Radial Mode Analysis – Institute of Turbomachinery and Fluid Dynamics

Radial Mode Analysis (RMA)

Sound fields in closed pipelines and in turbomachinery result from the superposition of complex modal structures. These propagate in both radial and circumferential directions. Knowledge of the individual components of the sound field is crucial for understanding its propagation characteristics. With the help of the so-called Radial Mode Analysis (RMA), a decomposition of the total sound field can be achieved so that the individual modal components can be determined.

Field of Application

Determination of acoustic mode distributions in turbomachinery and pipe systems
Applicable in non-flow geometries as well as in ducts with uniform or swirling flow
Analysis of noise reduction systems (silencers)

Description

For the RMA, the sound pressures are first recorded at various circumferential and axial positions of the turbomachine using microphones inserted flush with the wall in the flow channel. Using special evaluation algorithms, the transient pressure signals can be used to infer the modal structure of the sound field. The essential components of these algorithms are a spatial Fourier analysis (FFT), which uses the circumferential pressure distribution to determine the azimuthal orders of the sound pressure modes. By solving a linear system of equations, the order of the sound pressure modes in the radial direction is then calculated. The highest order of the mode that can be detected using the RMA is determined by the number of microphones in the circumferential direction and by the axial measurement positions.