The Acoustic Doppler Velocimetry Biology Essay

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About to decades ago the united state army engineer was designed and developed the acoustic Doppler velocimetry (ADV). An instrument is used to measure three components of the velocity vector in laboratory and field environments (Lohrmann, et al 1994). The apparatuses now available commercially and previous flow meters such as electromagnetic current meters, propeller meters…etc., had been replaced with ADV. The apparatus consists of three modules: the module processor, the probe instrument, and the module state. An acoustic sensor of length of (5-10) cm is mounted on a stiff stem 40 cm long, or at the toe of a 1m cable, and is composed with three receiving transducers and one transmitting transducer. A significant benefit of ADV is able to measures the flow in a very small volume (approximately 0.25 cm3) that is (5-10) cm away from the sensing element in addition no recalibration required. For these reasons the ADV very common among researchers dealing with both laboratory and field flows.

To reduce a Doppler noise influence Nikora, V. I. et al (1998) developed a simple technique. The procedure contains Flow measurement under consideration, Doppler noise components measurements in immobile water taken from the flow, and via both sets of measurements to evaluate the characteristics of turbulence and relationships urbanized, and then by this technique three typical environments are tested. It was shown that even for high-level turbulence flows the Doppler noise can modify expansively the measured turbulence features.

Previous study

Two-dimensional (2D) depth-averaged numerical model over the past decades, have been broadly used to simulate unsteady free surface in open. The approach is with exchange of discharging depth-related information computationally well-organized, but not sufficiently expert sympathetic mechanism of the natural occurrences in which vertical variations of velocities are the critical information, such as flows in channel bends, junctions, obstructions such as piers and abutment existence, density currents in reservoir, seawater intrusions in river estuary,…..etc.

Beside laboratory experimentation and field measurements, and after the rapid increase in computer power computational fluid dynamics (CFD) in current years has appeared as a powerful hydraulic engineering analysis and design tool, quite a lot of three-dimensional (3D) numerical models solving the Reynolds-averaged equations have been developed to simulate flows in natural rivers. Despite of most important current steps and many successful goings-on, the reason is because flows mainly of interest happen in subjectively complex, till now most of numerical simulation of real-life hydraulic engineering problems continues to compose a big challenge to even the majority advanced CFD techniques available today. Because as flows of interest mainly occur in arbitrarily complex, both artificial and natural domains, are highly turbulent and three-dimensional (3D), large-scale, coherent vortices are dominated unsteady shedding.

The big challenge in modelling such flows is complicated by the large inequality in scales between the relatively small-scale artificial hydraulic structures, which contain the large-scale topography of natural river reaches, and complex, local hydraulics surrounded by which such structures are embedded and which determine the approach flow hydraulics in the vicinity of the structures.

Bijvelds, M. D. J., et al (1999) tested the stationary flow in square harbour with 3D numerical model. The harbour was feds by a constant discharge in adjacent river. Two length- scale turbulence models were implemented to account flow in the mixing layer at the transition from harbour to river. The bottom friction caused to generate turbulence, and by using two separate ( ) turbulence model, turbulence generated by horizontal shear are accounted. It can be stated that the flow in this case can be predicted reasonably well. In the field of hydraulic engineering environment, Junction flow is encountered commonly. There are quite amount of data available to help researchers who required investigate the various coefficients related with junction flows in closed conduits. Ramamurthy, A. S., et al (2006) applied the Reynolds averaged Navier-Stockes equations to investigated junction flows in 900 conduit combined with rectangular cross-sections. Both mean flow pattern and energy loss coefficients are calculated then by comparison with experimental data validated. Of course computation modelling is less expensive and less time required to find the different flow parameters needed for engineering design. They are proofed that the three-dimensional two-equation turbulence model closely calculates the mean flow characteristics of junction flows for 90° combines of rectangular cross-section closed conduits. They obviously showed that the computational results consent well with the experimental data related to pressure and velocity fields. The separation flow zone which calculated by the numerical model adequately agrees with the experimental data. Also the numerical model can be used to calculate results such as discharge ratios, and coefficients coefficient of energy loss for different area ratios. In comparisons with the experimental procedures, numerical modelling is cheaper and less time-required.

A multi block structured grid approach for simulating flows in river with islands developed by (Sinha et al. 1998). A curvilinear, body-fitted coordinate's employed to discretize the complex river bathymetry. By using a patched multi block approach the existence of islands in the domain was handled. The computational core area was divided in to set of smaller sub areas, each area separately discretized with a set of curvilinear coordinates, and neighbouring sub areas were correlate together exactly at their respective interfaces. Excellent result for the reach under consideration, but its applicability is restricted for more complex situations.

The commercial code FLOW-3D employed by (Richardson, and Panchang, 1998) to simulate steady flow past a cylindrical bridge pier using a structured, single-block grid. In most cases, single-block structured-grid techniques are restricted due to complexity of the geometries Multi block approaches with patched grids are rather more elastic, but the necessitate to completely patch grids at interfaces limits considerably their applicability to complex geometries. Unstructured mesh is more flexible in complex, 3D flow, and their applicability to steady hydraulic engineering flows (Lai et al. 2003).

An unsteady incompressible turbulent flow solver for simulating flows past complex bridge foundations developed by (Liang, Ge. and Fotis Sotiropoulos, 2005). The method solves the unsteady Reynolds average Navier-Stocks equations URANS and turbulence closure equations a finite-volume method that is second-order accurate both in space and in time. The method was arbitrarily complex geometries it can be considered as a powerful hydrodynamic model for developing an efficient, robust, and reliable numerical model of bridge foundation scour.

Demuren and Rodi (1983) urbanized a 3D model using the 'rigid-lid' approach, which add the free surface into relation as a symmetry plane. A 2D shallow water equations solver assuming hydrostatic pressure not able to compute the free surface correctly because of assumption of hydrostatic pressure, so instead that (Hervouet and Van Haren 1996) proposed a 3D Navier-Stokes solver in a domain with a moving boundary method, of course the pressure field calculated by 3D Navier-Stokes solver locally differ from the hydrostatic pressure and more reasonable. A procedure employed to correct the free surface given by 2D shallow water equations according to the 3D pressure field.

A new code solving Reynold-averaged Navier-Stokes equations presented by (Meselhe and Sotiropoulos 2000) in which by allowing the computational mesh to deform during the iterative solution procedure the free-surface elevation is determined, so that the proper kinematics and dynamic conditions are satisfied at convergence. Due to the mesh regeneration In case of unsteady simulation the model was modified by (Meselhe and Sotiropoulos 2000) will be rigorously computationally time demanding Simulation of free-surface flow in computational fluid dynamics (CFD) and chemical engineering has been broadly used by volume of fluid method (Hirt, et al 1981) and the CFX is one of the mainly popular commercial CFD packages. Faure et al (2004) used CFX code to fix the handling of inlet/outlet boundary conditions problems to simulate flow with free-surface. Due to handling the lack of field velocity investigations to set up the boundary conditions is obligatory to the application of ordinary 3D CFD CFX codes.

For the shallow water free-surface flow computation a semi-3D layer integrated numerical model depended on boundary continuity of velocity and shear stress is conducted by (Hung, M. c., et al 2008). To guarantee continuity of velocity and shear stress at boundary, they applied a quadratic shape function as an auxiliary function. The model was verified by comparing the simulated velocity profiles to the analytical solution. Also they investigated the developing process of water surface and velocity profile. Also they are investigated variations of velocity profile due to alter of viscosity distribution and near bed velocity. Furthermore, an easy relation between discharge, water depth, and eddy viscosity is derived as the end of the governing equations.

To study the flow over a subjective three-dimensional (3D) surface a latest situate of depth-averaged equations is invented by (Anh, T. N., and Hosoda, T. 2007). The derivation of equations depended on a sweeping of curvilinear coordinate system attached to the 3-D bed surface; existence of these equations make good possible to analyze flows over complex environment with no the restriction assumption of mild slope urbanized in conformist depth-averaged models. The derived procedure is the pressure distribution is derived as a combination of hydrostatic pressure with the centrifugal force owing the curvature of the bottom surface effects. They applied the model to calculate the water surface profile on, flow over a cylindrical surface; flow over a circular surface; and at a vertical intake the flow with an air-core eddy. Validation for numerical model by comparison with laboratories data, and the result obtained from the model are showed an excellent harmony with laboratory data and computation by an empirical formula. As a result, it shows the applicability of the model in belongings of flow over a vastly curved bottom.

As a rule open channel flows of real problems are harshly three dimensional (3D). Though, this characteristic is frequently of secondary important, particularly when the ratio of width to depth is considerable large. and due to strong vertical mixing occurred by the bottom shear stress the vertical deviation of the mean-flow quantities is not considerable. Depending on this evidence, the depth-averaged modeling adequate to effectively analyze flow features. Under some assumptions (Steffler and Jin 1993) urbanized the depth-averaged modeling, and it is adequate to successfully analyze flow features. For straightforwardness and considerably reduces the depth-averaged modeling urbanized computational attempt to derive the equations sacrifices flow information over the vertical dimension. The depth-averaged models are limited in use due to the assumption of moderate slope of the channel bed. Even though the effectiveness and sensible accuracy of conventional.

Hos, and Kullman, (2007) are investigated the ability of computational fluid dynamics (CDF) commercial codes to solve a numerical model of a two-dimensional free-surface channel flow with a bottom obstacle existence. They urbanized ANSYS CFX 10.0 with its built in tow-phases flow model. The results are showed ANSYS-CFX10.0 present the option of analysing free-surface flows without differentiate between or separately modelling subcritical, Transcritical and supercritical cases. Also they obtain that the cost is the computational is very expensive, on a 2.4 GHz PC with 1GB RAM steady state case run a numerical study on free-surface channel flow for the problem will need 10-15 hours for computation. Such flow models include a non-hydrostatic pressure distribution and weak hydraulic jumps (undular jump) also studied which can not be studied with assumption of hydrostatic pressure. It can say by many researchers that the classic methods are based on the shallow water equations give only a limited explanation of the flow but cannot handle e.g. supercritical cases. In addition, there are parameters (remarkably the friction coefficient), which are difficult to approximation. The classic methods are appropriate only for simply subsonic or supersonic flows and more complicated models are needed for transonic flows and hydraulic jumps.

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