Acoustic Modal Analysis of cylindrical-type cavities

Abstract

The acoustic modes of closed cavities play an important role in the characterization of combustion chamber behavior. On automotive internal combustion engines, an undesired spontaneous ignition phenomenon, called knock, can occur and the detection of this phenomenon can be based on the acoustic modes of the combustion chamber. This is possible, once the high pressure levels inside the combustion chambers excite internal standing waves, causing the lubricant oil dilution and, hence, damaging the liners, pistons and rings. On the other side, in the space industry, combustion instabilities have become a serious problem in gas turbines and dynamics of chambers must be characterized, since the vortical motions can couple directly to an acoustic field, or may produce local acoustic sources if they impinge on a surface. This paper describes a methodology to determine the acoustical longitudinal natural frequencies and modes of a generic cylindrical cavity that is surrounded by a metallic surface. The Finite Element (FE) model was validated through Experimental Acoustic Modal Analysis (EAMA). The Frequency Response Functions (FRFs) were accessed using a volumetric acoustic source and a microphone. The results showed that the numerical natural frequency of each mode agreed with those measured in EAMA. The visual presentation of the experimental extracted modes seems to be poor, compared to the FE results, due to the difficulty of showing the mode shapes using a restricted number of measurement points. However, a reasonable mode characterization can be obtained with EAMA. In addition, a coupled Frequency Response Analysis (FRA) of the referred structural-acoustic system was calculated, in order to evaluate the vibro-acoustic coupling, considering a FE/FE model.

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