About acoustic field characteristics in the test section of a cavitation tunnel

Abstract

Noise measurements in a cavitation tunnel are the common procedure widely used by most of the institutes for predicting the underwater radiated noise (URN) of a boat propeller. The accuracy of such a measurement is strongly linked to the “acoustic response” of the facility. The knowledge of this acoustic behaviour is then one of the key point to perform an accurate measurement. Due to the geometrical shape of a test-section, the acoustic propagation is similar to propagation in duct. This propagation could be described at low frequencies by modal propagation. For higher frequencies, the total number of propagating modes increases and the acoustics inside the duct becomes statistical. Schroeder described the limit between these two domains (modal/statistical) in 1954 for closed space applications like room acoustics. The formula derived in 1954 was completed and improved few years after to fit with results obtained by experiments and Monte Carlo simulations. Following the same effort, the principle of this limiting frequency (called here and henceforth Schroeder frequency) is derived to comply with particularities of the acoustic propagation in duct and to the environment of a test-section of a cavitation tunnel. Because it exists cavitation tunnel with rectangular or circular test-sections, the adaptation of the Schroeder equation is performed for both configurations. The description of the statistical features of the acoustic field is also derived. Finally, experiments have been carried out in our cavitation tunnel to assess the accuracy of the equation and more particularly the threshold used to delimit the two domains.