مطالعه پروفیل متوسط دوگانه سرعت طولی در کانال روباز با زبری نسبی متوسط

نوع مقاله : مقاله پژوهشی

نویسندگان

1 دانشجوی کارشناسی، مهندسی عمران و محیط زیست، دانشگاه صنعتی امیرکبیر، تهران، ایران

2 استادیار/دانشگاه خوارزمی

3 استاد، مهندسی عمران و محیط زیست، صنعتی امیرکبیر، تهران، ایران

4 دانشجوی دکتری، مهندسی عمران و محیط زیست، صنعتی امیرکبیر، تهران، ایران

چکیده

جریان با زبری نسبی متوسط (نسبت ارتفاع زبری به عمق جریان بزرگتر از1/80 و کوچکتر از1/5 ) در رودخانه های کوهستانی دیده می‌شود. با وجود مطالعات فراوان، شکل پروفیل سرعت طولی در عمق برای این نوع جریان ناشناخته باقی مانده است. در مطالعه حاضر، سرعت لحظه‌ای جریان در یک کانال مستطیلی روباز با بستر زبر به روش سرعت سنجی تصویری ذرات اندازه گیری شد. به منظور تحلیل نتایج از روش متوسط گیری دوگانه (متوسط گیری مقادیر لحظه‌ای سرعت در زمان و در یک صفحه نازک موازی با بستر کانال) بهره برده شد. مشاهده شد که در نزدیکی بستر، بردارهای سرعت به شدت در مکان تغییر می‌نماید، لذا نمی‌توان رفتار مشخصی را بدون متوسط گیری مکانی برای این سرعت در نظر گرفت. همچنین بررسی‌ها نشان داد که مقادیر متوسط دوگانه سرعت نسبت به متوسط گیری زمانی-مکانی یا مکانی-زمانی حساس نبوده و می‌توان بدون توجه به اولویت متوسط گیری زمانی و یا مکانی مقادیر متوسط دوگانه را حساب کرد. به منظور بررسی پروفیل سرعت جریان از سه رویکرد پروفیل لگاریتمی، پروفیل خطی و نیز معادله لایه اختلاطی بهره برده شد. بررسی‌ها نشان داد که با وجود تطابق نسبی هر سه دیدگاه، پروفیل لگاریتمی، به جهت بهره گیری از یک پشتوانه تئوری مستحکم و تطبیق مناسب با نتایج آزمایشگاهی، مناسب‌ترین دیدگاه برای این نوع جریان می‌باشد. البته در این دیدگاه ثابت ون-کارمن و ثابت انتگرال گیری معادله لگاریتمی با زبری نسبی تغییر می‌نمایند. براین اساس توصیه می‌گردد در مطالعات آتی از این رویکرد جهت تحلیل و مطالعه جریان با زبری نسبی متوسط بهره برده شود.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Characterization of Double-Averaged Velocity Profile in an Open-Channel With Intermediate Relative Roughness

نویسندگان [English]

  • Khatereh Kashmari 1
  • Seyed Hossein Mohajeri 2
  • Amirreza Zarrati 3
  • Maryam Azarpira 4
1 BSc student, Dept. of Civil and Environmental Engineering, Amirkabir University of Technology
2 Department of Civil engineering, Faculty of Engineering, Kharazmi university
3 Professor, Dept. of Civil and Environmental Engineering, Amirkabir University of Technology
4 PhD student, Dept. of Civil and Environmental Engineering, Amirkabir University of Technology
چکیده [English]

Flow with intermediate relative roughness (the ratio of roughness height to water depth higher than 1/80 and lower than 1/40) is common in most of the mountainous streams. Despite this fact and numerous studies on flow with intermediate relative roughness, it is still unclear how the profile of the streamwise velocity varies along with water depth. In this study, the instantaneous velocity of flow in an open-channel with the rough bed has been measured using Particle Image Velocimetry (PIV). In order to analyze the profile of streamwise velocity, double averaging method (spatial averaging of time-averaged values in a thin slab parallel to the channel bed) was used. It was observed that near the rough bed, vectors of instantaneous velocity showed strong spatial variations that make extraction of unique behavior for velocity profile impossible without double averaging. Results also showed that values of double averaged velocity were not sensitive to priority in averaging (i.e. time and then spatial averaging or spatial and then time averaging); thus double averaging regardless of priority can be used. To investigate double averaged velocity profile, three approaches including a logarithmic profile with a variable parameter, linear profile and mixing layer profile were employed. Results showed that all three profiles could be fitted properly to our experimental data. However, logarithmic profile with variable Von-Karman constant and integration constant that was supported by strong scientific background was the most suitable profile and therefore can be recommended.

کلیدواژه‌ها [English]

  • Open-channel
  • Intermediate relative roughness
  • Particle image velocimetry (PIV)
  • Double averaging
  • Streamwise velocity profile
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