Current advancements of numerical methods and experimental means for the integration of future propulsion systems.

To integrate advanced propulsion systems and to assess and verify the related benefit (e.g. fuel burn, noise) suitable design, evaluation and measurement tools are required. For that reason, the so-called Cross-Capability-Demonstrator (XDC) has been set up as one major activity of the Large Passenger Aircraft (LPA) Platform 1 of the Clean Sky 2 initiative. The XDC is intended to demonstrate high-fidelity CFD-tools, further developed prediction tools for noise and aero-elastics as well as advanced testing tools for measuring e.g. the flow field, the deformation and the acoustics. The article will provide an update on activities within the XDC and presents some examples of recent accomplishments related to this demonstrator

A Method for local Damping Identification

The increase of computer power enables the analysis of detailed effects using refined numerical models based on the Finite Element Method. But even though refined models for stiffness and inertia properties can be obtained, damping modelling is mostly disregarded or included on a modal basis which does not reflect the governing mechanisms for damping or their location in a structure. For accurate response prediction of lightweight structures, especially when they are exposed to broadband excitation, proper damping modelling is required. More accurate response predictions will improve the accuracy of dynamic loads analysis which is the baseline for detailed structural design and sizing of structural components. The work presented here is intended to support damping modelling as it is required for accurate response analysis or dynamic loads analysis. It addresses the identification of local damping sources and their relative contribution to the global damping of an assembled structure. As input, measured dynamic responses are required, e.g. in the form of frequency response functions. As output, a map of damping distribution over the structure is obtained. These results uncover areas of interest at which damping could be introduced into the structural model, for instance in terms of local physical damper elements. The approach for local damping identification is inspired by methods used in acoustics which analyze the reverberation time in an acoustic cavity. The method presented here uses a hybrid time-frequency domain processing for the estimation of structural damping and spatial distribution of damping. The new analysis strategy allows for estimation of damping over a wide frequency range. At low frequencies, where the modal density is low, the identified damping matches the values obtained with the classical modal analysis tools. At high frequencies, where modal analysis is no longer possible due to high modal density, the presented method is able to estimate equivalent damping values for pre-defined frequency bands. Thus, a variation in damping with frequency can be provided based on experimental observations even in the higher frequency range typically not covered by experimental modal analysis. It is also possible to use these damping ratios for improved accuracy of numerical simulations compared to other approaches such as structural damping or Rayleigh damping. Numerical examples are presented in order to compare and validate the results obtained. Finally, experimental data from a fuselage-like structure is presented

Experimental characterization of vibro-acoustic properties of an aircraft fuselage

Modern turbo-prop engines have been identified as an efficient alternative to jet engines for the propulsion of next generation short range aircraft. However, the benefit in operating efficiency comes along with disadvantages related to engine noise and noise transmission into the aircraft cabin. Numerical models used for response analysis often show significant deviations in the medium frequency range up to 500 Hz. Model validation based on experimental modal data is often not possible due to the high modal density that aircraft fuselage structures exhibit in this frequency range. A method is presented that provides a meaningful correlation of the results of vibration tests on aircraft fuselage structures with corresponding numerical predictions. The correlation criterion used is inspired by statistical energy analysis and is based on kinetic energies integrated over frequency bands and spatially integrated over surface areas of the fuselage structure. The objective is to indicate frequency bands where the finite element model needs to be adjusted to better match with experimental observations up to 500 Hz and to locate the areas where these adjustments should be applied. The application of the kinetic energy correlation criterion comes along with several requirements on the test equipment, test installation and testing procedures. An effort has been spent to extent well defined testing procedures stemming from aircraft ground vibration testing into the medium frequency range. As a result, a test method is presented that has been applied in a test campaign on a full-scale aircraft fuselage structure to validate the proposed correlation criterion based on kinetic energies

Classification of the mid-frequency range based on spatial Fourier decomposition of operational deflection shapes

Aircraft structures are characterized by their lightweight design. As such, they are prone to vi- brations. On numerical side there are several tools suitable for the response analysis of dynamic structures. Each numerical tool is more adequate for different frequency ranges. The Finite El- ement Method, for example, is the state-of-the-art numerical tool for the low-frequency range where modal density is still low and methods based on the modal approach are still appropriate. For the high-frequency range, the Statistical Energy Analysis is more adequate. This method anal- yses the energy transmission between substructures and requires high modal density of the sub- structures. The intermediate section between low- and high-frequency is called the mid-frequency range and is characterized by strong interaction of vibrations and acoustics. The mid-frequency range typically suffers from a lack of tools available. Improvement of tools for the mid-frequency range is highly important e.g. for predicting the acoustic comfort inside an aircraft cabin, where interaction between vibrations and acoustics can be observed. However, as a prerequisite it is nec- essary to determine the beginning and the end of the mid-frequency range. This paper addresses a new method to be applied in acoustic comfort analysis of aircraft fuselage. It utilizes dynamic response measurements conducted on an aircraft fuselage, i.e. a stiffened cylindrical shell which exhibits global and local dynamic behaviour in the mid-frequency range. Existing criteria for the determination of the mid-frequency range are reviewed, but fail in this specific application. Therefore, a new method for the classification of the mid-frequency range for a stiffened cylindri- cal shell test-structure is proposed in this paper. It is based on wavenumber spectra obtained from spatial Fourier decomposition applied to operational deflection shapes extracted from experimen- tal dynamic responses. Analysing the response characteristics of the different components of the aircraft fuselage a classification of frequency ranges is possible