بررسی آزمایشگاهی اثر شکل بستر شکنج در دو حالت موازی و پولکی بر تنش برشی بستر

نویسندگان

چکیده

رودخانه‌ها، در طی زمان برای رسیدن به پایداری، شکل‌‌های بستر متفاوتی را با رسوب‌گذاری یا فرسایش در بستر خود ایجاد می‌کنند. از راه‌های شناسایی رفتار رودخانه‌ها، مطالعه و بررسی ساختار جریان و شیوه‌ی شکل‌گیری شکل‌های بستر در آن‌ها است. در رودخانه‌های آبرفتی بخشی از تنش برشی مربوط به شکل بستر است. در این پژوهش به بررسی تنش برشی بستر، روی دو نوع شکل بسترشکنج (موازی و پولکی) پرداخته شده است. بدین‌منظور آزمایش‌هایی در چهار شیب و چهار دبی روی شکنج‌های مصنوعی ساخته شده در آزمایشگاه هیدرولیک دانشگاه شهرکرد انجام شد. برای اندازه‌گیری تنش و سرعت جریان از دستگاه سرعت‌سنج سه‌بعدی ADV استفاده شد. نتایج نشان داد میزان تنش در تمام آزمایش‌ها، روند ثابتی داشته است؛ به‌گونه‌ای که روی وجه بالادست شکنج، ماکزیمم مقدار را دارد و با نزدیک شدن به تاج از مقدار آن کاسته می‌شود و در تاج، کمترین مقدار را دارد. همچنین نتایج نشان داد شکنج‌های موازی و پولکی به‌ترتیب ?? و ?? درصد از میزان کل تنش برشی را تشکیل می‌دهند. میزان تنش برشی بستر، در روی شکنج‌های موازی، ?? درصد بیشتر از مقدار تنش برشی بستر روی شکنج‌های پولکی است.

کلیدواژه‌ها


عنوان مقاله [English]

Experimental study of the effect of the parallel and flake ripple bed forms on the bed shear stress

نویسندگان [English]

  • pari maleki
  • hossein samadi boroujeni
  • Rohollah Fatahi nafchi
  • Darioush maleki
چکیده [English]

Interaction between water flow characteristics and bed erodibility plays an important role in sediment transport process. In order to reach stability, rivers with deposition or bottom erosion make different forms in the bed .The bed forms create extra resistance, which is called the bed resistance. The mutual interaction between the flow and the erodible bed through sediment transport phenomena in a sand-bed channel causes a variety of bed forms. Starting with ripples and gradually increasing in shear stress or water velocity, dunes, washed out dunes, flat bed, anti-dunes, and standing waves are formed. The most common boundary conditions in alluvial rivers are the mobile beds covered with Ripple and Dune. These forms, in many alluvial systems, play a critical role in contrast between the flow, discharge of sediment and morphology of bed. One way to identify the behavior of the rivers is to study the structure and the formation of bed forms within them. Ripples are among the smallest of the bed forms. The longitudinal cross-sections of ripples are usually asymmetrical. The upstream face of ripple is long and has a gentle slope, and the downstream face is short and steep. The height of ripples is usually between ?.? cm and ? cm, but not more than ? cm. The wave lengths normally do not exceed to ??cm, and they are usually within the range of ? cm to ?? cm. Some ripples that form in deep-water regions are symmetrical. Ripples are the smallest of the bed configurations. They are related to physical parameters near the river bed and have little correlation with the water depth. Their occurrence is the result of the unstable viscous layer near the boundary. They can form in both shallow and deep water. In plan, they either are parallel to each other or have a shape like fish scales. With increasing the flow velocity, the plan form of the ripples gradually develops form straight line to curves and then to a pattern like fish scales, symmetrical or unsymmetrical Resistance is a function of the geometrical dimensions of the bed forms and depth of water. Estimating of the flow resistance is one of the most important matters in planning, designing and operating of water resources projects, including water transfer and river system management.
In this research, the effects of two different types of ripples (parallel and flake shape) on the hydraulic characteristics of flow were experimentally studied. The experiments flume located at the hydraulic laboratory of Shahrekord University, Iran. The flume used in this study was a Straight type that had the dimensions of ?.? m wide and depth and ?? m long. This flume has vertical PVC sidewalls. Generally ?? tests in variety slopes of ?.???? to ?.??? and variety discharges of ?? to ?? lit/s were conducted. Velocity and the shear stress were measured by using an Acoustic Doppler Velocimeter (ADV). Velocity measurements were performed with a frequency of ??? Hz, which provided accurate statistics on the mean flow and turbulence characteristics. Detailed velocity measurements were performed in ? cross-sections in the Straight flume. The cross-sections were determined at: before crest, crest and after crest. Generally, in each section velocity was measured in ?? point (?, ?? and ?? cm from flume side and ? points from bed). For All tests, flow depth was kept constant. In this study generally ?? test were tested under different hydraulic conditions.
It was observed that the peak value of the bed shear stress appeared on the midpoint of upstream surface (before crest ripple) and the crest of the ripples had the lowest value of the bed shear stress. From the crest to trough (after crest) of the ripples, the general bed shear stress was in an increasing trend. It was generally found that with increasing Froude number and the bed shear stress increased. Also, in the case of parallel ripple bed form, the shear stress was about ??% more than that of plane bed and in the case of flake ripple bed form the increasing rate was ??%. It indicated that the shear stress was much affected by the parallel shape of ripple bed form compared with the flake shape, as it was ??% more than the flake shape.

کلیدواژه‌ها [English]

  • Alluvial river
  • Bed form
  • Bed shear stress
  • Ripple