Unsteady turbulent properties in negative waves in open channels

In an open channel, a sudden drop in free-surface elevation is associated with the development of a negative wave. While some simple analytical solution is widely described in textbooks, little research was conducted to date on the unsteady turbulence properties beneath negative waves. A series of new physical experiments were conducted in a rectangular channel. The unsteady free-surface profile and turbulence characteristics were measured in a negative wave propagating upstream against an initially steady flow using non-intrusive acoustic displacement meters, video imagery and acoustic Doppler velocimetry (ADV). For one set of flow conditions, the experiments were repeated 25 times at two longitudinal locations and four vertical elevations to yield ensemble-averaged data. The wave leading edge propagated upstream with a speed which was a function of time and space. The velocity data showed that the upstream propagation of the negative wave was linked with a gentle drop in water elevation associated with an acceleration of the flow, while some increased turbulence occurred beneath the wave associated with large velocity fluctuations and large Reynolds stress components. The velocity fluctuations and turbulent stresses were significantly larger than in the initially steady flow and in the final flow motion.

► Unsteady turbulence in negative waves was investigated physically.
► The wave leading edge propagation speed was a function of time and space.
► The negative wave yielded a gentle drop in water level and a flow acceleration.
► An increased turbulence occurred beneath the wave.
► The unsteady turbulent flow motion was three-dimensional.