Evaluation of Acoustic Doppler Velocity meter (ADV) for determining velocity and concentration profile in turbidity currents  Fulltext 

[ Seyed Abbas Hosseini ] - PhD. Candidate of Civil Engineering Department, Sharif University of Technology
[ Aboulfazl Shamsaei ] - Professor of Civil Engineering Departemnet, Sharif University of Technology
[ Seyed Reza moosavi ] - Phd. Candidate of Mechanic Engineering Departemnet, Azad University

خلاصه مقاله:

Turbidity currents are flows driven by density differences caused by suspended fine solid material. They belong to the family of sediment gravity currents. These are flows of water laden with sediment that move down slope in otherwise still waters, such as oceans, lakes, and reservoirs. Their driving force is gained from the suspended sediment, which renders the flowing turbid water heavier than the clear water above. Turbidity currents are often encountered in fluvial hydraulics when sediment-laden discharge enters a reservoir. During passage in, it may unload or even resuspends granular material. The pattern of fluid motion within turbidity currents has been described by several authors in general terms. However there exist no detailed measurements of the velocity and concentration of turbidity currents using modern high-resolution flow measurements techniques, and current knowledge of the internal dynamics of turbidity currents and other forms of density currents is based mainly upon flow visualization. The present study is a preliminary attempt to quantify such currents, using Acoustic Doppler Velocimeter (ADV) to measure three component of flow velocity. The ADV measures mean flow and turbulence compared favorably with other independent from a Laser Doppler Velocimeter in a laboratory flume and in ocean boundary layer. Also the gathered data are used to investigate the concentration distribution of turbidity currents based upon backscatter intensity. Experiments were carried out in a transparent tilting flume with a total length of 12.0 meter 1.5 m wide, and 70 centimeter high, with bottom slope of 1.0 ≤ S (%)≤ 3.0. the entire flume is divided into two section. One head tank for turbid water was installed. A cylindrical mixing tank with volume of 2.0 m3 was used for mixing sediment and water, which can supply all of the turbid water inflow during experiment. The turbid water pumped from mixing tank to constant head tank. A gate with variable opening allows the controlled release of turbidity current into the flume. The turbid water overflow from the head tank into mixing tank. Turbid water was supplied An important advantage of the ADV is that it measures the flow in a small sampling volume (approximately 0.25 cm3) that is 5 (or 10) cm away from the sensing elements. This enables measurements to be taken without interfering with the flow. This important advantage is very suitable for velocity measurement in turbidity currents, because sensing element places in still water and measures velocity of current beneath it without interfering with current. Experimental result for velocity and concentration show good agreement with others. Result show turbidity current has a shape similar to a wall jet the distinguish with two regions: a wall back scattering analysis was introduced. It is very new technology and was used for first time in turbidity currents. Concentration distribution showed good agreement with sampling measurements. Experiments were performed at the Hydraulic laboratory of department of Mechanic engineering at Sharif University of technology. Guidance by Dr. Firoozabadi and professor Rad is greatly appreciated. region and a jet region as shown by Altinakar et al. The situation of maximum velocity (hmax) separates theses regions. Result show in wall region velocity profile follows from logarithmic relation and concentration distribution express by Power relation. In jet region, velocity and concentration profile follow from Gussian relation. For determining concentration profiles,