Another uncertainty in computing turbulent flows is the unknown value of turbulence quantity at the boundaries. However, they are needed for simulations as inputs. In this section effect of the turbulence intensity was compared, again SST k-ω turbulence model was used in the computations. The turbulence intensity both in the fan and core streams were doubled and decreased to half. In other words, 3%, 6% and 12% turbulence intensity values were used as parameters.
As shown in the Figure 32, different turbulent intensities do not affect the axial velocity distribution; however, there is a slight influence is observed at downstream locations as can be seen in the Figure 33. Similar trend occurs for the turbulent kinetic energy distribution as shown in the Figure 34 and Figure 35, below. According to the results, turbulent flow field is slightly influenced only at downstream regions of x=25 m and further downstream locations.
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Axial Velocity at x=1 m Axial Velocity at x=2 m
Axial Velocity at x=4 m Axial Velocity at x=8 m Figure 3.32 Axial velocity distribution in radial direction at x=1, 2, 4, 8 m for different turbulent intensities.
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Axial velocity at x=25 m Axial velocity at x=32 m Figure 3.33 Axial velocity distribution in radial direction at x=25, 32 m for different turbulent intensities.
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Turbulent Kinetic Energy at x=1 m Turbulent Kinetic Energy at x=2 m
Turbulent Kinetic Energy at x=4 m Turbulent Kinetic Energy at x=8 m Figure 3.34 Turbulent kinetic energy distribution in radial direction at x=1, 2, 4, 8 m for different turbulent intensities.
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Turbulent Kinetic Energy at x=25 m Turbulent Kinetic Energy at x=32 m Figure 3.35. Turbulent kinetic energy distribution in axial direction at x=25, 32 m for different turbulent intensities.
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CONCLUSION
In this study, jet flow emanating from a turbofan engine exhaust was computed by using Reynolds-averaged Navier-Stokes method with different turbulence models of FLUENT solver. Turbulence modeling issues and a parametric study were considered.
First, a 2D ejector problem was solved to find out the most appropriate turbulence model. The results of 2D ejector problem showed that turbulence model plays an important role to define the real physics of the problem. All four turbulence models; Spalart-Allmaras, realizable k-ε, k-ω and SST k-ω used here lack to predict accurately especially the initial jet growth region. The models should be carefully evaluated and necessary modifications should be done. For example, calibrating the constants used in the models can be a choice to have better results.
For the parametric study, SST k-ω turbulence model was used by taking the turbulence model sensitivity studies for both ejector and turbofan engine exhaust analyses results into account. It is seen that boundary layer thickness effect becomes important in the jet flow close to the lips of the nozzles. At far downstream regions, it does not affect the flow field. For different turbulent intensities, no significant change occurred in both mean
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and turbulent flow fields. However, higher intensities might be possible for real engine conditions which might impact the flow characteristics.
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