Incompressible flow assumption in hydroacoustic predictions of marine propellers

Abstract

Incompressible flow assumption is an essential step that simplifies numerical simulations for objects inside flowing water. However, incompressibility assumption creates a conflicting situation for sound propagation as the propagation speed is given by 0 = √ ⁄ where denotes the pressure and denotes the density. When = 0 is assumed to relieve simulations, speed of sound theoretically becomes infinite and therefore, induced pressures should be corrected with the acoustic analogy. This approach is called the "hybrid method" that combines hydrodynamic solver with hydroacoustic solver. Hydroacoustic solver adds compressibility effects to the incompressible hydrodynamic solver and uses hydrodynamic pressure to calculate acoustic pressure. Time step size is an important parameter to calculate acoustic pressure field in the fluid domain and an approach to determine the minimum value (for at least capturing the first blade passage frequency) is presented in this study. Another purpose of this paper is to investigate the effect of incompressibility on hydroacoustics and to analyze the necessity of utilizing the famous Ffowcs Williams-Hawkings (FWH) equation for predicting marine propeller noise; both for cavitating and non-cavitating cases. Our results are in line with other researchers; hydrodynamic pressure is sufficient to assess the hydroacoustic performance of marine propellers in the near-field due to having very low acoustic Mach numbers. Near-field results from the hydrodynamic solver are then extrapolated to the far-field by adopting ITTC distance normalization equation. However; this equation, which is actually the inverse distance law, is only valid for point noise sources in stationary flow. It is found out that eliminating FWH equation by coupling the incompressible hydrodynamic solver with ITTC distance normalization equation fails to produce satisfactory results. For cavitating cases, numerical results in this study show that implementation of FWH is required even in the near-field.

Numerical investigation of marine propeller underwater radiated noise using acoustic analogy Part 2: The influence of eddy viscosity turbulence models

“…ITTC (ITTC, 2014a) formulation is generally applied in the experimental facilities to transfer the measured value from the confinement region to free field conditions. As clearly stated in our previous study (Sezen and Kinaci, 2019), extrapolation of the noise results from the near field to the far-far field by the aid of ITTC distance formulation does not yield satisfactory results for far-field noise predictions.…”

Hydroacoustic and hydrodynamic investigation of bio-inspired leading-edge tubercles on marine-ducted thrusters

“…Open Sci. 8: 210402 conducted in numerous studies within the literature for validation of the FW-H acoustic analogy [4,5,43].…”

“…RBM was selected as it is considered the most accurate methodology to predict unsteady propeller flow [ 43 ], where a time-step of 1 degree of rotation per time-step was selected which is roughly 1.85 × 10 −4 s, a time-step of between 0.5 and 2 degrees is recommended by ITTC [ 44 ]. The second-order scheme was applied to the temporal discretization.…”

“…In order to validate the FW-H acoustic analogy, the near-field direct hydrodynamic and hydroacoustic pressures were compared for the KA4–55 open propeller at the radial receiver, M0. This has been conducted in numerous studies within the literature for validation of the FW-H acoustic analogy [ 4 , 5 , 43 ]. As can be shown in figure 8 , there is good agreement between the direct hydrodynamic and hydroacoustic (FW-H) pressure.…”

Prediction of cavitating propeller underwater radiated noise using RANS & DES-based hybrid method

“…Early investigations in the open literature were concentrated on the non-cavitating propeller URN to understand the mechanism of the URN and sensitivity of the FWH acoustic analogy (e.g. Ianniello et al 2013;LLoyd et al 2015;Testa et al 2018;Sezen and Kinaci 2019). Nowadays, the investigations have been focusing on the cavitating URN studies using advanced numerical techniques and FWH acoustic analogy (e.g.…”