بررسي عددي آثار ارتفاع تاج روزنه ي جانبي مستطيلي بر الگو و مشخصات جريان در اطراف روزنه

Numerical Investigation of Effects of Rectangular Side Orifice Crest's Height on Flow Characteristics around Orifice

روزنه هاي جانبي از جمله سازه هاي انحراف جريان محسوب مي شوند كه بطور گسترده در مهندسي هيدروليك و مهندسي محيط زيست مورد استفاده قرار مي گيرند. بررسي مشخصات و الگوي جريان عبوري از آنها از قبيل نحوه توزيع مولفه هاي مختلف سرعت و سطح آزاد جريان در مجاورت روزنه جانبي از اهميت زيادي برخوردار است. در مطالعه حاضر جريان عبوري از روزنه ي جانبي لبه تيز مستطيلي واقع در ديوار جانبي كانال باز با استفاده از نرم افزار FLOW-3D و مدل آشفتگي RNG k- شبيه سازي شده است. در اين مطالعه، ابتدا نتايج دبي عبوري از روزنه ي جانبي و الگوي جريان در اطراف آن در مدل عددي با نتايج آزمايشگاهي مورد مقايسه قرار مي گيرد و سپس اثرات ارتفاع تاج روزنه ي جانبي بر توزيع مولفه هاي مختلف سرعت و سطح آزاد جريان در مجاورت روزنه ي جانبي در كانال اصلي مورد مطالعه قرار گرفته است. نتايج نشان مي دهد در مدل هاي شبيه سازي شده با ارتفاع تاج هاي متفاوت، ماكزيمم و مينيمم سرعت طولي به ترتيب در ابتدا و انتهاي روزنه ي جانبي رخ مي دهد. با كاهش ارتفاع تاج روزنه، ماكزيمم و مينيمم سرعت طولي به ترتيب افزايش و كاهش مي يابد. باكاهش ارتفاع تاج روزنه جانبي، ماكزيمم سرعت جانبي در طول روزنه افزايش مي يابد، كه نشان مي دهد با كاهش ارتفاع تاج، دبي عبوري از روزنه افزايش مي يابد. همچنين در خصوص سطح آزاد جريان، در حالتي كه ارتفاع تاج روزنه پايين مي باشد تغييرات سطح آزاد جريان ناچيز مي باشد. اما با افزايش ارتفاع تاج روزنه، تغييرات قابل توجهي در سطح آزاد جريان در اطراف روزنه ي جانبي رخ مي دهد.

Structures such as side orifices, side weirs, and side sluice gates are known as flow diversion structures among which side orifices have wide application in hydraulic and environmental Engineering. These flow diversion structures have been extensively used in irrigation and drainage networks, wastewater treatment plants, sedimentation tanks, etc. Therefore, Studying the pattern and characteristics of the flow -such as flow velocity components and free surface- adjacent to the side orifice would be important. In this paper, the flow over a sharp-crested rectangular side orifice in an open channel is simulated by FLOW-3D software. RNG k - Ɛ turbulence model is used to apply the Navier-Stokes equations and the VOF method is used to model the free surface profile changes. In the present study, the side orifice discharge and flow patterns are obtained by numerical simulation and are compared with experimental data of Hussian et al (2011) for model verification. The amount of the discharges through the orifice (both predicted by the present numerical simulation and recorded by the experimental research) are reported along with the relative errors which are about 8-9%. This shows relatively good agreement between numerical and experimental results. Therefore, the numerical model can be employed as a powerful tool for studying flow through side orifices in open channels. The effects of the side orifice crest's height (H) on the flow velocity components and free surface adjacent to the side orifice are also investigated. Results indicate that the discharge ratio (ratio of the discharge through the side orifice to the inlet discharge of the main channel) is increased withdecreasing the height of the side orifice crest. Maximum and minimum values for longitudinal component of the velocity -for all heights of the side orifice crest- is reported at the beginning and end of the side orifice, respectively. By decreasing the height of the side orifice crest, these maximum and minimum values are respectively increased and decreased. Decreasing the height of the side orifice crest, the longitudinal component of the velocity in the vicinity of the side orifice is negative because of the reverse flow formed in this area. Examining the variation of lateral velocity component shows that this component is increased with decreasing the height of orifice crest. That is why the amount of discharge through the side orifice is increased with decreasing the height of orifice crest. The flow direction is upward at the height level of 0.25H; therefore, vertical component of velocity trough the orifice length is positive in all cases. On the other hand, the flow direction is downward at the height level 0.75H; thus, vertical component of velocity trough the orifice length is negative in all cases. Absolute value of the vertical velocity is increased by decreasing the height of the side orifice crest (H) because more flow is diverted to the side orifice. By increasing the height of orifice crest significant changes are reported in the free surface profiles especially in the vicinity of the side orifice.