Aerodynamic noise in aero-engine LP turbines - Application of CFD techniques

Along with studies on the generation mechanism of turbine noise, there have been efforts to alleviate the annoyance of turbine noise for decades. When one adopts a noise control approach, not only the noise reduction effects but also the penalties due to these controls such as weight increase, complexity of the structure, manufacturing and maintenance costs, performance deterioration, etc., should be considered.
The sources control, the acoustic treatment, and the combination of both are employed in practice. The sources control here is to alter the noise generation mechanism primarily by improving the design of the component. It involves a turbine design that weakens or delays the aerodynamic interaction.
One of the traditional design guidelines is to make the rotor-stator axial distance large enough to decay the wakes and to fully mix them before arriving at neighboring stator vanes or rotor blades. The larger distances between the rotor (stator) and the downstream stator (rotor) are also effective in reducing potential interaction. However, if the separation between the rotor and the stator is too large, the benefits is lost on the aerodynamic point of view, so that the best solution often corresponds to a choice that balances these effects. Another possible way to reduce the sound is of course to reduce the rotational speed, since the intensity generally scales with a characteristic speed. This trend leads to larger blades with larger camber or angle of incidence, hence increased self-noise.

Another way is certainly to tune the number of stator vanes to the number of rotor blades. They are chosen so that lower circumferential modes are cut-off and so cannot propagate along the duct (see next chapter for more details on this matter). Otherwise, the number of blades may be chosen so that, for a given harmonic, mode number m is negative, that is counter rotating with respect to the rotor. In this case the rotor itself acts as a shield obstructing the spiralling modes to leave the duct. Anyway the aeroacoustic design, in particular the number of blades and vanes, cannot be separated from other considerations, like aero-elastic ones. Other techniques, which do not concern the generation, but the wave damping and energy absorption, consist in the use of acoustically absorbent linings. Noise absorbing linings material converts acoustic energy into heat, so that the noise is attenuated while it propagates through the duct. The absorbent linings usually consist of a porous skin supported by a honeycomb frame and are installed both upstream and downstream of the fan section.

The materials used to this purpose are lightweight composite material, suitable in the lower temperature regions and fibrous metallic materials that are used in the higher temperature regions. The disadvantage of liners is the increase in weight and skin friction, then in the fuel consumption. The typical methods for reducing turbine noise, so far described, are classified as passive control. Passive noise control, for instance the employment of an acoustic liner, makes use of the properties of shape, material and other physical phenomena. As seen from the widening the axial distance between blade vane rows or selecting blade and vane numbers, the designs for avoiding or delaying noise generation are also parts of passive noise control. Active noise control (ANC), whose idea comes from Leug’s patent in the 1930s, has long been studied, but is not in use. The recent developments in electronics and digital signal processing technology have pushed for this idea to be applied. The most remarkable feature of the active noise control is the use of additional sound input to primary noise. The additional sound causes acoustic interference between the original noise and the additional sound. This acoustic interference is the phenomenon where different sounds from different sources affect each other.