Cavitation dynamics and underwater radiated noise signature of a ship with a cavitating propeller

The paper presents SSPA’s work in the EU project AQUO to predict underwater radiated noise (URN) generated by a coastal tanker with a cavitating propeller. A CFD method, consisting of a multi-phase Delayed Detached Eddy Simulation (DDES) and a Ffowcs Williams-Hawkings (FWH) acoustic analogy, is applied to predict the cavitation, pressure pulses and radiated noise for the ship at model and full scale. In comparison with the data obtained from the model test and full scale measurement, it is found that the predicted sheet cavity correlates quite well with the observed ones in the experiment and sea trial. Some success is made in predicting the collapse and rebound of tip vortex cavitation (TVC) at model scale, yet the extension of TVC is under-predicted.The predicted pressure pulses agree reasonably well with the measured ones at the first three harmonics, deviation becomes larger at higher harmonics.The tonal noise has fairly good agreement with the measured signal at both scales up to 5th harmonics. The simulation however under-predicts part of broadband noise that is caused by the TVC, mainly due to an under-resolution of the flow in the tip region and the propeller wake. The agreement with the data for the model scale case is slightly better than that for the full scale case

Noise measurements of a cavitating propeller in different facilities : results of the round robin test programme

Mitigation of shipping noise is a topical issue in marine engineering because of the dramatic increase in the levels of anthropogenic underwater noise and its impact on marine life. In recent years, hydro-acoustic research has focussed on the development of reliable methods for predicting underwater radiated noise (URN) due to cavitation, which is known to be the dominant contribution to the overall radiated noise spectrum of ships above the cavitation inception threshold. Model-scale measurements are currently considered the most reliable approach to study URN problems in marine engineering and are crucial for the verification and validation of numerical methods. However, their reliability is affected by several uncertainty sources for which suitable test procedures and post-processing techniques are needed. As a means to better understand the accuracy and reliability of underwater radiated noise measurements, a round-robin (RR) test programme for an open water propeller setup was organized within the Community-of-Practice “Noise” of the HydroTesting Forum, with the aim of comparing results among several institutes (i.e. University of Genova UNIGE, University of Newcastle UNEW, NMRI, SSPA, KRISO, CNR-INM and MARIN). This paper reports an overview of the RR programme and compares the different approaches and results

Influence of Acoustic Interaction between Cavities That Generate Cavitation Noise

The thesis concerns high frequency cavitation noise generated by marine propellers, for example. The collapse of a cavity is forced by the surrounding pressure which comprises the pressure in the undisturbed flow, the pressure disturbance associated with the local non-cavitating flow and, finally, the pressure disturbance from the presence and motion of neighbouring cavities. This last contribution, dealt with here mainly as an acoustic interaction from the motion of neighbouring cavities, is the main subject of the present work. The primary aim is to make a preliminary study of the role of interaction between a few cavities on the generation of high frequency cavitation noise. Different cavitation processes, statistical distributions of parameters and initial conditions for interacting cavities were observed by high-speed films from experiments. The numerical part of the study concentrates on the central example of acoustic interaction between two spherical cavities. The influence of various parameters on an interaction model and on the interaction itself were studied. Monte Carlo simulations with random input data were made with the model to study the influence of interaction in collapse processes identified from experiments. Scaling laws were analysed and applied to interacting cavities. A main engineering conclusion drawn from the numerical simulations is that, provided the levels at the very highest frequencies are disregarded, the acoustic interaction between medium-sized dominating structures is not very important. This indicates that, if a numerical method can generate an acceptable size and time distribution of cavities and a realistic pressure forcing the collapse, it can also be expected to generate realistic noise levels up to medium-high frequencies. For prediction by model tests, the present study implies that, except for the very highest frequencies, the exact distribution in space and time of nearby cavities is not very critical. The numerical simulations indicate that, if a good estimate of the noise at the very highest frequencies is desired, an accurate simulation of both the acoustic interaction and the statistical properties of cloud cavitation is required

Correlation of acoustic emission to cavitation erosion –model tests with hull model M3405-A22

The tests reported here covers acoustic emission measurements on a typical model scale set-up with a model propeller in behind condition on a hull model in the cavitation tunnel. This report is the final report from three rounds of tests. This project has shown that with an AE-sensor, a correlation between the distribution of AE peak amplitudes and the cavitation volume and intensity can be established. Thus, the method can detect potentially erosive cavitation. However, whether the cavitation implodes on the blade surface and cause erosion or if it collapses slightly off the Surface with no erosion cannot yet be distinguished. The paint test is still the best option to distinguish this.More work needs to be done on analysis of the recorded data to see if we can differentiate erosive cavitation from potentially erosive cavitation. Another question is if we can achieve similar results with sensors not mounted on the rotating shaft; either with high-frequency pressure transducers in our standard pressure transducer positions above the propeller or by accelerometers mounted on the shaft-line supports. This would simplify the set-up significantly.Bestämning av risk för kavitationserosion på fartygspropellra

Influence of Acoustic Interaction between Cavities That Generate Cavitation Noise

The thesis concerns high frequency cavitation noise generated by marine propellers, for example. The collapse of a cavity is forced by the surrounding pressure which comprises the pressure in the undisturbed flow, the pressure disturbance associated with the local non-cavitating flow and, finally, the pressure disturbance from the presence and motion of neighbouring cavities. This last contribution, dealt with here mainly as an acoustic interaction from the motion of neighbouring cavities, is the main subject of the present work. The primary aim is to make a preliminary study of the role of interaction between a few cavities on the generation of high frequency cavitation noise. Different cavitation processes, statistical distributions of parameters and initial conditions for interacting cavities were observed by high-speed films from experiments. The numerical part of the study concentrates on the central example of acoustic interaction between two spherical cavities. The influence of various parameters on an interaction model and on the interaction itself were studied. Monte Carlo simulations with random input data were made with the model to study the influence of interaction in collapse processes identified from experiments. Scaling laws were analysed and applied to interacting cavities. A main engineering conclusion drawn from the numerical simulations is that, provided the levels at the very highest frequencies are disregarded, the acoustic interaction between medium-sized dominating structures is not very important. This indicates that, if a numerical method can generate an acceptable size and time distribution of cavities and a realistic pressure forcing the collapse, it can also be expected to generate realistic noise levels up to medium-high frequencies. For prediction by model tests, the present study implies that, except for the very highest frequencies, the exact distribution in space and time of nearby cavities is not very critical. The numerical simulations indicate that, if a good estimate of the noise at the very highest frequencies is desired, an accurate simulation of both the acoustic interaction and the statistical properties of cloud cavitation is required

Correlation of acoustic emission to cavitation erosion – second round of model test

The tests reported here covers acoustic emission (AE) measurements on a typical model scale set-up with a model propeller in behind condition on a hull model in the cavitation tunnel. The existing ship model M3336-A2 equipped with stator S28 and propeller model P2754 was used for the AE measurement tests. The AE measurements were performed simultaneously with erosion paint tests and high-speed video recordings. The propeller model P2754 was used in the original cavitation testsfor DSME in year 2012 and showed at that time weak indication at the blade tips after an extended 60 minutes cavitation erosion test.In the first round of tests within the current project, SSPA Report RE40178293-01-00-A, P2754 showed weak indication of erosion at a few bladed tips in the erosion tests at the tested loading conditions. Thus, the new tests confirm the 2012 results.The variation of advance ratio and cavitation number was not enough to generate much erosion in the paint tests. For the tests within this report, the PSS configuration was varied to create more intensive cavitation. Thus, we were able to produce a graduation from no erosion to severe erosion in the paint.The cases with strong erosion give a higher number of high peak-amplitudes. The atmospheric conditions with very little cavitation and no indication of erosion give lower peak-amplitudes and fewer number of peaks. The measurements show a clear correlation between the distribution of peak amplitudes and the amount of paint erosion.Bestämning av risk för kavitationserosion på fartygspropellra

Correlation of acoustic emission to cavitation erosion – initial model tests

The tests reported here covers acoustic emission measurements on a typical model scale set-up with a model propeller in behind condition on a hull model in the cavitation tunnel. An AE transducer was mounted in rear end of the propeller shaft on an existing Aframax model. Two propellers were tested at several operating conditions. P2754 showed weak indication of erosion at a few bladed tips in the erosion tests. P5091 did not show any sign of erosion.The number of AE peaks at each amplitude are approximately proportional to the amount of cavitation. Even atmospheric condition, there are some, but very few, peaks up to 3.9 V amplitude. Whether these are due to sporadic cavitation incidents or some disturbance in the measurement is not clear at this moment. At the cavitating conditions, peaks up to 4.0 V amplitude are detected. All these conditions are judged to be barely erosive with just a few pit marks on a couple of blades. The question is if there would be even higher amplitudes in a highly erosive case? It is obvious that thetests must include conditions that are significantly more erosive.LR and DSME have suggested a threshold amplitude of 3.6 V for erosion based on systematic Sonotrode tests with erosion paint samples. It seems such amplitude threshold must be combined with a minimum number of incidents to be classified as erosive, otherwise non-cavitating conditions would also be classified as erosive.Bestämning av risk för kavitationserosion på fartygspropellra

Evaluation of methods to measure acoustic transfer functions in cavitation tunnels

The interest for cavitation noise studies and prediction is increasing in the field of naval architecture, mainly because of attention to environmental issues. Model scale tests in cavitation tunnels are commonly considered as one of the most effective tools for cavitation noise studies. However, despite being carried out since long time, model scale experiments still present many challenges. These are mainly related to scale effects on propeller hydrodynamics, cavitation behaviour and scale effects on noise generation and propagation. Besides these phenomena the effect of the confined environment in which tests are carried out may be of great importance. In present work, an acoustic characterization of the SSPA large cavitation tunnel section #3 is presented, with the aim to obtain suitable transfer functions in order to take into account (at least partially) this phenomenon. The acoustic characterisation is performed considering two different underwater transducers and different signals and post processing techniques. The obtained transfer functions are shown and discussed in order to analyse advantages and shortcomings of the different procedures and to generally identify main problems related to this kind of activity

Design of low drag-to-power ratio hydrokinetic turbine

The abundant power in tidal current is a reliable source of renewable energy for electricity production in coastal areas. With Deep Green technology, Minesto has presented a cost-effective unique approach to extract energy even from low speed water currents and increase the potential of ocean energy extraction. In PowerKite project, funded by EU Horizon 2020, we designed and tested a special type of horizontal axis hydrokinetic turbine for Minesto’s tidal kite. The operational condition for this turbine is very different from stationary hydrokinetic turbines due to correlation between the kite’s maneuvering speed, turbine drag and power production. In this paper, we present the procedure of turbine blade design for reducing drag forces while producing maximum torque. We used OpenProp, a lifting line method open-source code, combined with CAESES modeler to establish the design space. To select the suitable design parameters with regard to cavitation and viscous effect, we also used RANS CFD solver OpenFOAM and improved the turbine performance. Computational studies as well as experimental test in cavitation tunnel are presented and the result is compared with a baseline turbine performance. The new design is currently being evaluated in sea-trial with a quarter-scale kite