کاهش ابعاد ناحیه جدایش جریان در آبگیر 90 درجه با ایجاد زبری و اختلاف تراز ورودی

بررسی الگو و ساختار جریان در بسیاری از سازه های هیدرولیکی از جمله آبگیر‌‌ها بسیار ضروری است‌، زیرا میزان دبی جریان و رسوب ورودی به آبگیر تا حد زیادی به خصوصیات این الگو بستگی دارد. در آبگیر جانبی بخاطر وجود گرادیان فشار جانبی و نیروهای برشی و جانب مرکز، جریان ثانویه ای ایجاد شده که مکانیسم تشکیل آن شبیه جریان ثانویه در قوس‌ها می‌باشد. جداشدگی جریان در دیواره بالادست کانال آبگیر از جمله مشکلاتی است که همواره در آبگیرها وجود داشته و باعث ایجاد ناحیه‌ای با جریان گردابه ای در ورودی آبگیر می‌شود. این ناحیه عرض موثر جریان عبوری و راندمان آبگیری را کاهش و رسوبگذاری در دهانه آبگیر را افزایش می‌دهد. بنابراین اقداماتی جهت کاهش ابعاد ناحیه جداشدگی جریان در آبگیرها حائز اهمیت است. در این پژوهش، با تغییر زبری در دیواره ورودی آبگیر و تغییر تراز کف آبگیر، تاثیر آنها بر ناحیه‌ی جدایش جریان بررسی شده است. بدین منظور، 4 دبی، 7 زبری و 3 اختلاف تراز متفاوت و جمعا 84 آزمایش، انجام شده است. تنها با افزایش زبری دیواره‌ی کانال آبگیر در ورودی آن، ابعاد ناحیه‌ی جدایش جریان تا 38٪ کاهش یافت. که این کاهشدر سازه آبریز 10 سانتی متری به 41 % رسید. در هر زبری، با ایجاد سازه آبریز، ضمن افزایش نسبت دبی، ابعاد ناحیه جدایش جریان کاهشیافت به طوری که در حالت بهینه، در زبری 032 / 0 و سازه آبریز 10 سانتیمتری به صورت توام ابعاد ناحیه جدایش جریان تا 63 % کاهش یافته است.

Declining Separation Zone Dimensions at 90 ° Lateral Intakes by Enhancement of Roughness Coefficient and Drop Implementation

Introduction Studying the flow pattern at lateral intakes where separation occurs is a critical issue. The flow rate and sediment trap is highly dependent on the flow structure in this area. Flow separation occurs due to the detachment of stream lines from the channel side walls. It creates a secondary flow similar to what happens at river bends. Flow separation, reduces the turnout efficiency by decreasing the flow effective area at lateral intakes. It also creates a region with horizontal vortices where is prone to sedimentation. Hence, application of methods to reduce the separation zone dimensions is of important significance. Different techniques have been introduced in the literature such as installation of submerged vanes, deployment of different intake angles to main channel, construction of the entrance part as a part of circles with various radiuses, etc. This study examines the effect of roughness coefficient and drop implementation at the entrance of a 90degree lateral intake on the dimensions of the separation zone. As far as the knowledge of the authors show, these two variables have not ever been studied as methods to decrease the separation zone dimensions and enhancing the turnout efficiency. Methodology In order to investigate the effect of roughness coefficient and drop implementation on the separation zone dimensions, four different discharges (16, 18, 21, 23 l/s) in subcritical conditions, seven manning roughness coefficients (0.009, 0.011, 0.017, 0.023, 0.030, 0.032) and 3 invert elevation differences between the main channel and lateral intake (0, 5 and 10 cm) at the entrance of the intake were considered. Totally 84 tests were performed in a concrete flume with 15 m length 0.5 m width and 0.4 m depth. The intake structure was made at a 90degree angle to the main channel with 0.35 m width. The Manning roughness coefficient values were selected based on available and also feasible value for real condition, so that 0.009 is equivalent to galvanized sheet roughness and selected for the baseline tests. 0.011 is for cement with neat surface, 0.017 and 0.023 are for unfinished and gunite concrete respectively. 0.030 and 0.032 values are for concrete on irregular excavated rock. (Chow, 1959). The roughness coefficients were created by gluing sediment particles on a thin galvanized sheet which was installed at the upstream side of the lateral intake. The values of roughness coefficients were calculated based on Srickler’s formula. For this purpose, some uniformly graded sediment samples were prepared and the manning roughness coefficient of each sample was determined with respect to D50 value pasted into the Strickler’s relation. All the experiments were recorded and photographs were taken severally during the experiment and after steady flow conditions were established. The photos were then imported to AutoCAD to measure the separation zone dimensions. The velocity values were also recorded by a onedimensional velocity meter at 15 cm distance from the intake entrance and in transverse direction (perpendicular to the flow direction). Results and Discussion Negative velocity values were recorded in the separation zone indicating dominant secondary currents at the intake entrance. The velocities were intensified by moving toward the intake midway showing that the effective area is scaled down. The velocity values were almost equal to zero near the side walls as expected. Results were presented as dimensionless separation zone area (ratio of the separation zone area to intake area). Analysis shows that by increasing roughness coefficient alone, separating zone dimensions reduce up to 38%. This technique requires minimal changes in intake geometry and is definitely an inexpensive method to be applied for intakes under operation. Besides roughness coefficient studied were quite accessible. Implementation of drop can decline this area respecting the roughness coefficient value differently. Naturally there is an invert elevation difference between the feeding canal and the areas to be irrigated. This idea was developed base on this issue. A minor part of this difference can be compensated at the intake entrance. This method increases the discharge ratio (ratio of intake to main channel discharge). The results are compatible with literature. Some other researchers reported that intensifying the discharge ratio can scale down the separation zone dimensions (Rumamurty et al., 2007. Keshavarzi and Karami Moghaddam, 2007). However, these scientists employed other methods to enhance the discharge ratio. Employing both techniques simultaneously can decrement the separation zone dimensions up to 41%. A comparison between the new methods introduced in this paper and traditional methods such as installation of submerged vanes, and changing the inlet geometry (angle, radius) was performed. The comparison shows that the new techniques can be highly influential and still practical. Conclusion This study introduces practical and still easy and costly beneficial methods for enhancing intake efficiency by declining the separation zone dimensions. Increasing roughness coefficient and implementation of inlet drop were considered as remedied for reduction separations zone dimensions. Results showed that enhancing roughness coefficient can decline the separation zone dimensions up to 38% while drop implementation effect can scale down this area differently based on roughness coefficient used. Combining both methods can descend the separation zone dimensions up to 41%. It is proposed to investigate the effect of roughness and drop implementation on sedimentation pattern at lateral intakes for further researches.