Acoustics and hydrodynamics of circle and square moonpool - An experimental research

In the present study, hydrodynamic forces, resonance ranges due to flow induced oscillations and shear layer self sustaining oscillations for circular and square shaped moonpool; beneath a low speed, laminar boundary layer were studied through model test experiments in a towing tank. The research concentrated upon determining the effects upon resonance and comparison of hydrodynamic forces by rotating the leading edge of the cavity away from normal to the flow direction. Cavity resonance was identified through spectra of flow fluctuations sensed with hydrophone installed inside model. Pressure fluctuations were monitored using pressure sensors installed inside circular and square moonpool separately. Square moonpool was rotated for different attack angles varying from 0 similar to 45 degrees with an interval of 15 degrees. Data analysis was conducted using 'Empirical Mode Decomposition Method'. The natural frequency of water governs the water oscillations inside the moonpool which remains the same (0.7 Hz) from Froude number 0.26 to 1.3 in calm water with forward model speed condition for circular shape, while the Froude number for square moonpool was from 0.23 to 1.17. 'Piston Phenomenon' is the condition of resonance when free surface water oscillation frequency and shear layer frequency are equating. Piston phenomenon was observed between Fr=0.26 similar to 0.39 for circular shaped; while for square shaped this phenomenon was different for changing attack angles; ranges were; 0 similar to 30 degrees Fr=0.23 similar to 0.47; and 45 degree Fr=0.23 similar to 0.58. The resonance frequency was found to decrease gradually as the cavity leading edge was rotated up to 45 degrees from normal to the flow. Hydrodynamic forces increased at angles to the flow direction. Moreover, hydrodynamic noise is distinguished from structural noise and relationship between St and Fr number is presented. Noise generated is highest (154.14 dB) at beginning of resonance in circular moonpool, while it is lowest between Fr=0.52 similar to 0.63. This paper gives complete understanding for the physics in moonpool related to the underwater noise and hydrodynamic forces. The drag on surface can be altered due to the location of moonpool, and forces exerted by fluid on the cavity walls can be strong enough to cause structural failure, if the location of moonpool is not appropriate and optimized.