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Investigation of Turbulent Jet Noise with Instability Analysis Method

Arablu, Saeed | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48215 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Afshin, Hossein; Farhanieh, Bijan
  7. Abstract:
  8. Acoustic noise from turbulent jet flow discharged to the einvironment is called turbulent jet noise which is a problem for turbojets, turbofans, missiles, rockets and everywhere the discharge of a jet at high speed exists. In order to achieve the jet noise reduction methods, understanding this phenomenon with modeling on a detailed theory for the future developments in this field is required. In this study the noise generation from coherent large scale structures for a subsonic jet with acoustic Mach number of 0.5 is studied using linear stability analysis. To get the base flow used in instability analysis, the jet flow is modeled using different two equation k-ε models and after comparission with the experimental results, the standard k-ε model has been chosen due to better agreement. Using the base flow profiles obtained from this model, the stability equation governing the pressure disturbances is solved via shooting numerical method and the parameters of eigenvalues and eigenfunctions is presented. In the following the pressure fluctuations obtained from Direct Numerical Simulation (DNS) is compared to the results of linear stability analysis method and good agreement between these two methods in the linear hydrodynamic region is shown. Then the results obtained from linear stability for amplitude and phase of pressure fluctuations were compared with experimental results and the validity of the linear stability method is shown. In the following the effects of turbulence intensity and boundary layer thickness at nozzle exit on the results of stability is discussed. The results show that with increasing the turbulence intensity at nozzle exit, the frequency range that the values of spatial growth rates for them is positive gets smaller and except for very low frequencies, values of growth rates for axisymmetric and first helical modes are decreased. Increasing the boundary layer thichness at nozzle exit has led to decrease of spatial growth rate values of disturbances for both axisymmetric and first helical modes in the studied frequency range
  9. Keywords:
  10. Linear Stability ; Turbulence Intensity ; Boundary Layer ; Turbulent Jet Noise ; Coherent Large Scale Structures ; Boundary Layer Thickness ; Nozzle Exit

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