Weirs are one of the most common hydraulic structures, playing a significant role in water resource management, controlling water level, mitigating flood risk, and protecting hydraulic infrastructure. Research on weirs, particularly on hydrofoil weirs, is of great importance due to their use in hydraulic engineering and water resource management. Several studies have been conducted on the hydraulic characteristics of flow over the hydrofoil weirs, varying parameters such as the relative eccentricity, angle of attack, curvature, and the weir upstream face slope. Former researchers have also investigated the flow regimes, downstream of the weirs, overfalls, drops and other hydraulic structures. However, the influence of tailwater depth on hydraulic characteristics of flow and possible flow regimes downstream of the hydrofoil weirs has not been yet eva‎luated. Given the performance of hydrofoil weirs, the present study reposes on the former investigations, examining the effects of tailwater depth and weir submergence on the hydraulic characteristics and flow regimes downstream of the hydrofoil weirs. Furthermore, the present study explores features of the flow regimes and characteristics of the hydraulic jump, considering different tailwater depths below and above the threshold submergence depth. Experiments were conducted for eight different stages, gradually increasing the tailwater depth, recording and analyzing the possible flow regimes at each stage. Nine hydrofoil weir models with different geometric parameters were employed. In each experiment, the tailwater depth was increased by adjusting a sharp-crested rectangular weir, and the flow regimes were examined at various stages. The flow regimes, downstream of the hydrofoil weirs, were categorized into three major types; including free flow (various types of free hydraulic jump), submerged flow (incomplete or fully submerged hydraulic jumps), and surface flow (subdivided into bubble wave, surface wave, and surface jet flow) regimes. Dependent on the intensity of flow turbulence, the hydraulic jumps length varied between 6 to 10 times the tailwater depth. Transition of flow regime from surface flow regime to submerged flow regime and from submerged flow regime to free flow regime occurred at lower tailwater depths. According to the present results, an increase in the flow discharge resulted in higher values of t1 and t2 for both increasing and decreasing tailwater depth. For instance, in model 1 with a discharge of 10 l/s, t2r was found to be -3.4, and t2f was -1.9. The findings also indicated that the threshold submergence depth is not only dependent on the ratio of upstream head to the weir height (hu/P) but also on the Fu. Accordingly, an empirical equation is derived for the threshold submergence depth, being applicable for the ranges of 0.2 ≤ hu/P ≤ 1.6 and 0.1 ≤ Fu ≤ 0.45. The results suggest that the proposed equation has acceptable scatter and correlation coefficient compared to the experimental data. To consider the influence of upstream water head relative to the weir crest, the parameters S and Fu* were combined using the concept of incomplete self-similarity (ISS), defining the parameter t/Yc. This combination provided improved results in describing the discharge reduction coefficient Cs, indicating that as t/Yc increases, Cs decreases.

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محمدرضا چمني , مهدي اژدري مقدم