بررسي تاثير استغراق آبشكن بر پارامترهاي آشفتگي در اطراف آبشكن مستقر در قوس ᵒ90

Experimental study of submergence effect on turbulent parameter around spur dike located in a 90 bend

در تحقيق حاضر ميدان جريان اطراف آبشكن T شكل مستقر در قوس ᵒ90 با مطالعۀ پارامترهاي آماري بررسي شد. براي اين منظور از دو آبشكن با درصد استغراق %5 و %50 استفاده شد. آبشكنها در زاويه ᵒ45 نسبت به ابتداي قوس قرار گرفتند و آزمايشات در شرايط كف صلب (بدون حركت رسوبات) انجام شد. در اين تحقيق ميدان جريان اطراف آبشكن مستغرق بررسي و احتمال ايجاد پديده‌هاي چهارگانۀ آشفتگي، زاويه هاي اعمال پديده‌ها، تبديل پديده‌هاي آشفتگي به يكديگر و تنشهاي رينولدز در جهتهاي طولي و عرضي مورد بررسي قرار گرفت. نتايج اين تحقيق نشان دهندۀ تشكيل دو جريان ثانويه در امتداد عرض كانال مي‌باشد كه بر مكانيزم انتقال رسوب بسمت ساحل داخلي و آبشستگي ايجاد شده در ترازهاي بالايي جريان درساحل خارجي تاثير گذار مي‌باشد. در نزديك نوك بالادست آبشكن تنشهاي رينولدز برشي بزرگتري در آبشكن با %5 استغراق مشاهده شد كه نشان دهنده قدرت بيشتر گردابه نعل اسبي در اين ناحيه بدليل جريان روبه پايين قوي تر در اين ناحيه مي‌باشد. توسعه حفره آبشستگي بسمت پايين دست در امتداد لايه برشي مي تواند بدليل احتمال بيشتر پديده هاي آشفتگي بيرون‌راني و جاروبي، ناپايداري بيشتر پديده‌هاي اندركنشي، كاهش زاويه هاي اعمال پديده‌هاي بيرون‌راني و جاروبي و افزايش تنشهاي رينولدز برشي در اين ناحيه باشد. تنشهاي رينولدز برشي نيز در نزديك كف در لايه برشي اطراف آبشكن با %5 استغراق بيشتر از آبشكن با %50 استغراق مي‌باشد و عامل توسعه بيشتر حفره آبشستگي بسمت پايين‌دست در آبشكن با درصد استغراق كمتر مي‌باشد.

abstractIn this research the statistical parameter of the flow around T shaped spur dike located in a 90 bend was investigated. the experiments were conducted in a 90 bend at tarbiat modares university. the velocity was measured using ADV apparatuse. the frequency of the ADV set to 50 HZ this frequency was used by many previouse researchers. Two different submerged spur dike with submergence ratio equal to 5% and 50% were used. The submergence ratio is the ratio of the flow depth on crest of the spur dike to spure dike height. The spur dikes were located in 45 degree respect to the beginning of the bend and the experiments were done in freeze bed condition. The flow around the spur dike were investigated using parameters such as: Probability of the events (the ratio of the number of the events to the total of the events during the velocity measurment), variation of the events during the velocity measurments, shear Reynolds stresses in streamwise and lateral direction and angle of the events (the ratio of the vertical velocity fluctuation to the streamwise velocity fluctuation). The results showed that two secondary flow formed in lateral direction of the channel and the secondary flow affect on the upstream of the bend. These flow affect on sediment transport mechanism in lateral direction. In the upstream toe of the spur dike the Reynolds stresses in 5% submergence spur dike is greater compared to the 50% submergence spur dike since stronger horse shoe vortex due to the stronger downflow. The scour hole propagation along the shear layer in longitudinal direction may be due to the increasing the probability of the sweep and ejection events and variation of the interaction events, decreasing the angle of the events and increasing the Reynolds stresses. The Reynolds stresses along the shear layers located in 5% spur dike is greater that the 50% spure dike and larger scour process may be expected in this region. the stability of the events are more pronounced compared to the instability of the events. the angle of the sweep and ejection were minimum near the bed surface and maximum in the middle of the flow depth. Two different zone with maximum angle of sweep and ejection were observed that showed the fluid parcels deviate to the downstream recirculating zone. The angle of the events has different value near the over topping flow that passed from the crest of the spur dike. the flow measurments and analysis of the statistical parameters confirmes the previous researchers conclusions about larger scour hole around the spur dikes with lesser submergence. This may related to the difference of the flow structures around the submerged sour dikes especially near the upstram toe of the spur dike and sround the shear layer.