized. A model, which highlights the importa
nce of viscous flow near the sink surface, allows us to predict the ju
mp radius and the flow depth on the upstream and downstream sides of t
he jump as functions of viscosity, gravity, and flow velocity. In our
approximate treatment, the jump location is independent of gravity and
inversely proportional to the cube root of the viscosity.