بررسي ميدان جريان آشفته اطراف آبشكن T شكل مستغرق

Turbulent flow structure around submerged T-shaped spur dike

در اين مقاله ميدان جريان در اطراف آبشكن Tشكل مستغرق در مسير مستقيم بررسي شد. بررسي‌هاي انجام شده شامل سرعت‌هاي ميانگين، خطوط جريان و پارامترهاي آشفتگي جريان است. نتايج پژوهش حاضر نشان دهنده وجود يك جريان چرخشي در پايين دست آبشكن است. جهت جريان چرخشي، به دليل تاثير جريان روگذر از روي آبشكن، خلاف جهت جريان چرخشي مشاهده شده در آبشكن‌هاي غير مستغرق است. محدوده جريان چرخشي، نسبت به آبشكن‌هاي مستغرق ذوزنقه اي، تا محدوده وسيعتري از پايين دست امتداد مي يابد. پارامترهاي آشفتگي بررسي شده در اين پژوهش شامل تنش‌هاي رينولدز عمودي، تنش‌هاي برشي وارد شده بر كف، و احتمال ايجاد هركدام از پديده هاي آشفتگي است. بيشترين مقدار تنش‌هاي رينولدز عمودي در نوك بالادست آبشكن مشاهده مي شود. علاوه بر اين نيز در اين منطقه مشاهده شد. بررسي احتمال ايجاد هركدام از پديده هاي انفجار آشفتگي در راستاي جريان و عمق كانال نشان دهنده احتمال بيشتر پديده هاي بيرون راني و جاروبي در بالادست آبشكن است. در حالي كه در منطقه چرخشي پايين دست آبشكن، پديده هاي اندركنشي احتمال بيشتري دارند. احتمال ايجاد پديده هاي بيرون راني و جاروبي در نوك بالادست آبشكن، براي نوسانات سرعت در راستاي عرض و عمق كانال نيز بيشتر است.

Spur dikes are training structures that may be used for river bank erosion protections. These structures may increase the navigation depth. The spur dike creates stable pool for aquatic habitat. Many previous researches have been done to study the effect of non dimensional parameters on flow field and scour around the spur dike. Most of the previous researches focused on scour hole dimensions and the flowfield around the emerged spur dikes and few studies have been done to find the submergence conditions on scour and flow field around the spur dike. As the spur dikes may be submerged during the floods, in this paper, the flowfield around a submerged T-shaped spur dike was investigated using Vecterino app velocimeter. The experiment was conducted in a channel with 8m length, 60 cm width and 80 cm depth at Tarbiat Modares University (Tehran- Iran). The parameters that were investigated in this paper includes, mean velocities, turbulence parameters and the flow streamlines. The results of this study indicate that the high velocity region of the flow elongates to the far distance of downstream in the upper layers. There is a recirculating flow downstream of spur dike. Due to the effect of the spur dike overflow, the downstream recirculating flow, rotates in the opposite direction as compare to the emerged spur dike. The recirculating flow elongates to the far distance of downstream as compare to the submerged trapezoidal spur dike. Turbulence parameters analyzed in this research include normal Reynolds stress, bed shear stresses, the probability of each process and triple correlations. Maximum normal Reynolds stresses were observed at the upstream tip of the spur dike. In addition, the maximum shear stress was observed at the same region. The high stress region around the spur dikes showed a shear layer region around the spur dikes. Analysis of the probability of the turbulent bursting events in streamwise direction shows that ejection and sweep events are the most probable events in the upstream section of the spur dike. In the streamwise direction, the interaction events are the most probable process near the upstream tip of the spur dike, while ejection and sweep events in the widthwise direction are the most probable events near the upstream tip of the spur dike. In the downstream recirculation zone of the spur dike, the probability of the events approximately are the same in the streamwise direction and interaction events are the most probable events in widthwise directions. Triple correlations presented useful information about the turbulent bursting process. The triple correlation analysis in lateral direction showed that the ejection is the strongest event in the shear layer region. Triple correlation analysis in the streamwise direction, showed that the ejection was the strongest event in the upstream of the spur dike and in the region between the channel wall and spur dike wing. This causes sediment transport in streamwise direction as suspended load. The interaction events are the strongest events in the downstream recirculation zone, hence the sediments will be deposited in this region.