Hydraulic Jump as “Mixing layer”

Abstract

In free‐surface water flows the hydraulic jump has usually been considered a turbulent reverse roller supported by an underlying and expanding stream. The air‐entrainment aspect of the water surface at the jump is normally a predominant feature. In this investigation the character of the turbulence in the jump, and therefore of the air‐entrainment mechanism, is controlled by the use of drag‐reducing soluble additives. The resulting lower level of entrained air permits photographic and visual observation of a jump in a small glass‐sided laboratory flume. The photographs presented in the paper show that the remaining air bubbles act as flow tracers and reveal a turbulent structure in the upper part of the jump very similar in appearance to that observed for a turbulent mixing layer in a wind tunnel using shadow refractography. Measurements of expansion angles for the turbulence structures in the jump agree well with wind‐tunnel measurements of expansion angles for the turbulent mixing layer between coflowing airstreams with a velocity difference.