diplom_TNR14_final (Tupolev 154M noise asesment (Анализ шумовых характеристик самолёта Ту-154М)), страница 4

Figure 7.1 Schematic illustration of noise sources from turbofan engines

Figure 7.2. shows a typical farfield SPL noise spectrum generated by a turbofan engine at a landing-approach power setting. Below 800 Hz, the spectrum is controlled by noise from the primary jet exhaust. The spectrum between 800 and 10000 Hz contains several discrete frequency components in particular that need to be attenuated by the linings in the inlet and the fan duct before they are radiated to the farfield.

Figure 7.2 Engine-noise spectrum

The objective in applying acoustic treatment is to reduce the SPL at the characteristic discrete frequencies associated with the fan blade passage frequency and its associated harmonics. Noise reductions at these frequencies would alleviate the undesirable fan whine and would reduce the perceived noise levels.

A promising approach to the problem has been the development of a tuned-absorber noise-suppression system that can be incorporated into the inlet and exhaust ducts of turbofan engines. An acoustical system of this type requires that the internal aerodynamic surfaces of the ducts be replaced by sheets of porous materials, which are backed by acoustical cavities. Simply, these systems function as a series of dead-end labyrinths, which are designed to trap sound waves of a specific wavelength. The frequencies for which these absorbers are tuned is a function of the porosity of flow resistance of the porous facing sheets and of the depth or volume of the acoustical cavities. The cavity is divided into compartments by means of an open cellular structure, such as honeycomb cells, to provide an essentially locally reacting impedance (Fig. 7.3). This is done to provide an acoustic impedance almost independent of the angle of incidence of the sound waves impinging on the lining.

The perforated-plate-and-honeycomb combination is similar to an array of Helmholtz resonators; the pressure in the cavity acts as a spring upon which the flow through the orifice oscillates in response to pressure fluctuations outside the orifice.

Figure 7.2 Schematic of acoustic damping cavities in an angine duct. The size of the resonators

is exaggerated relative to the duct diameter.

The attenuation spectrum of this lining is that of a sharply tuned resonator effective over a narrow frequency range when used in an environment with low airflow velocity or low SPL. This concept, however, can also provide a broader bandwidth of attenuation in a very high noise-level environment where the particle velocity through the perforations is high, or by the addition of a fine wire screen that provides the acoustic resistance needed to dissipate acoustic energy in low particle-velocity or sound-pressure environments. The addition of the wire screen does, however, complicate manufacture and adds weight to such an extent that other concepts are usually more attractive.

Figure 7.3 Acoustical lining structure.

Although the resistive-resonator lining is a frequency-tuned device absorbing sound in a selected frequency range, a suitable combination of material characteristics and lining geometry will yield substantial attenuation over a frequency range wide enough to encompass the discrete components and the major harmonics of most fan noise.

7.1.2 Duct Lining Calculation

First we have to determine the blade passage frequency:

,

where z is number of blades, n is RPM.

Blade passage frequencies for different engine modes are given in table 7.1

Next we determine the second fan blade passage harmonic frequency, which is two times greater than the first one: .

Table 7.1 Fan blade passage frequencies for different engine modes.

Using experimental data, we determine lining and cell geometry:

For the first harmonic, parameters will be:

• Distance between linings 28.5 cm;

• Lining length 45 cm;

• Lining depth 2.5 cm;

• Cell length 2 cm..

For the second harmonic, parameters will be the following:

• Distance between linings 4.5 cm;

• Lining length 5 cm;

• Lining depth 2.5 cm;

• Cell length 0.4 cm.

Figure 7.4 shows the placement of the lining in engine nacelle.

Figure 7.4 Lining placement in the nacelle.