Avital E., Yu G., Williams J.

Sound generation by a vertically mounted cylinder that pierces a free surface is studied computationally using the Large Eddy Simulation and acoustic analogy approaches. This case study is taken as a representative model for sound generated by an off-shore structure or a submarine mast. Therefore the free surface is taken as an interface between two fluids of which the upper one is air and the lower one is water. Both fluid flows are assumed to be incompressible and are simulated using the Monotonically Integrated Large Eddy Simulation (MILES) technique where the interface surface is simulated using the VOF technique. The resulted pressure distribution along the cylinder is used to calculate the dipole-generated sound propagation inside the water phase. This type of sound generation is taken as dominant over the quadrupole type due to the very low Mach number of the flow. The Lighthill-Curle acoustic analogy is reformulated to account for a zero pressure surface on the upper side of the water phase and a reflecting surface on the lower side, modelling the effects of the interface and bottom surfaces respectively. Simulations are carried out for a sub-critical Reynolds number ReD of about 30.103, Froude numbers up to two and water height between 2D to 4D, where D is the cylinder diameter. A symmetric wake is found to dominate the interface surface and signals the start of the Kelvin surface wake. Von Karman rollers are found to dominate the wake near the bottom. As a result, a variation in the pressure distribution along the cylinder is recorded. Consequently, a dependence on the water height is found in the sound field pattern resulting in a variation in the near field penetration length, directivity pattern and frequency spectra. These effects are further analysed by a comparison with the 3D compact version of Lighthill-Curle’s formulation.