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Experimental and Theoretical Investigation on Entrainment Coefficient in Density Current and Introducing Modified Richardson Number

Maleki Tehrani , Mahdi | 2009

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 39578 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Firoozabadi, Bahar
  7. Abstract:
  8. Dense underflows are continuous currents, caused by interaction of two or more fluids of different density. Density currents move down-slope due their density being heavier than that of the ambient water. In this work, entrainment in 3-D density currents in a straight channel was investigated by a set of experimental studies and Theoretical calculations. Velocity components were measured using Acoustic Doppler Velocimetry (ADV). First of all, the bed shear stress was calculated by the velocity profile method and Reynolds stress method as well and showed a good agreement. By increasing the Richardson number, the bed shear stress decreases. On the other hand, the interface shear stress was calculated by turbulence stress method. By increasing the Richardson number the interface shear stress decreases. Also the relation between the bed and interface shear stresses is investigated. In this work, the momentum equation is used to calculate the entrainment coefficient theoretically. A new formula is derived for entrainment coefficient by considering the interface shear stress and the momentum of entrained water. It can be observed that by increasing the Richardson number the entrainment coefficient decreases. But there are some ambiguous points in experimental results. First of all, there is no specific Richardson number that determines the location of hydraulics jump exactly. On the other hand, in the chart of entrainment versus Richardson number, scattering and dispersion of the data are large. This scattering is obvious in other researches. A modified Richardson number is defined in this research to solve these ambiguous points
  9. Keywords:
  10. Density Current ; Shear Stress ; Entrainment Coefficient ; Richardson Number ; Interface Shear Stress

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