B1.9 - Validation of turbulent boundary layer-induced sound transmission through a fuselage section

This project addresses the validation of broadband noise transmission through shell structures. Specifically, we determine the sound transmission through a generic fuselage section under stochastic loading within a wind tunnel experiment and by finite element simulations. A scaled full fuselage (A320 type, 4:1) including the cockpit section is considered to generate a realistically evolved TBL in the experiment. Under flow, the vibrations of representative outer skin panels within the wind tunnel model are measured contact-less in order to yield a validation basis for the simulations. Extensive numerical studies of the representative panel are conducted for the study of a mid-fidelity sound field generation beneath the TBL to be used in finite element models. By testing different modelling approaches such as a superposition of plane waves and a variation of modelling parameters, a validated model for a realistic and stochastic excitation of airframes without high-fidelity aeroacoustic computations is aimed for. The validation is based on the shell vibrations instead of measures in the flow, which states an important step towards a validation of cabin noise simulations of full aircraft under complex and realistic loadings and clearly avoids any disturbance of the flow itself.

Objectives

Investigation steps in the project — Three consecutive phases of the project plan

A major challenge within the modelling process is the highly complex and stochastic characteristics of sound sources beneath the TBL. Hence, a study of a representative airframe structures under well-defined conditions is conducted for in order to deliver recommendations for a mid-fidelity modelling in early design phases as high-fidelity simulations are hardly possible for an entire airframe. As in-flight measurements are not suitable for a validation due to non-ideal conditions and manufacturing uncertainties, a scaling must be applied for wind tunnel measurements. This scaling must follow the objective of representability in order to obtain transferable validation results. For the experimental setup, the characteristics of the acoustic coupling is aimed to be similar to cruise flight conditions. Based on the experimental data, a stochastic assessment and validation of models for the generation of sound fields beneath the TBL is the final objective.

In order to pursue the objectives explained above, which are mainly represented by the validation of TBL induced noise transmission, experimental setups and numerical studies of fuselage shells are combined. A wind tunnel experiment of a scaled full fuselage is planned to assess mid-fidelity excitation models beneath the TBL evolved in the cockpit section of the test specimen. As validation strategy, the applicants propose an indirect validation by the dynamic response of the deterministically well-known fuselage shell under stochastic flow excitation. As shown in the figure, the working plan consists of three essential and consecutive phases.

The preliminary studies aim to find a suitable test setup and yield a representative and validated test panel for the wind tunnel experiment. Representability stands for valid transferability to size-one aircraft under extrapolated cruise flight conditions, for which the scaling must fit the change of the flow conditions.
The wind tunnel experiment is conducted in the low-speed wind tunnel (NWB) at DLR site in Braunschweig. The consideration of a fuselage cutout for changeable outer skin panels allows for the study of different setting such as stiffened or damped panels.
Based on the experimental data in the wind tunnel experiment, different models for the TBL excitation are studied in the last Analysis and Validation step. Several approaches are tested, varied in its parameters and assessed. Finally, we deliver a validated model for a wave-resolving TBL excitation of the curved and stiffened shell and its transferability to cruise flight conditions.