Obstacles and Vegetation on Flood Flows - Research Paper Example

Obstacles and Vegetation on Flood Flows Table of Contents Table of Contents 2 Introduction 3 2. Aims and Objectives 4 2. Literature review 5 2.1. Introduction 5 2.2 Behavior of Floods Flows 6 2.2.1 Open Channel Flows 6 2.2.3 Types of Flows 7 2.3. Flow past two identical Cylinders (Side by side arrangement) 8 2.3. Further Flow Theory 11 2.3.1. Flow around a cylinder- Drag Coefficients and Boundary Layers 11 2.3.2. Fluid Force Estimation 11 2.4. Velocity Profile 12 3. Complex Flow that exist around obstacles 14 3. Materials and Methods 15 3.1. Needle Gauge. 15 3.2. Acoustic Doppler Velocimeter (ADV) 15 3.2. Strain Gauge 15 3.4. Set up of the experiment 15 3.4.1 Readings of Strain Gauge and Experiment 15 Bibliography 16 Abstract The main aim of the project is to understand the effects of a circular cylinder object on open channel flow that is considered to be subcritical and turbulence. In this case, there will be a fixed cylinder at a given point and the effect of it on the flow will be determined. Later, the other cylinder will be included either side by side or in front of the fixed cylinder, then the effect of the fixed cylinder on the flood will be determined as well. The Ascoustic Doppler Velocimeter will be employed in measuring the velocity while the strain gauge was used to measure the drag force on this fixed cylinder at different occasions based on the position of this other cylinder which is not fixed. The measurement of the force will be undertaken at different velocities and depth of water as well as at a constant rate of the flow (the strain gauge will be at fixed point; otherwise the measurements are not reliable). Basing on the results, it was observed that the fixed cylinder’s drag force coefficient was seen to decrease with increase in the flow’s Reynolds number. In case of laminar flow it was observed that the drag coefficient increased to a big extent. For the case of when the other cylinder which was not fixed when positioned side by side, it was observed that the distance between it and the fixed cylinder had an effect on the flow, that is, when the distance between the two was decreased, the effect on the flow was similar to as if it was one cylinder. When the cylinder was placed directly in front of the fixed cylinder, it was observed that the drag coefficient increased with decrease in flow’s Reynolds number. 1. Introduction The research topic is based on the obstacles and vegetation in flood flow. In this case, the flow of water on the fixed cylinder with the other cylinder placed at different positions around this fixed one was studied. By so doing this, the aim was to try and understand the effects that surround this fixed cylinder either in the presence or absence of the other cylinder placed around it at varying positions. This experiment setting simulates the real life incidence related to the obstacles and vegetations in flood flows. There are several obstacles and vegetation in flood flow observed in most parts of the world. A detail on the way either the obstacle or flow is affected is considered to be very critical regardless of where the obstacle is located. Floods are known to cost lives including homes and cities. Therefore, this justifies the need for studying of flood flow where the effects related to flood flow or the vegetation and obstacles is determined. This might contribute in the reduction of the impacts caused by these natural disasters which are considered to be so challenging in terms of management, more so by the developing countries. Another merit of understanding the relationship between the flood flow and the obstacles is that it enables construction of barriers to enable early preparation of the floods that have been predicted. In relation to this, research of this kind can contribute to the determination of the most prone areas that need quick attention since the flow do affect foundations for structural piles. This can largely be applied to areas that are considered to be more venerable to storms with limited capacity of managing natural disasters. It is important to study and understand the effects related to obstacles and flood flow at a basic level before considering the more complex factors that include complex liquids and shapes. This is because it is challenging in studying these effects when these complex factors are considered. In this regard, the major focus of the project is on the effects of the cylinder on the open channel flow, whereby this is a simulation of a real life situation and the cylinder is related to structural piles or trees. 1.2. Aims and Objectives The research aim is to understand how a fixed circular cylinder is affected as well as how it affects the flood flow. To meet this aim there are various objectives which were considered and are indicated as below; Review of literature in order to understand the methods related to the project as well as the project context. This will enable familiarization of other research carried out and that is related to this project to be undertaken. The water flow velocity determination at varying depth and positions relative to the fixed cylindrical object in the presence and absence of the other cylinder that is not fixed but placed at different positions at a time. Drag force measurements on the fixed cylinder in the presence or absence of the other cylinders at varying positions. In this case, the drag coefficient was determined. Data analysis and interpretation will involve results analysis and understanding in order to come up with the conclusion about the data based on the research objective. Future recommendations that will involve proposing future work which might be undertaken as a follow up of this current research. 2. Literature review 2.1. Introduction The main aim of this section is to explore literature related to the topic of study as well as the project objective. This section is divided into eight sections. The first section explains a flow past two identical cylinders which are arranged side by side. The second section of the literature review explains open channel flow. It is noted that fluid mechanics open channel flow is considered to be a very critical field for civil engineers. The third section explores in details the types of flows that are available. In this case, it is noted that there are different types of flow and they include steady or unsteady, Uniform flow, rapidly-varied flow and gradually varied flow. The fourth section covers the properties of flow whereby it is noted that the flow that has Reynolds number that is less than 500 in open channel flow is considered to be laminar.On the other hand,it is considered to turbulent when it is atleast 12,500.The fifth section explains about the flow around a cylinder. In this case, the focus is on drag Coefficients and Boundary Layers. That is, as the drag coefficient slowly decreases, the more the turbulence of the flow. This goes on up to a level where the Reynolds number drops. The sixth section explains about the fluid force estimation and it has been highlighted that it can be used to estimate the drag coefficient on the surface of the cylinder. Part seven of the literature also talks about the flow over cylinders arranged side by side. In this case it has been indicated that in fluid dynamics, a flow over side by side arranged pair of cylinder is seen to be a big challenge of interest due to the complexities observed in relation to vortex-vortex integration that emerges as a result of the cylinder proximity. The second last section explains about velocity profile and the key point is that in case the cylinder is staged at the open channel, there will be a turbulent boundary layer separation of the flow upstream. The last part explains about complex flow that exists around obstacles. In this case, it is noted that various studies have been undertaken on complex flows around obstacles. 2.2 Behavior of Floods Flows 2.2.1 Open Channel Flows In fluid mechanics open channel flow is considered to be a very critical field for civil engineers. It defines the flows in manmade channels, rivers and pipes that are partially full. This also includes the hydraulic structures behaviors like sluices, spillways and weirs. There are several examples of open channel flows and include drainage, canals and floods. The open channel flow is observed to be driven by gravity unlike pipe flow (Kang, 2003) 2.2.2 Properties of Flow The following equation defines the flow or discharge rate; Q=U*A Where Q represents the flow rate,the average velocity is represented by U and it is over the section.The cross-sectional area is represented by A.This equation can be applied in velocity calculations incase the flow rate as well as the cross-section is known or has been calculated. The Reynolds number can be described as below: In this case,h represents the flow depth that is measured from the point that is the lowest Circular (radius R) Where It is known that a flow that has Reynolds number that is less than 500 in open channel flow is considered to be laminar.On the other hand,it is considered to turbulent when it is atleast 12,500,in this case,gradual changes are observed more between the flow types than the pipe flow.Analysis of tabulance methematically is known to be complex.The tarbulance alse seems to be irregular as well as fast (Lumanog,2013). Froude number is used to determine the properties of open channel flow that include supercritical,critical or subcritical.It can be difined as below; For a rectangular or wide channel mean is considered to be equal to h. While for channels that are not rectangular is considered to be the mean depth and it is described as the rectangle depth that has same cross sectional area and the width of the surface. Where bs represents the width of the surface while A represents the cross section of the water. Subcritical level is described by Fr being less than one hence referred to as tranquil and slow. For Fr greater than one it implies that the flow is rapid and on the basis of gas dynamics, it is analogous to supersonic. In case Fr is equals to one, it implies that it is a critical flow; hence, the water velocity can be equated to the speed of the wave. The project majorly focuses on the subcritical flow. 2.2.3 Types of Flows There are different types of flow and they include steady or unsteady, Uniform flow, rapidly-varied flow and gradually varied flow. There is no variation on the velocity profile of the cross stream and depth in the flow direction for uniform flow. For rapidly –Varied flow, there is occurrence of flow when the flow adjusts over distances that are relatively short. Examples to these include hydraulic jump. For the gradual varied flow; there is a slow change in water depth over distances that are several times the flow due to the imbalance that exists between the frictional and gravitational forces. Open channel flow is considered to be steady in case all the properties of flow are time independent. There are unsteady example examples considered to be very important and include surge waves as well as tidal flows. For our case we shall focus on steady flow. For uniform, steady flow referred to as normal flow can be described by the equation below: Where represents the average wall friction and the hydraulic radius is represented by Rh .In this case, it can be defined by the area of cross sectional which is divided by the perimeter that is wetted. In case of the channel that is rectangle, and then Rh is expressed as: By skin-friction coefficient (cf ) definition, the stress of bed shear could be associated with the mean velocity v as; Thus, when the two expressions are equated as well as rearranged, V is equals to; In this case,Manning’s equation is used to get the actual value of cf as shown below: Where; 2.3. Flow past two identical Cylinders (Side by side arrangement) (see article titled: characterization and analysis of flow over two side by side cylinders In fluid dynamics, a flow over side by side arranged pair of cylinder is seen to be a big challenge of interest due to the complexities observed in relation to vortex-vortex integration that emerges as a result of the cylinder proximity. Interactions of such kind results to induced vibration interactions. In relation to this, it is known that problems related to this have big importance in terms of engineering. Such cylinders structural design requires a deep understanding related to the flow physics. It becomes more problematic in cases where two or more cylinders are relatively in close proximity and hence there is an interaction that is a periodic as well as stronger. This interaction is not only based on the distance that exists between the cylinders but as well as the flow of Reynolds number. There has been exploration of the flow over cylinders that are arranged side by side by both experimentally and numerically. Unfortunately, full understanding of this flow has not been met. Flow investigations based on experimental and numerical on two side by side cylinders has been undertaken (Ding et al. 2007; Bearman and Wadcock. 1973; Chang and Song 1990; Williamson 1985).Several research is seen to majorly focus on the drag and lift coefficient variation as well as flow classifications into different patterns of wake as well as groups which have Reynolds number that varies and gap that exist between the cylinders. Current studies have used various tools in the analysis of the flow complex features as well as determining different regimes flow basing on the gap that exist between both cylinders. Vortices shedding from cylinders as well as their interactions generates on them time varying loads. Frequencies that are dominant have been generated from the time history spectra of coefficients of force with the use of FFT. The shearing and swirling regions in the cylinders vicinity as well as their wake are seen to be studied with the use of λ2 criterion. Stresses as a result of periodic flow field variation are known to have been investigated. Representation of phase space is seen to have been used in the analysis of the flow filed dynamic behavior. For below section: refer to article titled: first instability and structural sensitivity of flow…….) There are major concerns related to the flows past one or at least two cylinders in different arrangements. Such flows are considered to be prototypical and could be considered in several engineering applications like pilings of bridges, heat exchangers as well as offshore platforms. An example of prototypical configuration was observed in an experiment the included a flow past circular cylinders that were identical and arranged side by side. This was investigated both in numerical as well as experiments simulations. There could be complex interactions between the two cylinders despite the geometry being simple. In this case, the interaction will result to various patterns of flows (Summer, 2005).There are two parameters that are considered in this flow dynamics for this configuration. These parameters include Reynolds number (Re=U*D/v) as well as gap spacing that is non-dimensional between the surfaces of the two cylinders (g=g/D (fig 1).In this case, represents the velocity of free-stream, D* represents the diameter of the cylinder while v* represents kinematic viscosity. Figure1. Flow past two cylinders in arrangement of side by side computational domain The work related to most experiments is known to be based on previous researches (Spavack 1946; Bearman and Wadlock 1973) as well as researches that are more recent (Alam and Zhou 2007; Alam, Moriya and Sakamoto 2003).Research related to these as based on turbulent regime and it had a range of 103 Read More