Aeroelasticity and Structural Dynamics
Fixed-Wing Aircrafts and Helicopters: Internal Vibroacoustics in
the LF, MF and HF Range
Internal Noise Prediction of Internal Noise in
Fixed-Wing Aircraft Fuselages
The DADS department at ONERA is involved in the development of numerical tools for the prediction of internal noise in aerospace structures. They are based on the finite element method in the low (LF) and mid-frequency (MF) ranges on a one hand, and on energetic approaches in the high-frequency (HF) range on the other hand: Statistical Energy Analysis (SEA) or Vibrational Conductivity Analogy (VCA). In the LF and MF ranges, the finite element formulation for the structure and internal fluids is coupled to a boundary integral equation for the external fluid. A dedicated SEA software, called PEGASE, has been developed at ONERA for structural dynamics and internal/external structural acoustics applications in the HF range.
In collaboration with Airbus of France and Dassault-Aviation, measurements of internal noise in the MF and HF ranges were carried out on a naked cell of a Falcon plane and on the fuselage of an ATR-42 aircraft. Low and mid-frequency response simulations were performed by the finite element method for the Falcon cell, and high-frequency response simulations were performed by the SEA method for the Falcon cell and the ATR-42 fuselage. The Falcon cell was excited in the band [0, 1250 Hz] by four point forces applied to the engine fasteners (see Figure 1). Four configurations of the ATR-42 fuselage were tested in the range [0, 3000 Hz]. One of these configurations is the so-called "naked plane" (cabin without equipment, without commercial preparing, neither fitted carpet nor glass wool on the internal wall), and another configuration is the so-called "green plane" (for which glass wool is added on the internal wall of the fuselage).
Figure 1: mechanical excitations applied to the Falcon cell structure
Prediction in the MF Band by the Finite Element Method
The structural finite element mesh of the Falcon cell, mainly made up of beam, shell and bar elements, was provided by Dassault-Aviation; see Figure 2. It is adapted to the ADINA-ONERA software, which is used to perform all computations in the LF and MF ranges. The structural model is complemented by a finite element mesh of the acoustic cavity using (incompressible) fluid tetrahedrons. The overall mesh includes about 54,000 degrees of freedom.
Figure 2: finite-element half-mesh of the Falcon cell
Computations of accelerations of the structure and pressures in the fluid cavity were performed in the LF-MF band [20, 400 Hz]. The algorithm used for the analysis is a dedicated mid-frequency resolution process. This so-called MF-method, as implemented in ADINA-ONERA, does not proceed by modal projection and avoids solving a frequency-by-frequency linear equation to compute the frequency response functions. The average pressure computed in the cabin for one of the four mechanical excitations applied to the fasteners is compared with measurements done on the test cell on Figure 3. These numerical results agree well with measurements and validate the whole set of parameters chosen for the simulations.
Figure 3:
average pressure in the
passenger-cabin of the Falcon cell
Prediction in the HF Range by the SEA Method
SEA computations were carried out using the PEGASE software. The SEA model of the Falcon cell is a simplified model which includes seven mechanical subsystems and two acoustic subsystems (the passengers cabin - CAV1, and the compartment - CAV2, see Figure 4). The main feature of this model is that the SEA parameters of the mechanical subsystems, namely the average modal density, the radiation loss factor, and the coupling loss factors, were identified through numerical simulations performed using the ADINA-ONERA MF-method mentioned above. Loss factors of the acoustic cavities were identified by experiments. Comparisons between SEA computations and measurements of the different noise levels recorded in both cavities are displayed on Figure 5. A 10dB noise level decrease between the cavities is apparent. It is due to the fact that part of the vibratory energy is absorbed by the partition of pressurization located between both cavities, and is not transmitted to the passengers cabin. This phenomenon is correctly captured by the SEA model. All SEA predictions are significant in the HF range, at least above 300 Hz.
Figure 4: the Falcon cell divided into SEA subsystems
Figure 5: internal noise in the cavities of the Falcon cell
The SEA model of the ATR-42 fuselage is a simplified model which contains four main subsystems, including the passengers cabin. All SEA parameters as well as the coupling loss factors were estimated from direct measurements. Experiments done at Airbus France in the range [0, 3000 Hz] were used to estimate the internal loss factor of the passengers cabin, the loss factor of the fuselage, and the coupling loss factors between them, on the one hand, and between the fuselage and the foil, on the other hand. A validation of the model is done for mechanical excitations (vibrating pots on the fuselage and then on the aerofoil at the level of the fasteners) and for acoustic excitations (internally in the cabin, and then externally in order to simulate the acoustic loading coming from the propellers). A comparison between measurements and SEA internal noise predictions in the passengers cabin for a mechanical excitation on the fuselage is presented on Figure 6. Black lines correspond to the "naked plane" and the green ones to the "green plane". A significant reduction of internal noise in the cabin is achieved by the simple addition of a glass wool layer on the fuselage.
Figure 6:
average acoustic pressure in the ATR42 passengers cabin
