The key element of the MoSy-testrig is a duct segment with a loudspeaker array. The MoSy-testrig is reffered as Mode-Synthesizer or Mode-Generator including the required hardware and software for the operating of this segment. The Mode-Generator is used mostly to excite and simulate the tonal and broadband noise, which commonly occur in ducted systems connected to turbomachinery components. The excited sound field can be used to generally investigate the sound propagation within and through the ducted piping systems. Further, such excited sound fields can be used as test scenarios for newly developed analysis methods or noise reduction technologies. Any required adaption of the Mode-Synthesizer is easily possible, due to its modular design
The loudspeaker array consists of three rings, in each ring up to 16 speakers can be placed with equidistant spacing. In contrast to turbomachinery components, which excite sound fields with specific mode content depending on the given blade/vane numbers and the flow field characteristics, the design of the Mode-Generator allows the selective excitation of tonal and broadband noise components. By controlling the amplitude and phase relations of the individual speakers the spatial structure, i.e. the azimuthal and radial mode order of modes, can be adjusted explicitly. In the case of broadband noise generation further the coherences of the acoustic modes can be influenced by appropriate processing of the loudspeaker source signals. Thus the acoustic transfer functions of flow duct elements can be analyzed in a comprehensive manner.
A DSP (digital signal processing) system with up to 32 D/A channels is available to drive the loudspeakers seperatively. Beside the onboard signal generation within the DSP environment, it is possible to feed external signals into the DSP system. Such a signal processing software tool is available and is used in experiments regarding the active noise control of the rotor-stator interaction noise.
Fields of Application
Most of the investigations at the MoSy-testrig are conducted to study the sound field propagation in ducted systems as they appear typically with turbomachinery components as well as in aeroengines. As an example, the transmission through blade rows can be studied. Further, installation effects are investigated caused by a general change of the shape or the cross sectional area of the ducted
system due to installed bifurcations or pylons. By using the Mode-Generator with excited test sound fields, an evaluation of damping modules as for an example acoustic liners can be achieved. All the experimental data can also be used for a validation of numerical methods which calculate the in-duct sound propagation.
Another field of application for the MoSy-testrig is the assessment of newly developed analysis methods. An evaluation of their accuracy can be achieved and compared to simulated results, due to the well known excited sound fields as a reference. The MoSy-testrig is also used for active noise control experiments, where the secondary sound field is excited with the Mode-Generator and superimposed with the primary field in the duct. Therefore, all the components in the testrig as the microphone array, DSP-system and the Mode-Generator are coupled together within a control loop.
Modular Testrig Design
The MoSy-Testrig consists of various modular segments. Its configuration is highly flexible and adaptable for an individual objective of each study. Especially the positions of the various modules can be interchanged in any way. The installation of a hub, bifurcations or pylons in the duct is feasible to reproduce geometrical features comparable to realistic conditions in turbomachinery components.
The in-duct sound field can be measured by using various microphone array modules. Modules are available with up to 100 sensor positions distributed at different azimuthal, radial and axial positions. For the analysis of very high frequencies, some sensor arrays can be traversed in the circumferential direction to realize a fine measurement grid. By using anechoic termination modules in the testrig, the reflections in the MoSy-testrig can be suppressed as defined in the Norm ISO 5136.
The recorded sensor data is used with various mode analysis methods to investigate both the noise generation as well as the propagation processes in detail. By using multiple microphone arrays with an enhanced analysis method based on correlations, a transmission and a reflection of the acoustic modes at the individual turbomachinery components can be determined. To determine the influence of the flow properties on the acoustics, various axial fan-stages are available, which can be integrated additionally in the MoSy-testrig.