The stability of the circular hydraulic jump

We present the results of an experimental investigation of the striking flow structures that may arise when a vertical jet of fluid impinges on a thin fluid layer overlying a horizontal boundary. Ellegaard et al. (1998, 1999) demonstrated that the axial symmetry of the circular hydraulic jump may be broken, resulting in steady polygonal jumps. In addition to these polygonal forms, our experiments reveal a new class of steady asymmetric jump forms that include structures resembling cat’s eyes, threeand four-leaf clovers, bowties and butterflies. A parameter study reveals the dependence of the jump structure on the governing parameters. The symmetry-breaking responsible for the asymmetric jumps is demonstrated to result from a capillary instability of the circular jump. For all steady nonaxisymmetric forms observed, the wavelength of instability of the jump is related to the surface tension, σ, fluid density ρ and speed U of the radial outflow at the jump through λ = (68± 7) σ/(ρU).