Experimental determination of transfer functions for a coated, ring stiffened cylinder as a function of hydrostatic pressure

Abstract

Coating effectiveness, as measured by the change in a ring stiffened cylinder's transfer function, is experimentally determined in the acoustic far field for increasing hydrostatic pressure. Polymer coating response characteristics are a function of temperature, frequency, molecular structure, chemical crosslinking systems and fillers. A sample coating, of unknown composition, is provided for analysis to gage performance as a function of frequency and filler (indirectly through hydrostatic pressure). Limited instrumentation assets (16 channels) required a pre-lake structural test program. Results of this test program determined the shell placement of accelerometers channels and provided an evaluation of bulkhead and shell radiated power levels. Results were highly subjective to boundary conditions and close proximity of nodal/resonant points thereby making 1/6th octave sampling unsuitable for the lake test program. Deep water testing was conducted at the Acoustic Research Detachment, Pend Oreille, Idaho between 3 to 12 March 1997. The far field power levels were measured using an equally spaced 5 element line array placed 25 feet from the cylinder's beam. Measurements were made for 3 operating depths; 50 ft, 200 ft and 450 ft. Twenty one frequency bins using an LFM sweep over a pass band of 400 Hz to 10.5 kHz were digitized then analyzed using MATLAB. Results show a 7 to 8 dB reduction at 50 feet; 3-5 dB at 200 feet and 3 dB for the maximum 450 foot depth. For each depth, the reduction in hull excitation was greater than far field leading to an increase in transfer function even though far field is reduced. Decay of excitation with distance from forcing points is significant for bare hull and much more rapid for a coated hull