Instability in a slightly inclined water channel

Abstract

An examination was made of the laminar travelling waves that disturbed the glassy surface in an open water channel, fitted with a trumpet entry and inclined downwards over the range 1 to . The Reynolds number at their first appearance was observed, and measurements were obtained of their velocities and lengths, the latter being highly irregular. Comparison was made with the theoretical values after corrections had been introduced to allow for the higher velocities of flow near the walls due to the increased depth caused by surface tension. At greater discharges the onset of turbulence occurred in a random manner, leading to the production of intermittent bores moving more slowly than the laminar stream. Transition was found to have usually begun at a Reynolds number R = 2500, R being defined as the discharge per unit width divided by the kinematic viscosity. Except close to the inlet, the channel was continuously occupied by turbulent water when R was in the range 4000 to 4500.