Hydraulic performance of fish guidance structures with curved bars – Part 1: head loss assessment

Abstract

The hydraulic performance of fish guidance structures such as louvres and angled bar racks is an important design criterion to achieve high fish guidance efficiencies with a minimum impact on hydropower production. The current geometrical designs of such structures result in high head losses and asymmetric turbine admission flow and hence need to be optimized. We therefore developed an innovative curved bar design for fish guidance structures and experimentally investigated different rack configurations in a laboratory flume. The present paper (Part I) focuses on the hydraulic performance of the novel curved-bar racks with regard to head losses, while in the companion paper (Part II) we report the results on the flow fields. The effects of the main rack angle, bar angle, bar spacing, bar depth, and top or/and bottom overlays were systematically studied. The curved bar shape leads to significantly lower head losses compared to straight bar shapes. An existing head loss prediction formula was extended to account for the new bar shape and the application of overlays.

Head Losses of Horizontal Bar Racks as Fish Guidance Structures

“…This flow velocity was an optimum value, thereby representing prototype conditions. Furthermore, the selected parameters were in excess of the threshold values h o t b −1 ≥ 40 and R b ≥ 1500 to avoid model effects for different rack types specified in the literature [11,12,18,19]. Figure 7 shows the rack head loss coefficient ξ R as a function of (a) the bar shape S, (b) the blocking ratio (BR), and (c) the approach flow angle α for all tested rack configurations without overlays and D b = 7.5.…”

“…This modified flow field usually negatively affects the head losses and the turbine admission flow [6,11]. These negative effects can be mitigated by using curved instead of rectangular bars [12,13], but their fish guidance efficiency and practical application have yet to be proven. Physical barriers are characterized by small clear bar spacings ranging between 10 and 30 mm, such that they are physically impermeable for the majority of fish.…”

Conceptual Approach for Positioning of Fish Guidance Structures Using CFD and Expert Knowledge

“…Considering FGS in real-scale numerical 3D-simulations of HPP is usually restricted to a smaller section, because the required grid resolution to resolve the whole FGS would lead to very large computing time making the conceptual approach inexpedient. Therefore, the hydraulic conditions of the present state without FGS are used as a reference, while the effects of FGS on the flow fields and on rack-near fish behavior known from detailed etho-hydraulic modelling [6,7,10,13,17,19,20,[44][45][46][47] are taken into account separately in the assessment phase, see Section 2.3.…”

“…For evaluation purposes, hydraulic head loss can be estimated with conventional formulas, like the approaches of Raynal et al [13], Albayrak et al [44], and Beck et al [19] for vertical angled racks or the methods of Albayrak et al [45] for horizontal angled racks. For inclined FGS the method of Raynal et al [46] can be applied.…”

“…In addition to the classical bar rack, there exist further developments that are not arranged orthogonally to the rack axis, such as those with straight (Modified Bar Rack, MBR [10,17]) or curved bar cross-section shapes (Curved Bar Rack [18][19][20]). First investigations of this FGS type show a lower flow deflection and more symmetric flow field in the tailwater as well as lower hydraulic losses compared to conventional FGS.…”