مطالعه آزمایشگاهی سرریز کلید پیانویی با تغییر نسبت عرض کلیدها

سرریز کلید ‌پیانویی سازه‌ای است که سطح و دبی جریان را تنظیم می کند و قابلیت استقرار در مخازن سد‌ها و کانال‌ها را داراست. ویژگی اصلی این سرریز توانایی عبور دادن دبی زیاد به ازای بار آبی اندک است. این نوع سرریز با توجه به خصوصیات هندسی، در چهار نوع a، b، c و d طبقه بندی می شود. در پژوهش حاضر برای بررسی تاثیر عرض کلید‌های ورودی و خروجی بر مشخصه های هیدرولیکی جریان، 16 مدل فیزیکی از سرریزهای کلید پیانویی نوع a و d ساخته شد. برای دستیابی به اهداف این پژوهش، بیش از 300 آزمایش روی یک کانال آزمایشگاهی اجرا و نشان داده شد که به ازای  بار آبی مشخص، سرریز نوع a، نسبت به سرریز نوع d، ظرفیت تخلیه بالاتری دارد. همچنین معلوم شد هندسۀ خاص سرریز نوع a ، مانع تشکیل پدیدۀ مخرب جریان موجکی روی آن می شود. بر اساس یافته های پژوهش، مشخص شد که مدل سرریز با نسبت عرض کلیدهای ورودی به خروجی برابر  1.66، در مقایسه با مدل های دیگر، تا 30 درصد عملکرد بهتری دارد.

Experimental Study of Piano Key Weir by Changing Width Ratio of the Keys

IntroductionPiano key weir (PKW) is a hydraulic structure that can be installed at dams and canals for regulating the flow surface and discharge. The main feature of this weir is the ability to pass a large flow of water over a small head.Regarding its geometrical properties, this weir is classified into four types of A, B, C and D. PKW with symmetrical upstream and downstream overhangs is classified as Type A, one with only upstream overhangs as Type B, one with only downstream overhangs as Type C, and one without overhangs as Type D.Despite various studies on piano key weirs (some of which were mentioned in the text), the flow behaiviour over these structures is very complex, unpredictable and threedimensional.So further laboratory investigations on PKWs are needed. In the present study, the effect of the width of the inlet and outlet keys on the hydraulic characteristics of these weirs is investigated. In addition, the occurrence of undular flow on these types of weirs is reported for the first time. To achieve the aims of this study, more than 300 experiments were conducted at a laboratory flume. MethodologyThe experimental tests of this research were conducted in the hydraulic & water structures laboratory of Shahid Bahonar University of Kerman, on a laboratory flume with 8 m length, 80 cm width and 60 cm height, having glass walls and metal bottom.The flow discharge was adjusted in a range of 5 to 60 liters per second by a valve. In total, 16 physical models of PKWA and PKWD weirs were constructed to investigate the effect of width of inlet and outlet keys on the hydraulic characteristics of flow over such weirs. Seven discharges were tested for each model of weirs, and experiments were repeated three times for each discharge. Results and DiscussionThe results demonstrated that at low water loads (Ht <3cm), both types of weirs have almost the same performance. But as the water load increases, the performance of PKWA weir deviates from the type PKWD weir, i.e. it was able to pass larger discharges for an identical flow head. The superiority of PKWA weir was more evident in small inlettooutlet ratios of keys.For low flow heads, the performance of weirs with different Wi/Wo ratios in terms of discharge capacity is almost similar. Anderson and Tullis (2013) found similar results in their experiments.At higher flow heads, the PKWA1.66, compared to other models, had the largest discharge capacity in same flow heads, indicating its better performance. At maximum flow head (5.5cm), this weir was able to discharge 52 liters per second, which is 24% more than PKWA0.6 weir. Similar trend exists for Dtype piano key weirs.It was observed that undular flow is formed on Dtype piano key weirs. Although, this phenomenon was previously observed on rectangular broadcrested weirs (Chanson, 1976; Madadi et al, 2013), but for the piano key weirs, this is the first report on formation of undular weir flow above Dtype PKW. ConclusionsAccording to the results of this study, the ratio of the width of the inlet keys to the width of the outlet keys is a very effective parameter in the discharge capacity of the piano key weirs, and the presence of a forehead can also play a role in enhancing the performance of the weir.Also, the results indicated that for a given head, not only the Atype PKW has more discharge capacity comparing to Dtype weir, but also due to its special geometry, the undular flow phenomenon cannot be formed above such weir. Furthermore, the PKW weir with inlet to outlet key width ratio of 1.66 demonstrated 30% higher performance comparing to the other investigated models.