Discharge characteristics and structure of flow in labyrinth weirs with a downstream pool. (March 2020)

• Record Type: Journal Article
• Authors: Dizabadi, Shahram; Hakim, Sara Seyed; Azimi, Amir H.
• Is Part Of: Flow measurement and instrumentation. Volume 71(2020)
• Publication Date: 2020-03
• Subjects: Labyrinth weir -- Flow measurement -- Discharge -- Free flow -- Submerged flow -- Turbulent flow -- Quadrant analysis -- Exuberance ratio; Fluid dynamic measurements -- Periodicals; Flow meters -- Periodicals; Fluides, Dynamique des -- Mesure -- Périodiques; Débitmètres -- Périodiques; 681.2805
• DOI: 10.1016/j.flowmeasinst.2019.101683
• Languages: English
• ISSNs: 0955-5986

Abstract

Abstract: A new design of a labyrinth weir is introduced in this study by adding a square pool to the vertex of a one-cycle triangular labyrinth weir with a sidewall angle of 45°. The addition of the square pool increased weir length without causing an excessive nappe interaction, and as a result, reduced the head water over the weir with the same discharge. Laboratory experiments were carried out to investigate the hydraulic performance of the new design with a potential application in pool-weir fishways. Mean and turbulence characteristics of flow for different weir geometries and in both free and submerged flow regimes were measured to be used for prediction of fish behaviour in the upstream of the proposed weir models. Discharge coefficients based on channel width and weir length were calculated. It was found that the new design can significantly increase the capacity of triangular labyrinth weirs and provide financial advantages in construction over triangular labyrinth weirs without pools in low discharges. In submerged flow conditions, the proposed model performed better than sharp-crested linear weirs in low discharges. Contour plots of the three-dimensional velocity components showed a region of strong mean flow around the neck of the new weir model. Turbulent characteristics such as turbulent kinetic energy, power spectra, exuberance ratio, and joint probability distribution functions of velocity fluctuations were extracted from instantaneous three dimensional velocities for different weir depths and flow regimes. Two vertical planes were identified based on the highest turbulent mixing in free and submerged flow regimes. The depths contributing the most to turbulent mixing were identified; active depths decreased as the flow regime changed from free to submerge flow regime. Highlights: A new design of triangular labyrinth weir is introduced. Discharge capacity of the new design is higher than the common triangular for h o / P ≤ 0.35. Negative modular limits were observed for h o / P ≤ 0.3 due to formation of surface jets. A dominant frequency of 0.2 Hz was obtained for the longitudinal and vertical velocity signals. Strong mixing zones were found in the neck of the weir with ERmin = 1/6.