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- Type of Document: Ph.D. Dissertation
- Language: Farsi
- Document No: 39985 (08)
- University: Sharif University of Technology
- Department: Mechanical Engineering
- Advisor(s): Firoozabadi, Bahar; Rad, Manouchehr
- Abstract:
- Dense underflows are continuous currents that move down-slope due to their density being heavier than that of the ambient water. This difference between the dense fluid and environment fluid can be due to temperature difference, chemical materials, solved materials or suspend solid particles. In these currents, the effect of buoyancy force is produced by this difference density. In this research, many experiments performed in different flow rates, slopes and concentrations to understand the current structure and turbulence specification of the salt solution density currents and particle-laden density currents. Acoustic Doppler Velocimeter is used to measure the velocity fluctuations. The height of density current in both density currents is defined by experimental results and also the effects of different parameter on velocity profile and height of both density currents are considered and compared. Also in the particle-laden density currents, the concentration profiles are considered. Because of lack information in understanding of turbulence feature and consequently how turbulence density current modeling, this research is defined based on recognition of turbulence mechanism and the role of disturbance velocity and concentration fluctuations in this process. Therefore, friction coefficient and friction velocity for both density currents are calculated and compared together. Results show that the friction coefficient in the supercritical region at particle- laden density current is greater than density current and in the subcritical region vice versa. Also, turbulence parameters such as kinetic energy and turbulence intensity were discussed for both currents. Results show that turbulence energy in density currents has two maximum, one of them near the bed and other near the interface. The minimum of turbulence energy occurred near the maximum velocity. Also, maximum of Reynolds stress occurred a little higher of the location of maximum velocity and it will be neared to the bed by increasing of concentration. Considering the secondary flow in the density currents with velocity fluctuations indicates that there is a large eddy near the inlet. Also, in this research, the entrainment between the environment and dense fluid has been calculated by using the vertical concentration fluctuation flux. The results show that the increase in the local Richardson number decreases entrainment in the subcritical region. Also, in this region, the rate of entrainment increases with increasing Richardson number. So that in the high Richardson number, entrainment will rapidly approach zero. Another important point is the fall velocity calculation of suspend particle in the particle-laden density currents. In this research fall velocity was calculated by velocity and concentration fluctuations. Fall velocity in the inlet of current was about 0.2 m/s. In this region due to hydraulic jump; fall velocity had the difference values in different heights. Also in this research, space-time structures of ejection and sweep for salt solution and particle-laden density currents in difference flow rates and concentrations for considering of turbulence produce mechanisms were analyzed near the bed. In all conditions, the behavior of the u-velocity and the w-velocity in ejection and sweep had the 180 degree phase difference. In the ejection phenomena, the amount of u-velocity until the value reaches its minimum gradually decreased and then increased rapidly. In all graphs ejection, this process was observed for different conditions. In zero hole size, the contribution of ejection and sweep in Reynolds stress produce were 80% and 100%, respectively. That amount due to the negative effects of interaction motions rate was more than 100%. In fact, inward and outward phenomena had a negative contribution 30% and 50 %, respectively in reduction of turbulence energy. Finally, three - dimensional turbulence salt-solution density current was simulated by different turbulence models such as Standard k-ε, Launder & Sharma k- ε and k-ω. The results showed that Launder & Sharma k- ε model was a suitable model to simulate three-dimensional turbulence salt solution density current. Also, for the two-dimensional laminar salt solution density current, the flow was simulated by VOF method of two phase flow. The results had a good agreement with the experimental data and the only disadvantage of this method was its computational cost. Since this method for having a high accuracy required a small time step.
- Keywords:
- Density Current ; Numerical Solution ; Turbulence ; Experimental Investigation ; K-Epsilon Turbulence Model ; Numerical Simulation ; Modified Volume of Fluid (VOF)
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محتواي پايان نامه
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