Three-dimensional flow structure at fixed 70° open-channel confluence with bed discordance

Abstract

This experimental study provides a characterization of the flow field created by bed discordance at a fixed 70${}^{\circ }$ open-channel confluence on the basis of detailed free surface topography and three-component acoustic Doppler velocimetry measurements, which are contrasted with the results for a concordant bed junction. Due to bed discordance, the jet-like flow from the tributary is bent downstream to be aligned with the main channel axis. Underneath the tributary backward-facing step, separation occurs, producing negative vorticity along the main channel axis. A strong secondary motion downstream of the tributary junction is related to flow separation at the step and to flow release from the jet-like flow induced by a difference in the curvature of jet-like motion. A quantification of the terms in the equation of conservation of the longitudinal vorticity shows that the vortex stretching terms are sufficient to explain the development of the helical motion, which is thus classified as a secondary motion of Prandtl's first kind.

Keywords:

• Acoustic Doppler velocimetry
• bed discordance
• hydrodynamics
• open-channel confluences
• secondary motion
• helical cell

Additional information

Funding

Olga Birjukova Canelas received a grant from Universidade Técnica de Lisboa (UTL) from January 2013 to February 2014 and a PhD grant [SFRH/BD/52482/2014] from the Portuguese Foundation for Science and Technology (FCT) as a part of the H2Doc Doctoral Programme. The work with ADV instrumentation and the three-axis traverse system was partially funded by FED-ER, program COMPETE, and by national funds through FCT [project PTDC/ECM/118775/2010].