Prediction of Noise and Acoustical Spectrum of Counter-Rotating Propellers
Authors: Güngör E. , Özdemir I. B. 
Published: September 2018

The acoustical performance of a counter-rotating marine propeller system is computed using the Ffowcs, Williams and Hawkings (FWH) algorithm. The solution procedures, where finite volume computational fluid dynamics calculations were exploited, was validated with a conventional propeller having experimental results (namely David Taylor Model Basin [DTMB] 4119). The large eddy simulation formulation and the sliding mesh technique are employed to build thrust and torque curves. Mesh dependency, turbulence model, and discretization schemes were all assessed to have repeatable and accurate results. Because the application under consideration involved inhomogeneous flow and blade interactions, DTMB 3684 and 3685 counterrotating propeller (CRP) models were used for comparisons. In terms of validation of numerical acoustics, the acoustical model evaluation study was first performed on a flow over a cylinder and a rain gutter model. The FWH approach produced good results for the spectrum at near-field and far-field locations. The performance of an inclined propeller, namely VP1304, in a noncavitating condition was investigated and the results were compared with the experiments conducted at the Potsdam Model Basin. Pressure pulses of this propeller in the cavitation tunnel were also attained and compared with experimental results. The amplitudes of the pressure fluctuations were in good agreement with the experimental data except for the weak third harmonics. Last, the generic form of the CRP system attached to an underwater vehicle body was considered to investigate the acoustic spectrum in terms of blade passing frequencies, interacted harmonics and radiated noise.

1. Introduction
Propellers are used to drive marine systems including underwater vehicles. To boost their performance and fuel efficiency, they were designed mainly by experimentations. Recent studies (Denny 1968; Liu & Hong 2011), however, show that useful information about the characteristics of different conventional propellers can be extracted with the computational fluid dynamics (CFD) methodology which generally produces results supporting the experimental findings. As, for example, Subhas et al. (2012) succeeded to reproduce the blade characteristics of the INSEAN E779a propeller-designed and manufactured by Marine Technology Research Institute in the field of naval architecture and marine engineering in Italy-using the CFD. Similar finite volume calculations (Güngör & Özdemir 2015b) of the performance of propeller P4382 agreed well with the available experimental data. Embedding a model (Xia et al. 2012) for cavitation occurring on blade surfaces has been shown to improve the numerical performance in comparison with the calculations that neglect the cavitation effects. In the present analyses, we take into account the single-phase nature of the flow, and the cavitation effects are neglected with the emphasis placed in the noncavitating regions where reliable test results are available. Thus, to produce meaningful results, propeller hydrodynamics needs be considered especially under inhomogeneous flow conditions. Experiments (Potsdam Experimental Team 2015) on propellers subjected to the inhomogeneous flow reveal a variable wake in front of the propeller, which can be conveniently modeled numerically although unsteady interactions make the convergence difficult. On the other hand, the counter-rotating propeller (CRP) tests might be necessary to understand the interactions between the blades and hubs. To observe the axial distance effect, the CRP tests were handled in a cavitation tunnel (Hecker & McDonald 1960) where thrust and torque comparison assists in validations of numerical computing for each propeller's performance.