Jet Noise Analysis of a Mixed Turbofan Engine

The demand for more efficient and quieter propulsion systems in civil aviation led to the development of mixed turbofans in the early 60s already. Since that time lobed mixers are widely used to improve the mixing process of hot core and cold bypass flows before they are expanded through a common nozzle.Mixing reduces both the maximum temperature and the maximum velocity in the jet of a nozzle. According to Lighthill this in turn decreases the emitted jet noise. As shown analytically by Tam et al. high density gradients amplify Kelvin-Helmholtz instabilities in shear flow which has a strong influence on pressure fluctuations known to be the sources of broadband noise in turbulent jets.To study these effects the stress blended eddy simulation (SBES) model by Menter in combination with a coupled pressure-based flow solver is used to simulate the time resolved turbulent jet of a mixed turbofan. Afterwards the Ffowcs-Williams Hawkings analogy is utilized to determine the noise level at a distant point from the jet. All CFD-simulations are carried out with the commercial CFD software ANSYS Fluent.