The requirements for fish protection at hydro power plants have led to a significant decrease of the bar spacing at trash racks as well as the need of an inclined or angled design to improve the guidance effect (fish-friendly trash racks). The flexible fish fence (FFF) is a new developed fish protection and guidance system, created by horizontally arranged steel cables instead of bars. The presented study investigated experimentally the head loss coefficient of an angled horizontal trash rack with circular bars (CBTR) and the FFF with identical cross sections in a flume (scale 1:2). Nine configurations of different bar and cable spacing (blockage ratio) and rack angles were studied for CBTR and FFF considering six different stationary flow conditions. The results demonstrate that head loss coefficient is independent from the studied Bar–Reynolds number range and increases with increasing blockage ratio and angle. At an angle of 30 degrees, a direct comparison between the two different rack options was conducted to investigate the effect of cable vibrations. At the lowest blockage ratio, head loss for both options are in similar very low ranges, while the head loss coefficient of the FFF increases significantly compared to the CBTR with an increase of blockage. Further, the results indicate a moderate overestimation with the predicted head loss by common head loss equations developed for inclined vertical trash racks. Thus, an adaption of the design equation is proposed to improve the estimation of head loss on both rack options.

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Zusammenfassung In den letzten Jahren hielt vermehrt die dreidimensionale (3D-)numerische Strömungssimulation im Bereich der Grundlagenforschung und der Planung von hydraulischen Anlagen Einzug. Rechen im Bereich von Kraftwerkseinläufen stellen jedoch für die numerische Simulation eine besondere Herausforderung dar. Dies ist der, im Vergleich zu anderen Bauteilen, kleinteiligen Rechenstabgeometrie geschuldet. Als alternativer Lösungsansatz zur Modellierung der Rechenverluste in der Software FLOW-3D bietet sich als Flächenobjekt eine Baffle an. Der Beitrag baut auf den Erkenntnissen von Waldy et al. (Österreichische Wasser-und Abfallwirtschaft 67:1-2, 2015) auf, welcher unterschiedliche Konzepte zur vereinfachten Modellierung der Verluste an Rechen untersucht hat. Dabei zeigt sich, dass die Verwendung einer Baffle als erfolgversprechende Variante gesehen werden kann. Diese Vergleiche erfolgten jedoch unter der Einschränkung eines gerade angeströmten Rechens, bei dem die Netzausrichtung immer in der Rechenebene lag. Der aktuelle Beitrag befasst sich mit der Modellierung der Rechenverluste mithilfe einer Baffle am schräg gestellten Rechen und erweitert somit den Beitrag Waldy et al. (Österreichische Wasser-und Abfallwirtschaft 67:1-2, 2015). Implementation of an angled trash rack in the 3D-numerical simulation with FLOW-3D Summary Over the last years, scientists and engineers have used more and more 3D-numerical flow simulations for basic research and the planning of hydraulic constructions. However, trash racks in front of the intakes of hydroelectric power plants represent a particular problem for numerical simulations. The reason for this is the especially fragmented geometry of the trash rack bars in comparison to other construction elements. As an alternative approach to include trash rack losses into a 3Dnumerical simulation with FLOW-3D a baffle can be used. Waldy et al. (Österreichische Wasser-und Abfallwirtschaft 67:1-2, 2015) demonstrated that such a baffle is a promising method to model the losses at trash racks. These comparisons of different concepts were limited to a vertical trash rack, which had its grid orientation along the computational plane. The actual paper focuses on the modelling of the trash rack losses by means of a baffle at an angled trash rack and thus upgrades the survey of Waldy et al. (Österreichische Wasser-und Abfallwirtschaft 67:1-2, 2015).