Numerical analysis of the flow near the aeration ramp of the tunnel overflow

Number of pages: 124 File Format: word File Code: 31469
Year: 2014 University Degree: Master's degree Category: Civil Engineering
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  • Summary of Numerical analysis of the flow near the aeration ramp of the tunnel overflow

    Dissertation for Master's Degree

    Civil Engineering - Hydraulic Structures

    Abstract

    In high dams, spillways, as one of the dependent hydraulic structures, play the critical role of discharging the incoming floodwaters into the dam reservoir with sufficient safety in free flow or under pressure downstream. In tunnel overflows where the flow is at a high speed and often under pressure, due to the roughness of the overflow concrete wall and the existence of seams and curvatures in the tunnel path, the possibility of the flow separating from the wall increases and as a result, the cavitation phenomenon and the damage caused by it are likely to occur. Researchers have done a lot of research on how to eliminate or reduce the possibility of cavitation phenomenon and the resulting damages in high overflows, and they have stated that flow aeration is one of the most effective methods to prevent this phenomenon. Numerical modeling of flow in pressurized tunnel overflows with aeration structure and studies related to two-phase flow of water and air in these overflows are very few. Therefore, more knowledge and understanding of the pressure fields and flow velocity near the pressurized tunnel overflow aerator structure and the effect of the aerator ramp geometry and different percentages of aeration on the flow need more studies.

    In this research, the flow passing over the ramp embedded in the pressurized duct floor is numerically modeled with the help of Fluent software in two-dimensional and three-dimensional mode. To validate the performance of the numerical model, the laboratory results of Manafpour (2004) were used. In order to model the flow turbulence in the non-aerated state, shear stress transfer (SST) turbulence model was used and in the aerated state, RNGk-? was used in the Reynolds number range of 3.1*104

    In the spillway of high dams, due to the high flow speed (more than 20 m/s), any change in the geometry of the channel, roughness of the bed and wall, curvature in the flow path, and the presence of operational seams in the wall cause the flow to separate from The wall of the duct and the local reduction of pressure is caused by the amount of water vapor pressure.In this case, with the occurrence of cavitation phenomenon, water at its ambient temperature changes from liquid state to vapor state, and cavitation bubbles (water vapor bubbles) are formed, which disappear with the movement of water to high pressure areas. The collapse of these cavitation bubbles, which is accompanied by the production of strong pressure waves caused by the explosion of the bubbles and the release of a significant amount of energy, if it occurs near the overflow wall, it causes erosion and corrosion of the concrete surface of the overflow and if it continues, it causes huge damages to the wall and the structure.

    Researches carried out in the field of investigating the phenomenon of cavitation and its prevention methods in The overflows have shown that the use of resistant concrete and the modification of the curvature and wall of the flow, stepped weirs and aeration of the flow are suitable methods to prevent the occurrence of this phenomenon and reduce the damages caused by it, and among them, the most effective and economical method to prevent and reduce the damages caused by this phenomenon is aeration. The change in the state of flow turbulence indicated that some of these changes are beneficial and others are harmful for the studied hydraulic system.

    The exorbitant cost and long duration of construction of hydraulic structures and the fact that testing is still considered as the most accurate method in investigating the issues and problems faced by such structures, has prompted researchers and designers to try to solve the mentioned problems and issues by simulating the real flow on physical models and performing various tests. For this reason, the use of numerical models has been greatly developed in recent decades.

    The most important advantage of a numerical calculation estimate is its low cost and considerable speed. Also, the numerical solution of the problems will give us complete information and the necessary details and will give the values ??of the relevant variables throughout the studied area. Unlike the unfavorable conditions that occur during the experiment, there are few inaccessible places in numerical calculation tasks. Obviously, no laboratory investigation can be expected to measure all the variables involved in the studied phenomenon in the entire flow field, so in order to complete the laboratory data, a simultaneous numerical solution becomes necessary. 1-2 Statement of the problem For many years, the incidents related to the cavitation phenomenon have focused the minds of engineers in different parts of the world. The phenomenon of cavitation is a well-known phenomenon that is considered as a major engineering risk and challenge in most hydraulic structures that are exposed to high-speed flows, including dam overflows. Of course, it should be noted that the effective factor on this phenomenon is not limited to the speed and the existence of a set of multiple factors leads to the occurrence of this phenomenon.

    In long overflows in the end areas, the speed of the flow increases tremendously and the depth of the flow decreases. The combination of these factors reduces the cavitation index [2]. As a result, a point of the structure with normal irregularity can become the starting point of cavitation in the structure. In such a case, it is possible to prevent this phenomenon by reducing the flow speed or by increasing the flow pressure at the borders. It is necessary to pay attention to the fact that the geometric shape of the overflow also plays a significant role in the occurrence or non-occurrence of the cavitation phenomenon. In order to prevent the occurrence of cavitation phenomenon, it is necessary to identify the location of the points where the pressure may decrease to the level of liquid vapor pressure with the increase in speed. Flow aeration as an effective factor in eliminating or reducing the damage caused by cavitation in free and tunnel overflows under pressure, has a significant impact on the hydraulic structure of the flow, including pressure and velocity fields, and in recent years, the correct understanding of the flow pattern and the effects of aeration in overflows Underpressure with an aerated system is one of the subjects studied by a number of researchers, and due to the specific complexity of two-phase flows and the lack of clarity regarding the relationship between flow aeration and the reduction of cavitation damage, extensive studies are still needed on this issue. 1-3 Necessity and purpose of research In order to store surface water for various uses, the design of high dams and spillways has been accompanied by a significant increase. The review of the references indicates that in recent years, many studies have been conducted on the phenomenon of cavitation, ways to prevent its occurrence and reduce the damages caused by this phenomenon on hydraulic and numerical models of free overflows.

  • Contents & References of Numerical analysis of the flow near the aeration ramp of the tunnel overflow

    List:

    1 Introduction 3

    1-1 Introduction 3

    1-2 Statement of the problem 4

    1-3 Necessity of conducting the research 4

    1-4 The purpose of the research 5

    1-5 Thesis structure 5

    2 Technical literature and subject background 7

    2-1 Introduction 7

    2-2  Cavitation in overflows 10

    2-3-1 Effects of cavitation 10

    2-3-2 The mechanism of destruction caused by cavitation 11

    2-4 Step overflows 13 2-4-3 Aeration 13 2-5 Flow aeration 14 2-5-1 General classification of flow aeration 15 2-5-2 Effects of flow aeration in weirs 18 2-6 Researches related to aeration 19

    3 Materials and methods 40

    3-1 Introduction 40

    3-2 Equations governing the flow 40

    3-3 Preparation of numerical model 42

    3-3-1 Model and laboratory conditions 42

    3-3-2 Introduction of Fluent software 45

    3-3-3 Geometry and numerical model meshing 51 3-3-4 Boundary conditions and initial conditions 54 3-3-5 Software settings 55 3-3-6 Sensitivity analysis 56 4 Results of numerical flow analysis - without aeration 59 4-1 Introduction 59

    4-2 Validation of the results of the numerical model 59

    4-2-1 Reconnection length 59

    4-2-2 

    4-3-1 The effect of increasing the height and angle of the ramp on the reconnection length 66

    4-3-2 The effect of increasing the height and angle of the ramp on CP min 68

    4-3-3 The effect of increasing the height and angle of the ramp on the intensity of turbulence 69

    4-4 The flow velocity profiles passing over the ramp 70

    4-5 Pressure distribution 72

    5 Results of numerical analysis of flow - with aeration 77

    5-1 Introduction 77

    5-2 Velocity profiles 83 5-3 The effect of the geometric dimensions of the ramp on the flow parameters 86 5-3-1 The effect of increasing the height and angle of the ramp on the cavity length 86 5-3-2 The effect of increasing the height and angle of the ramp on CP min 87 5-3-3 The effect of increasing the height and angle of the ramp on the intensity of turbulence 88 5-4 Effect of aeration on flow characteristics 90 5-4-1 Cavity length 90 5-4-2 Minimum pressure coefficient CP min 93 5-4-3 Turbulence intensity 95 5-4-4 Velocity profiles 96 5-4-5 Profiles Pressure 100

    5-5 The efficiency of two-dimensional and three-dimensional numerical models in simulating the flow passing over the ramp

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Numerical analysis of the flow near the aeration ramp of the tunnel overflow