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Model of Flow over Spillways by Computational Flluid Dynamics

Autor:   •  April 5, 2018  •  1,454 Words (6 Pages)  •  556 Views

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In addition to the explanation about spillways and overflows these will be evaluated using a CFD (in full meaning Computational Fluid Dynamics)rogram which in simple words is the use of applied mathematics, physics and computational software to visualize how a gas or in our gas a liquid flows, as well as how the gas or the liquid affects object as it flows past. Computational fluid dynamicsis based on the Navier Strokes equation, and these equations describe how the velocity, pressure, temperature and density of a moving fluid are related.

SOFTWARE

A 2D geometry was drawn in the CAD program within Ansys (which is the CFD program we used throughout the project) and exported to make calculations for solutions and results. FLUENT was the CFD solver that was used and it was also used for post processing.

LIMITATIONS

The calculations were done on a single computer which could be considered as a limited computational resource for such a CFD project. Regardless of the limitations and the limited time of the project the spillway was investigated with the help of above inlets and bottom outlets.

EXPIRIMENTAL DATA

The experimental data used for comparison was provided in the report named Model of flow over spillways by computational fluid dynamics written by Chaiyuth Chinnarasri, Duangrudee Kositgittiwong and Pierre Y. Julien 2012

The reason to why this paper was used for comparison was to examine how accurately the flow in the spillway could be computationally modelled. The experiment include measurements mainly for the pressure, volume and kinetic turbulence

PHYSICAL MODEL

Below shows a smooth spillway with the measurements we used which were similar to the research paper we used.

For the smooth spillway, flow discharges of 0.57, 1.13, 1.70, and 2.27 m3/s were used. The data on the five locations were measured in the direction normal to the spillway floor. They were at Ls/L ¼ 0.09, 0.28, 0.44, 0.60, and 0.76, as shown in

[pic 2]

Velocity and air concentration instrumentation was mounted on a carriage system. The manually operated carriage system allowed for two degrees of freedom, with movement along the spillway, and lateral movement within the width of the spillway. The remotely operated, motorised point gauge allowed for vertical movement normal to the pseudo-bottom. All profiles were taken along the centreline of the spillway. A back flushing Pitot-static tube was used to measure the velocity, due to its ability to work in non-homogeneous fluid. The output signal was scanned at 120 Hz for a duration of 20 s. The Pitot tube was patented by Leo et al. (1969) as a Strut mounted dual static tube No. US 3482445. The air probe was used to determine the air concentration. It acts as a bubble detector by passing a current through two conductors spaced approximately 2.0 mm apart and measures the change in conductivity that occurs when a bubble impinges on the probe tip. Its output signal was scanned at 15 kHz for 5 s per probe tip. The error on the vertical position of the probe was less than 0.025 mm. The accuracy on the longitudinal probe position was estimated as ˜x , _ 0.5 cm.

MULTIPHASE FLOW AND TURBLANCE MODELS

One of the main characteristics of turbulent flow is fluctuating velocity fields. These

fluctuations cause mixing of transported quantities like momentum, energy and species

concentration and thereby also fluctuations in the transported quantities. Because of the

small scales and high frequencies of the fluctuations they are too computationally expensive

to simulate directly in practical engineering situations.

Within the project the commercial fluent with the finite volume method was used with the uniform-sized, structured grids. A mesh was generated representing 2D grid which was later used for the final results. It was easier to use a 2D grid because it takes much less time than a 3D grid, also because it was used on the research paper we used for comparing our final results.

The inlet section is upstream which consists of the inlet of water at the bottom and the inlet of air at the top. Its water velocity was the initial condition and was set uniformly at the water inlet and flow through the spillway As mentioned above about the atmospheric pressure it was used for the air boundaries for the inlet pressure. While the outlet spillway at the downstream was defined as outlet pressure so that the water could flow out.

DETAILS ABOUT THE SIMULAION TIMES AND MEASUREMEMNTS USED LIKE BOUNDARIES OF VOLUME PRESSURE ETC (CHECK OUUT THE RESEARCH PAPER) AND HOW GEOMETRY WAS CREATED

- Volume

- Model used

RESULTS AND COMPARISIONS

CONCLUSION

Table of contents

The figure below shows an image of the created geometry using Ansys Designer Model. Which was the first thing that had to be done on the process to competing the project. The 2D geometry had to be done, then followed by the mesh generation and setup process where it included setting boundaries for example. From there calculations were ran plotting an iteration graph and later continued on to being used for the time step simulation run.

[pic 3]

[pic 4]

On the figure below, is after the geometry was used in mesh generation. Where a coarse like mesh was created for better results when solving the simulation. Depending on the grid generation on the mesh, the accuracy of the solution will be obtained.

The diagram below shows the first simulation using kinetic turbulence.

[pic 5]

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[pic 9]

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