بررسي تأثير ورودي گلبرگي شكل بر هيدروليك جريان آزاد در سرريزهاي مدور قائم

Effects of Daisy-Shape Inlet on Free Flow over Shaft Spillways

يكي از انواع بسيار مهم و كاربردي سرريزها، سرريز مدور قائم با فرم هاي مختلف ورودي نظير نيلوفري يا لاله اي است. يكي از راه هاي افزايش ظرفيت تخليه سرريز مدور قائم، اصلاح شكل ورودي به صورت گلبرگي شكل است. با استفاده از اين سازه مي توان طول تاج سرريز را افزايش داد. در رژيم جريان آزاد، سرريز گلبرگي با عمل كرد به صورت سرريز جانبي در مقايسه با انواع ديگر شكلهاي ورودي سرريز مدور، در هد يكسان دبي بيشتري را از خود عبور ميدهد. با وجود مطالعات و آزمايش هاي انجام شده در سال هاي اخير هنوز روابط جامعي براي تعيين ضريب دبي عبوري از اين سرريزها در حالت آزاد ارائه نشده است. در تحقيق حاضر به مطالعه آزمايشگاهي تاثير ورودي گلبرگي شكل بر هيدروليك جريان در سرريزهاي مدور قائم پرداخته مي شود. با انجام آناليز ابعادي پارامتر هاي بي بعد موثر بر هيدروليك جريان تعيين شده و در ادامه به مدل سازي آزمايشگاهي جريان در اطراف سرريز مدور قائم با ورودي مورد اشاره پرداخته شده است. آزمايش ها براي بازه ي وسيعي از پارامتر هاي هندسي و هيدروليكي انجام شد. نتايج نشان داد كه در نسبت طول سرريز به قطر شفت برابر با 3/75 و نسبت ارتفاع سرريز به قطر شفت معادل 1 بيشترين ضريب دبي جريان عبوري از ورودي هاي گلبرگي شكل در حالت آزاد حاصل ميشود. كاربرد ورودي گلبرگي شكل در سرريز مدور قائم كاهش 60 درصدي ارتفاع سطح آب را نسبت به سرريز مدور قائم بدون ورودي به همراه دارد.

Vertical shaft spillways have been widely used in a number of dam projects since the late 19th century around the world. Although a number of studies oriented to the shape of the inlet of shaft spillways including bell mouth inlet, there are lack of research studies regarding to the improved inlet shape of these hydraulic structures. A brief review of the former studies shows that most of those have focused on the bell mouth inlet to enhance the flow field thereby increasing the discharge capacity using complementary structural elements such as; anti-vortex plates, trash racks etc. Improving the hydraulic performance of different types of spillways was a major objective of several studies, resulting in different forms of spillway crests such as; piano key weirs and Daisy (Marguerite) shape inlets. Daisy (Marguerite) shape inlets which are the subject of the present study have been applied in some dam projects. Applying certain shape of inlets e.g. installing a Daisy (Marguerite) shape inlet over the shaft entrance is an alternation to avoid the swirling flow effects thereby to increase the shaft spillway discharge coefficient. Marguerite shape inlet has been used in different existing dam projects. Marguerite inlet is a unique inlet to increase the discharge coefficient compared to the other shapes of spillway crests. This is in part due to generation of spatially varied flow inside the Marguerite inlet blades, which makes it capable to pass greater flow discharges. Although different types of dam spillways have been the subject of different investigations, there is a lack of study on Marguerite-shape spillways. In this study, the effects of a Daisy (Marguerite) shape inlet on radial or crest control flow regime through shaft spillways have been investigated based on model experimentation. For the case of symmetry location of a vertical shaft spillway in the dam reservoirs, radial flow around the spillway should be considered to analyze the hydraulic performance of these spillways. Dimensional analysis has been used to determine the effective dimensionless parameters. Experimental study was conducted in a hydraulic model of vertical shaft spillway equipped with the Marguerite-shape inlets. Tests were performed in a circular cylinder of 2 meter diameter and 1 m high. The flow discharges through models of Marguerite spillways with different geometries (including; the length, the height and the number of blades) were ranged until a crest control flow was established. Tests were performed based on a wide range of geometric and hydraulic parameters to study as well as to evaluate the effects of each dimensionless parameter on flow hydraulic characteristics. To create a crest control flow condition, the flow was entered the main reservoir throughout a pipe inlet installed under the floor of the reservoir. Three-dimensional flow velocities were measured by an ADV installed over the reservoir with Frequency of 200 Hz. The water free-surface profile was measured using piezometers installed under the reservoir floor. Applying nonlinear regression analyses, empirical correlations were obtained for estimating the discharge coefficient and the threshold depth of orifice flow over Daisy (Marguerite) shape inlets.