Use of acoustic analogy for marine propeller noise characterisation

Being able to predict shipborne noise is of significant importance to international maritime community. Porous Ffowcs-Williams Hawkings acoustic analogy is used with cavitation model by Sauer & Schnerr in order to predict the noise signature of the Potsdam Propeller operating in open water. The radiation pattern is shown to be predominantly affected by a dipole source, in addition to less prominent sources at the propeller plane and in the wake. It is shown that the predicted sound pressure levels depend on the choice of the control surface and grid density. The unsteady RANS method is shown to be capable of capturing the blade harmonic noise components but lacks the ability to deal with the broadband part of the noise spectrum, both cavitation and turbulence induced, if no additional modelling is used

The influence of turbulence modelling techniques on the predicted cavitation behaviour on a NACA0009 foil

The work presented here forms part of a project focusing on the development of cost-effective measures of classifying the noise levels from ship propellers with the use of numerical techniques available in OpenFOAM software. It is also related to the on-going research within the Faculty of Engineering and the Environment at the University of Southampton, looking at underwater noise of tidal turbines. Ultimately, the aim of the complete study is to enable the assessment of the environmental impact of a ship on the marine ecosystems. In this work a set of results from numerical experiments applied to the NACA0009 foil is presented in the context of quantifying the noise levels produced by a cavitating body in a uniform flow. The simulations utilise both URANS and LES methods and provide a means of characterising the differences between the observed flow patterns from the cavitation modelling point of view. In particular, the interactions of the cavitation phenomena with the turbulence, both modelled and resolved, are studied. Furthermore, an overview of how the considered cavitation models may be used for the purpose of noise prediction is give

Assessment of underwater glider performance through viscous computational fluid dynamics

The process of designing an apt hydrodynamic shape for a new underwater glider is discussed. Intermediate stages include selecting a suitable axi-symmetric hull shape, adding hydrofoils and appendages, and evaluating the performance of the final design. All of the hydrodynamic characteristics are obtained using computational fluid dynamics using the kT - kL - ω transition model. It is shown that drag reduction of the main glider hull is of crucial importance to the ultimate performance. Suggested steps for achieving this are the encouragement of natural laminar flow, integration of sensors into the streamlined hull shape, and sound operational practic

Multi-scale modelling of cavitation-induced pressure around the delft twist 11 hydrofoil

A hybrid Lagrangian-Eulerian cavitation model based on the Schnerr-Sauer mass-transfer formulation is developed and then applied to study the flow around the Delft Twist 11 hydrofoil. The model uses volume-of-fluid approach to resolve large cavities and uses an interface reconstruction algorithm to identify vapour structures smaller than a grid-related threshold. These are then transferred to a Lagrangian framework and convected as particles acting as point noise sources. The underlying volume-of-fluid (VOF) model is shown to be in qualitatively good agreement with the experiment although it is found to under-predict the extent of cavitation. The combined model shows a substantial improvement in the prediction of near-field pressure fluctuations by accounting for the broadband contribution of bubbles smaller than the Eulerian grid size. In the pressure fluctuation spectra this is seen as a plateau extending to over a kilohertz beyond the low-frequency harmonics associated with the shedding frequency

Modelling natural transition on hydrofoils for application in underwater gliders

The paper explores the use of the Local Correlation Transition (LCTM) RANS model capable of predicting transition to turbulence in application to underwater glider performance prediction. Validation of the method is established by analysing the flow past SD7003 and SD8020 hydrofoil sections and comparing with experimental data. Finally, flow past a swept wing suitable for use on an underwater glider is predicted. The results indicate significant improvement in capturing the flow physics in comparison to a standard shear stress transport (SST) k-omega model. Three-dimensional effects are found to play a significant role in the flow behaviour due to the significant sweep of the foil considered

RANS computations of flow around a bulk carrier with energy saving device

The Fluid Structure Interactions group (University of Southampton) has been extensively involved in many research projects focusing on computations of ship wake field and the interactions between the propeller, rudder and the hull. A finite-volume RANS code, OpenFOAM (OpenFOAM, 2014) has been used mostly in majority of these works. The goal of the group has been to improve the in-house capability of prediction of ship stern flows using open-source software. In the present work OpenFOAM is benchmarked against a commercial code, Star-CCM+ (Star-CCM+, 2012), with the aim of exploring the differences in flow field results originating from particular features of both implementations.The Japan Bulk Carrier (JBC) has been chosen as a test case representative of the challenges faced in modern ship flow modelling. This vessel is fitted with an energy saving duct. The JBC case is part of the Tokyo 2015 CFD workshop and the latest in the series of benchmarking workshops to assess the state of art of marine CFD (Larsson et al., 2014). All computations are performed under steady state, fixed (even keel) conditions using identical grids and similar numerical setup. Presented analysis focuses on the mean flow, vortical structures and global hull forces

Development of an America's Cup 45 tacking simulator

This paper describes the development of an AC45 simulator conducted as a student Master’s project at the University of Southampton. The main aim was to be able to asses and improve the tacking skills of the helm and the crew through systematic training. The physical interface of the simulator replicates the seating position of the helmsman and the main trimmer and the graphical representation provides the users with visual cues of the simulated boat, boundaries and marks for a sample race course. The theoretical model uses hydrodynamic manoeuvring coefficients based on empirical formulae and experimental data. The aerodynamic forces are pre-calculated using a full-scale RANS CFD simulation. The accuracy of the model is verified against the AC45 racing tracking data to ensure that the speed loss during a tack, experienced by the users of the simulator, is as close to reality as possible. The ultimate aim of the project was to study the potential of the simulator to assess and train the crews, improving their skill in tacking the boat effectively. This has been done by examining the performance of two groups of users over a series of practice sessions. The simulator could be potentially used for training the helmsmen of the Youth America’s Cup Red-Bull teams, which have limited budgets, training days and sailing experience compared to the professional AC sailor

Feasibility study into a computational approach for marine propeller noise and cavitation modelling

There is increased interest in the ability to predict the noise associated with commercial ship propellers. Key components of the computational analysis process are considered for two test cases and the future direction in resolving the associated challenges is presented. Firstly, the Potsdam Propeller Test Case is used to compute tonal blade passage noise using the Ffowcs Williams–Hawkings acoustic analogy. Cavitation extents predicted using the Sauer and Schnerr mass transfer model agree well with the experiment but show little unsteadiness due to URANS being used. A complementary study of initial results from the study of cavitation noise modelling attempt is presented for a NACA0009 section, used as a simplified representation of a propeller blade. Large Eddy Simulation and FW-H acoustic analogy are used in order to estimate the cavitation-induced noise. Results indicate that the discussed approach provides the means for identifying low-frequency noise generation mechanisms in the flow, but does not allow for the fine-scale bubble dynamics or shockwave formation to be resolved. It is concluded that the discussed approach is a viable option to predict large parts of the marine propeller noise spectra but still further work is needed in order to account for the broadband component

Comparison of various approaches to numerical simulation of ship resistance and propulsion

The operation of a marine propeller dominates the flow interaction effects and alters the resistance on an upstream hull and the forces on a downstream rudder. A study is carried out into how these effects can be resolved by comparing four different methods. A classical prescribed body force approach in which an averaged nominal wake is used as input for the propeller model with prescribed thrust and torque; Two coupled BEMt-RANS solver which accounts for the non-uniform inflow into the propeller and a time resolved discretize propeller approach employing the use of an Arbitrary Mesh Interface model (AMI). The main differences between these four methods are also outlined quantitatively. The accurate results obtained using the two coupled BEMt-RANS approaches makes them fast and robust methods which can be used for ship resistance and self-propulsion estimation in the initial design phas

Outlook on marine propeller noise and cavitation modelling

Two computational studies are presented in this paper. First, the Potsdam Propeller Test Case which is used to demonstrate the capabilities of mass transfer cavitation models, more precisely the model by Sauer and Schnerr, in tackling the problem of marine propeller cavitation. It is shown that the extents of the predicted cavitation regions agree well with the experiment but suffer from the fact that the tip vortices and the associated low pressure regions are under resolved when URANS is utilised. Next, preliminary results from the study of cavitation noise modelling attempt are presented for a NACA 0009 section, used as a simplified representation of a propeller blade. Large Eddy Simulation and Ffowcs Williams-Hawkings porous acoustic analogy are used in order to estimate the cavitation-induced noise. Results indicate that the discussed approach provides the means for identifying low-frequency noise generation mechanisms in the flow, yielding sound pressure levels of the order of 40 dB re 20 mPa, but does not allow for finescale bubble dynamics to be resolved. One may conclude that the discussed approach is a viable option to predict large parts of marine propeller noise spectra but further work is needed in order to account for the high frequency components