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بررسي آزمايشگاهي اثر زبري بستر روي توزيع سرعت در بدنه جريان غليظ
عنوان (انگلیسی): Laboratory investigation of bed roughness effects on velocity distribution in the body of gravity currents
نشریه: پژوهش آب ايران
شماره: پژوهش آب ايران (دوره: ۱۰، شماره: ۳)
نویسنده: کاهه، مهدی ، قمشی، مهدی ، موسوی جهرمی، سید حبیب
کلیدواژه‌ها : زبري بستر ، توزيع سرعت ، جريان غليظ ، ناحيه جت ، ديواره
کلیدواژه‌ها (انگلیسی): Gravity Current , Jet and Wall Region , Velocity Distribution. , Bed Roughness
چکیده:

در اين پژوهش اثر زبري بستر روي توزيع سرعت در بدنه جريان غليظ در دو بخش جت و ديواره بررسي شده است. بدين‌منظور آزمايش‌هايي به‌صورت جريان غليظ نمکي با 3 دبي ورودي 7‎/0،‏ 1 و 3‎/1 ليتر بر ثانيه در محدوده شيب 0 تا 2‎/2 درصد انجام شد. براي زبر کردن بستر از ذرات شن طبيعي با اندازه‌هاي 4،‏ 8،‏ 12 و 15 ميلي‌متر (که در کف کانال چسبانده شد) استفاده شد. همچنين براي بررسي اثر تغييرات غلظت جريان ورودي بر مشخصات جريان غليظ،‏ از سه غلظت 10،‏ 16 و 20 گرم بر ليتر استفاده شد. توزيع سرعت در بدنه و بررسي روند تغييرات آن در زبري و شيب‌هاي مختلف با يک سرعت‌سنج پروفايلر ‌آلتراسونيک اندازه‌گيري شد. نتايج نشان مي‌دهد که ارتفاع سرعت حداکثر در پروفيل سرعت بدنه با افزايش زبري کف افزايش مي‌يابد. نسبت ارتفاع سرعت حداکثر به ضخامت بدنه جريان غليظ زماني‌که زبري نسبي (نسبت ارتفاع زبري به ضخامت بدنه جريان) ،‏ به سمت صفر ميل مي‌کند،‏ برابر مقدار ثابت 3‎/0 خواهد بود و با افزايش زبري نسبي ،‏ ارتفاع نسبي محل سرعت حداکثر ،‏ به سمت عددي ثابت 65‎/0 همگرا مي‌شود. نتايج نشان مي‌دهد که زبري عامل مهمي در توزيع سرعت در بدنه جريان غليظ است. بررسي تشابه توزيع سرعت در ناحيه جت و ديواره،‏ به ترتيب با توزيع‌هاي نيمه‌گوسي و تواني،‏ نشان مي‌دهد که ضرايب توزيع سرعت در بدنه با افزايش زبري بستر تغيير مي‌کند.

چکیده (انگلیسی):

Gravity currents form when a heavier fluid propagates into a lighter one in a predominantly horizontal direction. They are frequently encountered both in the environment and engineering applications. Gravity currents can be driven by density differences of the fluids involved, or by differential particle loading. In natural or human-made aquatic settings such as lakes, oceans or reservoirs, there is a myriad of possible contributors to these density differences including temperature differences, salinity contrasts, suspended material, both organic and inorganic, as well as combinations of these mechanisms. In water resources management, to prevent sedimentation and drain sediment from dam reservoirs, in most cases, is tried to remove fine sediments by using of hydrodynamic forces. One of these methods is removing sediment by the gravity current dynamics. The gravity currents are the most important effective events on reservoirs' sedimentary processes (transmission, distribution and deposition of particles). Therefore, identifying the factors affecting on this type of flow is very important. Both bed slope and surface roughness are the most important parameters on gravity current dynamics and have a considerable effect on velocity distribution of the body.
This study investigates the effect of bed roughness on the gravity currents characteristics and obtaining relations between effective parameter on gravity currents and finding their influences in body velocity distribution on rough and slope beds. In this study to determine the bed roughness effect on the velocity distribution, a series of gravity currents experiments was performed on beds with the rough natural element sizes of 4, 8, 12 and 15 mm and slopes from 0 to 2.2%. The experiments of saline gravity currents were carried out in three inflow discharges with amounts of 0.7, 1 and 1.3 (lit/s) and with three concentrations of 10, 16 and 20 (gr/lit). All Experiments was done on a Plexiglas flume with the length of 10 meters and width of 35 cm and in hydraulic models laboratory in Shahid Chamran university of Ahvaz (Iran). In all experiment, the velocity profile in the body of gravity current was acquired by an Ultrasonic Doppler Velocity meter device.
The results of this study can be summarized as an investigation on variations of velocity profile characteristics and analysis of velocity distribution in tow (wall and jet) regions of gravity current's body. Determination of general equations for velocity distribution in gravity current's body on rough beds showed that by increasing bed roughness the height of maximum velocity rose up to a higher position and the coefficient of velocity distribution equation changed consequently. Based on results of this study, the relationship between relative height of maximum velocity location in the velocity profile ( ), versus relative roughness ( ), was rational type; in which, hm is the height of maximum velocity location, ks is the bed roughness and h is the thickness of gravity current. The relative height of maximum velocity location was equal to 0.3 when was near to zero and by increasing , the value of converged to the constant of 0.65. The investigation on acquired velocity profile showed that the bed roughness was one of the most important factors on velocity distribution. The similarity of the velocity distribution, in both jet and wall flow regions, by Semi-Gaussian and power distribution equations showed that the coefficient of velocity distribution was varied by increasing the bed roughness.
For the velocity profile in jet region, the coefficients of Semi-Gaussian distribution (α and β) were varied from 1.2 to 2 and 2.2 to 1.8, respectively, by increasing the bed roughness. Also, for the velocity profile in wall region, the coefficient of Power distribution (n) was changed from 0.2 to 0.6.Gravity currents form when a heavier fluid propagates into a lighter one in a predominantly horizontal direction. They are frequently encountered both in the environment and engineering applications. Gravity currents can be driven by density differences of the fluids involved, or by differential particle loading. In natural or human-made aquatic settings such as lakes, oceans or reservoirs, there is a myriad of possible contributors to these density differences including temperature differences, salinity contrasts, suspended material, both organic and inorganic, as well as combinations of these mechanisms. In water resources management, to prevent sedimentation and drain sediment from dam reservoirs, in most cases, is tried to remove fine sediments by using of hydrodynamic forces. One of these methods is removing sediment by the gravity current dynamics. The gravity currents are the most important effective events on reservoirs' sedimentary processes (transmission, distribution and deposition of particles). Therefore, identifying the factors affecting on this type of flow is very important. Both bed slope and surface roughness are the most important parameters on gravity current dynamics and have a considerable effect on velocity distribution of the body.
This study investigates the effect of bed roughness on the gravity currents characteristics and obtaining relations between effective parameter on gravity currents and finding their influences in body velocity distribution on rough and slope beds. In this study to determine the bed roughness effect on the velocity distribution, a series of gravity currents experiments was performed on beds with the rough natural element sizes of 4, 8, 12 and 15 mm and slopes from 0 to 2.2%. The experiments of saline gravity currents were carried out in three inflow discharges with amounts of 0.7, 1 and 1.3 (lit/s) and with three concentrations of 10, 16 and 20 (gr/lit). All Experiments was done on a Plexiglas flume with the length of 10 meters and width of 35 cm and in hydraulic models laboratory in Shahid Chamran university of Ahvaz (Iran). In all experiment, the velocity profile in the body of gravity current was acquired by an Ultrasonic Doppler Velocity meter device.

فایل مقاله : [دریافت (777.7 kB)] ‏107 دریافت تاكنون
صاحب امتیاز:
دانشگاه شهرکرد
مدیر مسئول:
دکتر حسين صمدی
سردبیر:
دکتر منوچهر حيدرپور
مدیر داخلی:
دکتر محمدعلی نصراصفهانی