نوع مقاله : مقاله پژوهشی

نویسندگان

• محمدرضا زایری ¹
• مهدی قمشی ²

¹ استادیار گروه سازه های آبی، دانشکده مهندسی آب و محیط زیست ، دانشگاه شهید چمران اهواز، اهواز

² استاد گروه سازه های آبی، دانشکده مهندسی آب و محیط زیست ، دانشگاه شهید چمران اهواز، اهواز

چکیده

جریان گل‌آلود به‌عنوان یک جریان چند فازی، با حجم قابل‌توجهی از رسوبات معلق رودخانه و آبریز همراه بوده و باتوجه‌به دارا بودن سرعت و غلظت قابل توجه، بخشی از رسوبات ته‌نشین شده در دوره‌های قبلی را نیز فرسایش داده و با خود حمل می‌کند. این جریان در خط‌القعر رودخانه مسیرهای مستقیم یا پر پیچ و خمی را طی می‌نماید. لذا در این تحقیق به مطالعه آزمایشگاهی اثر نسبت شعاع قوس به عرض کانال با نسبت‌های 2، 4 و 6 بر نیم‌رخ سرعت و غلظت با استفاده از سرعت‌سنج داپلر پرداخته شد. با بررسی تأثیر افزایش شعاع انحنا بر نیم‌رخ سرعت جریان غلیظ در راستای قائم در انتهای قوس مشاهده شد در یک غلظت و دبی ثابت با افزایش سه برابری شعاع انحنا، سرعت در محور مرکزی کانال به‌دلیل کاهش اثر نیروی گریز از مرکز 40 درصد افزایش می‌یابد. همچنین نتایج نشان می‌دهد با 5/2 برابر شدن کدورت، حداکثر سرعت در نیم‌رخ سرعت جریان غلیظ در انتهای قوس 37 درصد افزایش می‌یابد. همچنین برای اعداد فرود دنسیمتریک جریان غلیظ در محدوده بین 55/0 تا 2/1 در پایین‌دست هر قوس جهت چرخش جریان ثانویه به‌صورت نرمال همانند مجاری روباز خواهد بود ولی با افزایش عدد فرود دنسیمتریک بالاتر از 2/1 جهت چرخش جریان ثانویه معکوس خواهد گردید.

کلیدواژه‌ها

• جریان چگال
• سرعت‌سنج داپلر
• شعاع انحنای نسبی
• نیم‌رخ سرعت

موضوعات

• کیفیت آب

عنوان مقاله [English]

Analysis of two-phase flow hydraulics using acoustic doppler velocimetry

نویسندگان [English]

• Mohammadreza zayeri ¹
• mehdi ghomeshi ²

¹ Department of Water Structures, Faculty of Water and Environmental Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran

² Department of Water Structures, Faculty of Water and Environmental Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran

چکیده [English]

Abstract

Introduction:

Accurate determination of sediment transport and settling rates in various hydraulic environments, such as rivers, lakes, estuaries, and bays, is of crucial importance for many aspects, including coastal morphology, navigation, water quality, pollutants, and biota. Sand particles and even finer particles can be transported in suspension in rivers, depending on the amount of energy in the flow, among other factors. In many sediment transport studies, due to the highly variable nature of suspended sediments, it is essential to use the most efficient methods and equipment possible when measuring and recording the mass concentration of suspended sediments. ADV velocimeters are used to measure instantaneous flow velocities in three directions. These instruments are easy to use and are widely used for field and laboratory research. Numerous studies have shown that the acoustic backscatter caused by flow turbidity can be a suitable alternative to direct measurement of suspended sediment concentration. The objective of this paper is to investigate the structure of sediment-laden flows in a laboratory flume, with a focus on examining the velocity and concentration profiles using an ADV velocimeter. The suitability and accuracy of the ADV technique for this type of flow have been investigated in previous research, but the effect of meander curvature on the hydraulic characteristics of sediment-laden flow is the focus of this study.

Methods:

The experiments for this study were conducted in a flume with a total length of 5.8 meters, a depth of 70 centimeters, a width of 20 centimeters, and a bed slope of 0.001, with three consecutive 90-degree bends with radii of curvature of 40, 80, and 120 centimeters in the Physical and Hydraulic Modeling Laboratory of the Faculty of Water and Environmental Engineering at Shahid Chamran University of Ahvaz. To conduct the experiments, salt and kaolin with a density of 2.63 g/cm3 and an average particle size of 4 microns were used as a non-cohesive sediment material, and a dye called coloring powder was used to create the dense fluid. The experiments were carried out at four flow rates of 0.5, 0.7, 0.9, and 1.1 liters per second and at concentrations of 8, 12, 16, and 20 g/l. The prepared dense fluid was transported from the dense fluid tank to a galvanized tank with dimensions of (100×50×125) centimeters and a constant height by a pump .the dense fluid and ambient water in the experimental channel were separated by a baffle sluice gate.



By opening the gate, the dense flow was directed into the still water. To keep the still water level constant, uncontaminated water with the required flow rate was introduced into the flume from the downstream end. Since the density difference between the dense fluid and the surrounding fluid is very small, especially in laboratory-scale dense flows, it is very important to control the temperature difference between the dense fluid and the surrounding fluid so that the specific gravity difference between the dense fluid and the surrounding still fluid is only due to the muddy fluid and the temperature difference does not play a role in the creation of the dense flow. In this study, a digital thermometer was used to ensure that the temperature difference did not exceed two degrees Celsius. Concentration and velocity measurements were taken after the complete discharge of the flow head and the uniformization of the flow body. Due to the subcritical nature of the flow in all experiments and the very low velocity of the dense flow, after passing each bend and entering the straight path, the maximum flow velocity returns to the center of the channel and the effect of the previous bend is eliminated; therefore, the short distance between the bends will not interfere with the experiment.

Results:

Sediment-laden flows can follow either straight or meandering paths in their movement through rivers or reservoirs. In this study, the behavior of saline dense flow in a flume with three consecutive bends with relative curvatures of 2, 4, and 6, a total length of 5.8 meters, a depth of 70 centimeters, and a width of 20 centimeters was investigated. An acoustic Doppler velocimeter (ADV) was used to measure flow velocity. This device measures water velocity based on the Doppler phenomenon. It also has the ability to measure flow concentration based on the magnitude of the returned pulse intensity. The results show that for all experiments, up to a concentration limit of 15 g/L, the ADV velocimeter can be used to measure the concentration profile pointwise. The steeper the curvature of the meandering path, the more linear the relationship between backscatter signal intensity and concentration becomes. Based on the results obtained, the effect of increasing curvature on the vertical profile of dense flow velocity at the end of the bend shows that the velocity:



- in the R/B=4 bend has increased by an average of 15% compared to the velocity in the R/B=2 bend, and

- in the R/B=6 bend has increased by an average of 40% compared to the velocity in the R/B=2 bend.

- For Froude numbers of dense flows in the range of 0.55 to 2.1, the direction of secondary flow in the downstream of each bend will be normal, similar to open channels. However, with increasing Froude number above 2.1, the direction of secondary flow will reverse.

کلیدواژه‌ها [English]

• dense flow
• doppler velocimeter
• relative curvature radius
• velocity profile