بررسي الگوي جريان اطراف آبشكن با ديواره‌ي شيب‌دار

Experimental study of flow around spur dike with side slope

در اين مقاله به مطالعه ي آزمايشگاهي ساختار جريان پيرامون آبشكن با ديواره ي شيب دار پرداخته شده است. بدين منظور، هيدروديناميك سه بعدي اطراف تك آبشكن مستقيم با ديواره ي شيب دار 75 درجه، بصورت آزمايشگاهي و در يك كانال مستقيم با بستر صلب مورد مطالعه قرار گرفت. مشخصات جريان با برداشت پارامترهاي آشفتگي اطراف آبشكن به وسيله سرعت سنج نقطه اي سه بعدي ADV، مورد بررسي قرار گرفت. در پشت آبشكن با كاهش سرعت جريان كه منجر به تشكيل ناحيه ي سكون مي شود، سرعت جريان در نواحي مياني افزايش مي يابد. به همين علت جريان به دو قسمت تقسيم شده و قسمتي از جريان به سمت بالا و قسمتي از جريان به سمت پايين كه فشار كمتر است، حركت مي‌كند. جريان پايين رونده عامل تشكيل گردابه ي نعل اسبي مي باشد. دو ناحيه ي تشديد سرعت، كه اولي مربوط به تشديد سرعت در هسته ي اصلي جريان و ناشي از كاهش عرض عبوري جريان بوده و ناحيه ي پرسرعت ديگر كه مربوط به تشديد موضعي سرعت در پايين دست آبشكن و در ناحيه ي بيروني لايه ي برشي مي باشد، تشكيل مي شد و حداكثر مقدار مؤلفه ي تنش ρ(u^ v^ ) ̅در امتداد لايه ي برشي رخ مي دهد. با توجه به مقادير منفي تنش هاي رينولدز (ρ(v^ w^ ) ̅) و (ρ(u^ w^ ) ̅)، تجمع رسوبات در ناحيه ي چرخشي پشت آبشكن اتفاق مي افتد.

Experimental study on effect of single spur dike side slope on flow structure carried out. In the study, 3D flow dynamics around the single spur dike with side slope of 75 degree has been studied experimentally in straight fixed bed channel. In order to analysis of flow characteristics, turbulence parameters flow around the scour have been measured using Acoustic Doppler Velocimeter (ADV). In behind of the spur dike, due to reduction of flow velocity that leads to formation of stationary zone, flow velocity will be increased in central zones. Therefore, a part of the flow is conducted upward and another part will be directed downward, where the pressure is lower. Downward flow is the cause of horseshoe vortex formation. Two velocity intensifier zones is formed, one is located in the main core of flow which is caused by reduction of flow width and the other one is a high velocity zone that is related to local velocity intensifying in outer layer of shear zone in the downstream of the spur dike. This is stemmed from increase in velocity of flow caused by reduction in flow width and leads to scour initiation from this zone. The maximum amount of “ρ(u^ v^ ) ̅” stress element in shearing layer direction is occurred. Regarding to negative amounts of “ρ(v^ w^ ) ̅” and “ρ(u^ w^ ) ̅” stress elements, accumulation of deposits is happened in circular zone behind the spur dikes. Increases in both the speed, a core area to increase the speed of the flow and reduce the width of the highspeed flow and other areas related to the intensification of local rapidly down the breakwater and in the area the outer layer the shear is formed. The maximum mean flow velocity at the bottom of the breakwater 55/1 is the mean flow velocity is approaching. In the upstream region of the breakwater a rotating flow in the leg is shaped breakwater. It has small dimensions and rotational flow near the breakwater body rotation power was concentrated in the can be high. Upstream of the nose, the lines of the horseshoe vortex, the rotation around the nose, the hands move downward and adjacent layers come to be intertwined with the shear. The average flow field, the maximum kinetic energy in the central part of the channel and away from the shear layer form is the maximum kinetic energy of turbulence along the shear layer will occur. Increase in kinetic energy along the shear layer plays an important role in the sediment bed holds it down, so that the kinetic energy due to the formation of vortex turbulence the total flow depth have been developed. In behind of the spur dike, due to reduction of flow velocity that leads to formation of stationary zone, flow velocity will be increased in central zones. Therefore, a part of the flow is conducted upward and another part will be directed downward, where the pressure is lower.