Abstract
The study analyzes the flow over bottom racks made of longitudinal T-shaped bars. A clear water flow is considered in a laboratory flume. Free surface profiles, wetted rack lengths, and discharge coefficients are measured, changing parameters such as longitudinal slope, void ratio, and approaching flow. The present work complements existing experimental studies, considering the influence of the approaching flow conditions. The velocity field measured with Particle Image Velocimetry (PIV) technique and the pressure field with Pitot tubes are quantified. Numerical simulations (CFD) are used to complement laboratory data. The energy head along the rack is calculated and compared with the hypothesis of horizontal energy level with minimum energy at the beginning of the rack. A discharge coefficient adjustment that considers the slope, the void ratio, and the position along the rack is proposed and presented with the results of other works. Theoretical proposals to calculate the pressure field along the flow are compared with measurements in the laboratory. The relation between the static pressure head in the space of bars and the discharge coefficient is used as an alternative method to define the discharge.
Highlights
Bottom intake systems made by racks and longitudinally disposed in the flow direction are classically used to derive mean flows in continuous mountains rivers with intensive sediment transport
This is due to the variation in the experimental conditions used to adjust the discharge coefficient, such as the shape of the bars, their separation and width, the void ratio, and the approaching flow conditions such as the initial flow depth, h1, or the longitudinal rack slope, tanθ
The current study has been done with the same type of racks as Noseda
Summary
Bottom intake systems made by racks and longitudinally disposed in the flow direction are classically used to derive mean flows in continuous mountains rivers with intensive sediment transport.Nowadays, work is ongoing to study these intakes in order to derive flash floods from ephemeral rivers in semi-arid zones, not to control flooding but to collect runoff and improve the availability of resources. Castillo et al [7], from experimental measurements in a flow with gravel-sized materials, found that a longitudinal rack slope around 30% minimizes the occlusion effect. The wetted rack length to derive a flow yields very different results depending on the study. This is due to the variation in the experimental conditions used to adjust the discharge coefficient, such as the shape of the bars, their separation and width, the void ratio (the ratio between the spacing between bars, b1 , and the total area, b1 + bw , where bw is the bar width), and the approaching flow conditions such as the initial flow depth, h1 , or the longitudinal rack slope, tanθ
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