Modal decomposition method in rectangular ducts in a test-section of a cavitation tunnel with a simultaneous estimate of the effective wall impedance

Abstract

The operational requirements for naval and research vessels have seen an increasing demand for quieter ships either to comply with the ship operational requirements or to minimize the influence of shipping noise on marine life. The radiated noise of a ship is estimated during the design stage by measurements with a scale model, generally realized in a tunnel or a depressurized tank. DGA Hydrodynamics owns its cavitation tunnel with low background noise that allows such measurements. Nowadays, a low background noise of the whole facility is not sufficient to perform an accurate acoustic measurement. Improving our knowledge about the acoustic response of the facility is then required to assess the relationship between the measurements and the noise radiated by a propeller for example. The confined geometry of the test-section imposes particular boundary conditions to the acoustic propagation and a reverberation-like behaviour. The acoustic response of the facility is then disturbed compared to a free-field configuration. This particular acoustic behaviour makes the absolute estimate of the underwater radiated noise power very difficult. A possibility to overcome this problem could be the use of an appropriated model coupled with the modal propagation theory. Following this approach, a method to estimate both modal magnitudes and wall impedance is proposed in this paper and confronted to experimental results.