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    American Institute of Aeronautics and Astronautics

    Optimization of structural countermeasures for noise attenuation in aircraft cabins

    Alejandro Bonillo Coll1

    Universitat Politcnica de Catalunya (UPC-BarcelonaTech), 08034 Barcelona, Spain

    A simulation methodology is proposed for the optimization of structural

    countermeasures to be integrated in the airframe of typical turbopropeller aircraft

    with the objective of providing significant cabin noise attenuation levels in the low-

    and mid-frequency range. A number of available countermeasures is considered,

    ranging from local structural modifications, i.e. local stiffening and punctual mass

    addition, to single- and multiple-degree-of-freedom passive vibration control

    devices such as dynamic vibration absorbers (DVAs). The optimization

    methodology benefits from separate modeling of the primary structure, i.e. the

    airframe, and structural countermeasures, thus allowing for the implementation of

    mathematical optimization algorithms which yield optimum countermeasure

    configurations at low computational cost.

    Nomenclature

    = speed of bending waves = bending stiffness = elastic modulus = frequency of oscillation = area moment of inertia = length = mass = viscous damping ratio = wavelength = surface density = angular frequency of oscillation = natural angular frequency

    I. Introduction

    IRCRAFT cabin noise or interior noise has been an active research field in structural dynamics and

    acoustics for the past 60 years1. The aircraft interior noise problem is generally known as the

    transmission of noise from aircraft sources, i.e. propulsion systems and turbulent boundary layers, into the

    cabin through both airborne and structure-borne transmission paths.

    The characterization of airborne noise sources varies notably with the type of aircraft which is subject

    to study as well as with the operating regime of its power plant. Thus, multi-engine propeller-driven

    aircraft, with maximum cruise speeds ranging from less than 240 km/h to about 450 km/h, present typical

    tonal excitations related to the blade passing frequency (BPF)2,3

    in the frequency range between 100 Hz

    and 250 Hz. The advent of the advanced turboprop, e.g. Airbus Military A400M, and open-rotor concepts

    has added extra complexity to this problem due to the effects of transonic and supersonic helical tip

    speeds4 and rotor-wake/rotor interaction effects in counter-rotating open rotor (CROR) systems

    5. Turbojet

    and turbofan aircraft, typically with cased and wing-mounted engines, are found to be prone to jet noise,

    which constitutes the dominant noise source under low-speed conditions such as those encountered in the

    climb stage of the flight profile. Interior noise for turbojet and turbofan aircraft under cruise conditions is

    generally dominated by boundary layer excitation of fuselage panels, although jet noise might also have

    an influence as the distance between engines and fuselage decreases6. Fan noise is another typical

    airborne noise source found in turbofan aircraft which presents significant contribution to interior noise

    only at low flight speeds, e.g. during takeoff and initial climb7, and is related to a tonal excitation at the

    BPF of the fan.

    1 PhD student, Department of Mechanical Engineering, ETSEIAT (UPC-BarcelonaTech), 08222

    Terrassa, Barcelona, Spain. AIAA Young Professional Member.

    A

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    American Institute of Aeronautics and Astronautics

    Structure-borne noise is principally associated to engine vibration and propeller wake-wing

    interaction. Engine-based structure-borne noise components are related to the BPF of rotational

    machinery elements, with fan contribution usually being dominant. Therefore, in the case of propeller-

    driven aircraft, structure-borne noise might be superposed to airborne propeller noise sources at discrete

    tonal frequencies. Results obtained for typical turboprop aircraft have shown that structure-borne

    components usually fall between 10 dB and 20 dB below airborne components in terms of interior noise,

    thus leaving structure-borne noise sources at a secondary role8-12

    .

    Figure 1 provides a schematic representation of aircraft interior noise sources which are typically

    found in turboprop and turbojet/turbofan aircraft.

    The present study focuses on the interior noise problem identified in turbopropeller aircraft related to

    BPF in the low-frequency range, i.e. up to 220 Hz. Cabin noise prediction and attenuation in

    turbopropeller aircraft has been addressed in previous research and development work in collaboration

    with EADS-CASA and Airbus Military14,15

    , which has arisen the need of taking a step further in

    simulation methodologies for higher attenuation of

    predicted and validated cabin noise levels.

    A simulation methodology is presented for the

    optimization of structural countermeasures

    implemented in aircraft fuselage sections with the

    objective of providing outstanding attenuation of

    interior noise levels predicted for turbopropeller

    aircraft. The range of structural countermeasures

    considered for application covers local modifications

    in terms of stiffness and mass, as well as the

    implementation of passive vibration control devices

    such as tuned and detuned DVAs. Such proposed

    countermeasures are selected as they are potentially

    applicable to the development of a vibro-acoustic

    solution kit which can be introduced in late stages of

    aircraft design or even during its service life, thus

    affecting at a much reduced scale the conventional

    design process.

    The optimization methodology is applied to a

    generic fuselage section of a typical turbopropeller

    aircraft which defines an acoustic cavity referred as

    the generic aircraft cabin. The fuselage section

    contains all primary structural elements, e.g. skin, frames, stringers, etc., and is henceforth referred as the

    primary structure. At a secondary level, all structural countermeasures, e.g. stiffeners, local masses,

    DVAs, etc., are referred as substructure or secondary structure. The optimization methodology is based

    on separate modeling of the primary structure and the substructure. Firstly, the primary structure is

    subject to conventional finite element modeling and vibro-acoustic simulation using currently available

    commercial software packages such as MSC NASTRAN and LMS Virtual.Lab. The effects of

    countermeasure elements are then applied to the primary structure using a method which computes

    equivalent dynamic forces16

    combined with frequency response functions (FRF) of the primary structure.

    The use of structural coupling techniques allows for vibro-acoustic simulation of structural

    countermeasure configurations at reasonably low computational cost, i.e. some milliseconds, using an

    appropriate MATLAB routine, which does not require recalculation of the primary structure. Therefore,

    simulation of multiple configurations within a reasonable period of time becomes possible, thus allowing

    for efficient application of optimization algorithms. As a final result of the proposed optimization

    methodology, an optimum configuration of structural countermeasures, which provides highest

    achievable noise reduction levels, is obtained based on any number of initial considerations and

    constrains related to the integration of such countermeasures in the airframe.

    In section II the primary structure is defined in terms of geometry and mechanical properties. The

    finite element model and the conventional methodology used for vibro-acoustic simulation are also

    presented together with baseline results in terms of cabin noise. Section III deals with the description and

    characterization of all structural countermeasures which are taken into consideration within the scope of

    the present study. These are divided into two major groups: local structural modifications and passive

    vibration control devices. Section IV constitutes a detailed mathematical approach to the structural

    coupling technique used to integrate the structural countermeasures into the primary structure. Section V

    is devoted to the formulation of the optimization algorithm which is implemented in combination with the

    Figure 1. Sources and transmission paths

    of aircraft interior noise (picture taken

    from Ref. 13)

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    American Institute of Aeronautics and Astronautics

    mathematical model for structural coupling developed in section IV. Finally, all simulation results are

    presented and compared in section VI, and conclusions are summarized in section VII.

    II. Primary Structure

    A. Definition The primary structure used for the implementation of the proposed optimization methodology consists

    of a generic fuselage section of a typical turbopropeller aircraft which integrates all primary constitutive

    elements: skin, frames, stringers and floor panels. A schematic view of the proposed structure is presented

    in figure 2.

    With respect to materials definition for the fuselage structure, a generic 2024 aluminum-copper alloy

    was used for the skin, frames and stringers, whereas floor panels were defined as a composite structure

    constituted by an internal honeycomb lay