**7. Nomenclature**

#### **Abbreviations**

flow path. Mode *m*= −20 is generated by the scattering of the HP‐stage interaction at the TMTF. **Figure 17** shows that the sound power levels of mode *m*= −12 decrease from clocking position 1 to 4 and increase again from clocking position 4 to 6. The minimum sound power level can be seen at clocking position 4. The mode *m*= −12 is either generated by the HP stator‐HP rotor interaction or by the HP rotor‐TMTF interaction. In case of mode *m*= −12 the strongest influence of the different clocking positions can be determined. This particular mode is reduced by 4 dB when changing the relative vane‐vane position from CP1 to CP4. For the modes *m*= + 12 and *m*= + 20 a similar trend to mode *m*= −12 can be observed. These modes also show significant changes of the sound power levels of up to 3 dB due to different relative positions of the HP vanes and the TMTF struts. Both modes reach their minimum sound power level at clocking position 5, whereas the level of the amplitudes is decreasing from 1 to 4. The mode *m*= + 12 is generated by the interaction of the HP vanes and the HP blades but also by scattering of the HP stage interaction modes at the TMTF. The mode *m*= + 20 is always generated in conjunction with the TMTF‐vanes, either with the HP rotor or with the HP stage. While the amplitude of mode *m*= −4 seems to be almost constant for all clocking positions, mode *m*= + 4 changes from CP2 to CP6 by 4 dB, whereas at the last clocking position the amplitude of m=+4 has its lowest value. Both modes are the result of the interaction of the HP‐stage and the TMTF. Summing up the sound power levels (depicted in **Figure 17**) reveals that there is a minimum sound power at clocking position CP4. A difference in sound power level of app. 2 dB between the acousti‐

Three different turbine exit casings with different turbine exit guide vane (TEGV) designs have been compared to a state‐of‐the‐art (reference) TEGV design. The possible reduction of sound power levels when applying the different designs and a rough estimation of the aerodynamic losses have been presented. When comparing the overall reduction of PWL (only considering the main airfoil interaction modes) it was revealed that the acoustically optimised inverse cutoff TEC has the largest reduction of sound power level of 14 dB. The aerodynamically optimised H‐TEC even increases the overall sound power level by about 2 dB. The leaned TEC also decreases the PWL by about 11 dB, but is still as twice as loud as the inverse cutoff TEC. However, for the operating point approach the aerodynamic losses are increased for all TEC designs. The losses measured at the aero design point are lower for the aerodynamically optimised H‐TEC and the inverse cutoff TEC than for the reference TEC. Both, the H‐TEC and the inverse cutoff TEC provide a much more uniform yaw angle distribution downstream of the trailing edge. These results give confidence that it is possible to design an aerodynamically and acoustically optimised TEC. Further, the effect of a change in stage design onto a TEGV with a compound lean was investigated and presented in this chapter. It was shown that while keeping the shaft power constant, the noise emissions downstream of the TEGV can be reduced by 0.7 dB for the given geometry and operating point. This decrease in sound power level is not caused by a decrease in interactions, but rather by acoustic modes not attributable to these interactions. The overall sound power level of the modes is increased by 0.3 dB. When having

cally best (CP4) and worst (CP2) clocking position is observed.

**6. Conclusions**

28 Recent Progress in Some Aircraft Technologies




C1, C2, C3 Configuration 1, 2, 3 CP1,…,CP6 Clocking position 1 to 6 EGV Exit guide vanes HP High pressure

30 Recent Progress in Some Aircraft Technologies

HPT High pressure turbine IGV Inlet guide vanes LP Low pressure

LPT Low pressure turbine OP1, OP2 Operating point 1, 2 PWL Sound power level RMA Radial mode analysis SPL Sound pressure level

TCF Turbine centre frame TEC Turbine exit casing TEGV Turbine exit guide vanes TMTF Turning mid turbine frame TTTF Transonic test turbine facility T&S Tyler and Sofrin modes

A Complex amplitude B Number of blades c Velocity, speed of sound F Normalisation factor f Modal shape factor h Channel height

i Imaginary unit

k Wave number k Integer number Ma Mach number

P Sound power

h Harmonic index, integer number

J Bessel function of first kind

m Azimuthal mode order

**Symbols**

STTF‐AAAI Subsonic test turbine facility for aerodynamic, acoustic, and aeroelastic investigations

⬚′ Stochastic fluctuations


‐ Propagation against flow direction
