International Research Journal of Engineering and Technology (IRJET) Volume: 04 Issue: 02 | Feb -2017
www.irjet.net
e-ISSN: 2395 -0056 p-ISSN: 2395-0072
Numerical Investigation of Head Frontal Velocity of Non-conservative Dense Flows in Small Inclined Beds Ehsan Hajibabaei1, Alireza Ghasmi1, Seyed Abbas Hosseini1 1Technical
and Engineering Department, Science and Research Branch, Islamic Azad University, Tehran, Iran
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Abstract - Non-conservative dense flow frontal velocity has
been simulated two dimensionally by fluent numerical code. The outcomes have been compared with experimental results. Numerical simulation was conducted as two-phase through Euler-Lagrange method. Reynolds-Stress Turbulent Model (RSM) with non-uniform grid and shredding mesh on the channel floor. The results obtained from numerical model of head frontal velocity show a good compliance with experiment results and greatly help analyzing the pattern of fluid movement in different scales. Key Words: dense flow, fluent, head frontal velocity
1. INTRODUCTION The specific area of head is in frontal edge of dense flow that has been deeper than tail area and have different characteristics of flow body. Dynamic identification of head is very important because it is generally considered as a boundary condition for the flow [1]. Dense flows which stream on horizontal surface have almost semi-permanent head and a depth twice the depth of the body of flow but in a flow which passes on a slope, the depth of head always increase. The shape of head in gravity flows aren’t generalizable and severely depend on physical conditions, environmental fluid depth and other cases. Figure (1) represents the first experiments that are formed for modeling the head of a cold flow of wind in atmosphere using temperature difference. In figures (1-a) and (1-b), the head of flow with low Reynolds number about 10, Rer=
Fig-1: The changes of radial velocity versus time
(Ur head velocity, hr is its height and v is kinematic viscosity) has been shown. When temperature gets more and the number of Reynolds increases, the form of head will change, its nose gets closer to the earth floor and severe mix occurs in front of and above the head. Figure (1-F) shows a flow whose Reynolds number is upper than 1000 in which some billows raise from high level and goes backwardly [2]. As it can be seen the head of flow is an area in which severe disorder and turbulent happen and there is high density gradient. When velocity and as result the number of Reynolds in flow increase, the form of head changes and mainly turn to more stretched mode. Whirlpool within head Š 2017, IRJET
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during rotation generates many vortices and pollens that in area above that breaks in three dimension and very complicated form and creates some instabilities. The velocity changes may create different types of damages such as erosion, abrasion and cavitation which are extensively reported for concrete and steel structures [3-5]. In head area because of great vortex, turbulence and mixing and inside mixture are also more and gradient of velocity and dense is stronger in this area. Mixture of dense flow with fluid of environment is an important process that occurs in head of flow through dense fluid suction behind and back of head as series of cross vortices [6]. Another example of turbulent flow with hydraulic jump, vortex and pressure fluctuations is considered by Hamedi et al. in stepped spillways with horizontal and inclined steps. They reported that inclination slightly increases the energy loss [7] Hamedi et al., also, mentoned that the combination of inclination and end sill increases the energy loss [8]. Also, velocity field in reservoir has been considered by Sarkardeh et al. in presence of an aircore vortex as another example of turbulent flow [9].
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Figure 1- transformation of head of experiment gravity flow through increasing Reynolds number a) Reynolds number less than 10, f) Reynolds more than 1000 [2] Using images with slow movement for tracking and following the form of billows in upper boundary of flow show that the wave and range of these billows can grow to the size of head and then they are collapsed and broken [1].
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