Author(s): Abdorreza Kabiri-Samani; Soroush Naderi
Keywords: Reynolds shear stress; subcritical flow; supercritical flow; transition; turbulent structure; velocity measurements
Abstract: In spite of some progress in the study of the transition from super- to subcritical flow without a hydraulic jump, many issues remain unclear, particularly in relation to the turbulence structure and dissipative mechanisms in the transition region. This paper therefore is focused on the experimental investigation of the turbulence characteristics in the transition region from super- to subcritical flow without a hydraulic jump. Instantaneous velocities were measured using an acoustic Doppler velocimeter and mean velocities, turbulence intensities, and Reynolds stresses were computed for different geometrical and hydraulic conditions. The turbulence intensities and Reynolds stresses were found to decrease in the transition region downstream and then approach the levels of the fully-developed turbulent open channel flow. The length of this transitional region was found to be three to seven times the tailwater depth for the range of Froude numbers between 1.3 and 4.5. The obtained results imply that the transition is driven by the longitudinal deceleration of the flow in the transition region.