اثرات فرآیند توسعه لایه سپر بر مقاومت جریان در رودخانه‌های درشت‌دانه

نوع مقاله: مقاله پژوهشی

نویسندگان

1 مهندسی سازه‌های آبی، گروه مهندسی آب، دانشکده کشاورزی، دانشگاه بوعلی سینا همدان

2 گروه مهندسی آب، دانشکده کشاورزی، دانشگاه بوعلی سینا همدان، همدان، ایران،

چکیده

در رودخانه‌های درشت‌دانه فاکتور اصطکاک به عنوان نمایه مقاومت جریان، وابسته به اندازه ذرات و شکل بستر است. وجود دامنه وسیعی از اندازه‌های ذرات در این رودخانه‌ها سبب توسعه لایه سپر و ساختارهای خوشه ذرات شده که اثرات قابل توجهی بر فاکتور اصطکاک دارند. در این مطالعه به بررسی اثرات ارتباط فیزیکی بین فرآیند سپرشدگی بستر و فاکتور اصطکاک به دلیل لایه سپر پرداخته شده است. بدین منظور با لحاظ نمودن ویژگی‌های قانون لگاریتمی توزیع سرعت، فاکتور اصطکاک کل تعیین شد. سپس فاکتور اصطکاک ذره با روش‌های کلگان و پارامتر شیلدز محاسبه شد و نتایج حاصل مورد مقایسه قرار گرفت. در پایان فاکتور اصطکاک ناشی از شکل بستر از تفریق فاکتور اصطکاک ذره از کل بدست آمد. تحلیل نتایج نشان داد که فاکتور اصطکاک مستقل از توزیع ذرات تشکیل‌دهنده بستر بوده و پارامتر اصلی کنترل‌کننده آن شیب خط انرژی است. این نتایج غیرقابل انتظار حاصل از ارتباط فاکتور اصطکاک با آستانه حرکت و بازآرایی ذرات درشت‌دانه در پوشش سپر می‌باشد. همچنین نتایج نشان داد که تشکیل و توسعه لایه سپر همراه با ایجاد خوشه ذرات با افزایش زبری نسبی بستر و کاهش استغراق نسبی جریان سبب افزایش میزان مقاومت جریان و پایداری بستر می­شود. مشاهدات میدانی حاکی از تغییر شرایط آستانه تنش شیلدز در رودخانه‌های درشت‌دانه به منظور ایجاد تعادل در نرخ انتقال ذرات درشت و ریز می­باشد

کلیدواژه‌ها


عنوان مقاله [English]

The Effects of Armoring Process on Flow Resistance in Coarse-Bed Rivers

نویسندگان [English]

  • Saeid Okhravi 1
  • Saeed Gohari 2
1 - PhD Candidate, Department of Water Engineering, College of Agriculture, Bu-Ali Sina University, Hamadan
2 Assistant Professor, Department of Water Engineering, College of Agriculture, Bu-Ali Sina University, Hamadan, Iran
چکیده [English]

Friction factor as an index for flow resistance depends on grain size and bed forms in coarse-bed rivers. A wide range of particle sizes located in these rivers causes development of the armor layer and cluster structures, which have great effects on friction factor. In this study, coupling effects between the armoring process and the friction factor due to an armor layer have been investigated. Therefore, considering logarithmic velocity distribution characteristics, the total friction factor was determined. Then, grain friction factor was determined by the Keulegan and the Shields parameter methods and results were compared. Finally, form friction factor was determined by subtracting the grain friction factor from the total one. Analysis of results showed that the friction factor is rather independent of grain-size distribution of the material forming the bed and the major controlling parameter is the slope of the energy grade line. These unexpected results are due to the coupling of the friction factor with the incipient motion problem and the rearrangement of the grains of the coarsest fraction in the armor layer. Also, the results showed that making and development of the armor layer accompanied with clusters of particles caused an increase in the relative roughness of the bed and a reduction in the relative submergence resulting an augmentation in flow resistance and bed stability. Field observations demonstrated that rivers could change the critical Shields stress due to making equal mobility of coarse and fine sediments.

کلیدواژه‌ها [English]

  • Relative submergence
  • Shields parameter
  • Armor Layer
  • friction factor
  • flow resistance

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