Using the FW-H equation for hydroacoustics of propellers

AuthorsLloyd, T.P., Lidtke, A.K., Rijpkema, D.R., Wijngaarden, H.C.J. van, Turnock, S.R. , Humphrey, V.F.
Conference/Journal18th Numerical Towing Tank Symposium (NuTTS), Cortona, Italy
DateSep 28, 2015
Marine propeller noise and vibration has been studied in order to reduce acoustic signature, improve fatigue life and comfort onboard, and, more recently, to quantify the impact of maritime noise on the environment. Therefore the importance of acoustic measurements and simulations has increased. Acoustic simulations offer the advantages of avoiding vibration effects on measurement data, as well as the free placement of probes. These are not without difficulty however, and require robust verification, especially in the maritime field, where hydroacoustic computations are relatively uncommon.
An attractive approach for making hydroacoustic predictions is an acoustic analogy, such as that of Ffowcs Williams and Hawkings (1969). The processes of sound generation and propagation are then conveniently separated, and acoustic predictions may be made as a post-processing of a hydrodynamic simulation. Since the precise location of the acoustic sources, such as cavitation and non-linear wake flow, are not known a priori in maritime applications (unlike in aeronautical applications, where, at low Mach number, they are assumed coincident with the blade surfaces), the porous formulation of the Ffowcs Williams-Hawkings (FW-H) equation is typically preferred (Di Francescantonio, 1997). In this case, the propeller is surrounded by a porous data surface (PDS) upon which velocity and pressure fluctuations representing the acoustic sources are extracted from a computational fluid dynamics (CFD) simulation.
Uncertainty remains, however, regarding how best to use this method in practice. This is motivated by research showing that the propeller tip vortex may be a non-negligible noise source for non-cavitating conditions (Ianniello et al., 2013; Felli et al., 2015). Ianniello et al. (2013) and Lloyd et al. (2015) used an open-ended PDS to avoid the wake penetrating the downstream end cap, but did not investigate the effect of this. While similar studies have been performed in the aeroacoustics community for jet flows e.g. see Rahier et al. (2004), general experience of using such methods for marine propellers is not widespread (Ianniello and De Bernardis, 2015; Lloyd et al., 2014; Lidtke et al., 2015).
In this paper we detail a study into appropriate use of the FW-H acoustic analogy for making hydroacoustic predictions of marine propellers. Two CFD codes (ReFRESCO and OpenFOAMR ) are used in order to provide additional verification. We focus on issues from the literature resulting from the simulation of a realistic propeller flow (e.g. Ianniello and De Bernardis, 2015), addressing two main questions, namely: how does closure of the PDS in the propeller wake affect the acoustic pressure prediction?; and what is the effect of the axial extent of the PDS in the downstream direction?
sustainable propulsioncfd developmentcfd/simulation/desk studiestime-domain simulationsnoise and vibrationresistance and propulsionmarine systemsresearch and developmentsimulationpropellerpropulsorresearch