Propeller Tip Vortex Cavitation Research Articles

Numerical study on cavitation reduction using PressurePores technology

The requirements in marine propeller design are rising in quantity and quality in recent decades. Besides hydrodynamic performance demand, the targets in reducing propeller cavitation and its side effects need to be considered such as underwater radiated noise and pressure pulses, to ensure comfort requirements and environmental regulations. Using numerical simulation, the present paper investigates the reduction of propeller tip vortex cavitation (TVC) as well as the effects on its performance by employing PressurePores technology. This study uses the Potsdam Propeller Test Case (PPTC) model with commercial code STAR CCM+. The CFD results are compared with the test results in the cavitation tunnel to validate the reliability of numerical simulation. In addition, cavitation mitigation of the propeller is estimated and an obtained average performance degradation is only 2% for different cases, in terms of pore diameters and drilling methods (cylindrical pore and conus pore). This research also provides the changes in pressure distribution, turbulent viscosity, and vorticity around the propeller to clarify the reason for cavitation reduction together with the hydrodynamic efficiency degradation of the propeller.

Numerical study on prediction and discrimination methods of propeller tip vortex cavitation inception

Tip vortex cavitation is typically the first type of cavitation in real ship propellers due to the scale effect. To lessen or eliminate propeller cavitation noise, it is necessary to effectively identify and predict the tip vortex cavitation initiation of the propeller and run the propeller as far as possible in the “non-cavitation area.” However, for the current numerical computation, it is impossible to determine the minimum pressure at the vortex core directly and correctly, making it difficult to anticipate the cavitation initiation of the propeller tip vortex. In this paper, based on computational fluid dynamics (CFD) numerical calculations and the tip vortex model proposed by Xin [Proceedings of 2013 Ship Hydrodynamics Conference (2013), pp. 211–218], we propose a new prediction method for propeller tip vortex cavitation initiation—“the tip vortex model method.” The propeller tip vortex is solved by CFD calculation in this model, and the minimum pressure at the downstream vortex core is examined based on the tip vortex model to determine the initiation of propeller cavitation. We examine the cavitation inception using the tip vortex model method and compare it to the results obtained using the minimum pressure coefficient method and the minimum vapor volume method. It is discovered that the cavitation inception number obtained by the tip vortex model method is closer to the experimental results. Furthermore, the effects of different turbulence models and grid settings on the prediction of tip vortex cavitation initiation are investigated, providing an effective reference for the prediction of propeller tip vortex cavitation initiation.

3D Off-Grid Localization for Adjacent Cavitation Noise Sources Using Bayesian Inference

The propeller tip vortex cavitation (TVC) localization problem involves the separation of noise sources in proximity. This work describes a sparse localization method for off-grid cavitations to estimates their precise locations while keeping reasonable computational efficiency. It adopts two different grid (pairwise off-grid) sets with a moderate grid interval and provides redundant representations for adjacent noise sources. To estimate the position of the off-grid cavitations, a block-sparse Bayesian learning-based method is adopted for the pairwise off-grid scheme (pairwise off-grid BSBL), which iteratively updates the grid points using Bayesian inference. Subsequently, simulation and experimental results demonstrate that the proposed method achieves the separation of adjacent off-grid cavitations with reduced computational cost, while the other scheme suffers from a heavy computational burden; for the separation of adjacent off-grid cavitations, the pairwise off-grid BSBL took significantly less time (29 s) compared with the time taken by the conventional off-grid BSBL (2923 s).

Influence of roughness on propeller performance with a view to mitigating tip vortex cavitation

This study explored the effects of uniformly and non-uniformly distributed biofouling roughness on hydrodynamic performance, particularly on the tip vortex cavitation (TVC) for model and full-scale marine propellers. The effect of roughness was investigated on a benchmark propeller belongs to the research catamaran, ‘The Princess Royal’. The investigation also explored the potential use of the roughness effect to mitigate this propeller's tip vortex cavitation with uniform, inclined and non-uniform flow conditions. In the numerical calculations, DES (Detached Eddy Simulation) approach was used to simulate the cavitating flow around the propeller. The Schneer-Sauer cavitation model was used to model the sheet and tip vortex cavitation. An advanced meshing technique called V-AMR (Vorticity-based Adaptive Mesh Refinement) was proposed to model the TVC in the propeller slipstream. The modified wall-function approach was utilised to implement the roughness effects in the calculations using the experimentally obtained roughness functions based on one of the authors' recent study. The results showed that the velocity components decreased inside the tip vortex due to roughness, resulting in a pressure increase and TVC mitigation. The suction side of the propeller blade tips was found to be an effective roughness application area for the TVC mitigation with a moderate level of loss in the propeller efficiency. The findings indicated that the cavitation volume reduction, mainly due to the TVC mitigation, was by approximately 6–38%, with a 5–10% efficiency loss in the model scale, while these figures were 4–10% and 2–5%, respectively, for the full-scale propeller under the uniform and non-uniform flow conditions.

Block-sparse two-dimensional off-grid beamforming with arbitrary planar array geometry.

For a sound field observed on a planar sensor array, compressive sensing (CS) reconstructs the two-dimensional (2D) direction-of-arrival (DOA) of multiple sources using a sparsity constraint. Conventional compressive beamforming methods suffer from grid mismatch, where true DOAs do not fall on the discretized angular search grid. This paper adopts a CS-based model, which can reconstruct block-sparse signals, and the model treats DOAs and the off-grid DOA compensation parts as blocks to deal with the off-grid 2D beamforming. The method is illustrated by numerical simulations and shows high estimation accuracy. Also, the approach does not require a specific array configuration and is suitable for arbitrary planar array geometry, which is practically useful. Since propeller tip vortex cavitation induces noise sources located sparsely near the propeller tip, the high-resolution of the method is demonstrated with experimental data from cavitation tunnel experiments.

Compressive two-dimensional beamforming for localization of tip vortex cavitation

Noises induced by propeller tip vortex cavitation (TVC) have sparse sources near the propeller tip. Localization of these noise sources with sensor arrays involves the direction-of-arrival (DOA) estimation. Provided the noise sources are sparsely located, compressive sensing (CS) is used for two-dimensional (2-D) DOA estimation, which estimates both azimuth and elevation angles of the sources. With a planar array configuration, the 2-D DOA estimation problem is formulated in the CS framework and CS has shown less ambiguity and higher resolution capabilities compared to traditional DOA estimation schemes even under a single snapshot data. The superior performance of CS is demonstrated on experimental data from cavitation tunnel experiments. Two experimental data are considered. For the first experiment with one known source location, the true DOA has been estimated using CS. The second case involves initially intended TVC noises, and CS has estimated DOAs near the upper downstream area of the propeller, where TVC noises are generally generated.

A Semi-Empirical Prediction Method for Broadband Hull-Pressure Fluctuations and Underwater Radiated Noise by Propeller Tip Vortex Cavitation †

A semi-empirical method is presented that predicts broadband hull-pressure fluctuations and underwater radiated noise due to propeller tip vortex cavitation. The method uses a hump-shaped pattern for the spectrum and predicts the centre frequency and level of this hump. The principal parameter is the vortex cavity size, which is predicted by a combination of a boundary element method and a semi-empirical vortex model. It is shown that such a model is capable of representing the variation of cavity size with cavitation number well. Using a database of model- and full-scale measured hull-pressure data, an empirical formulation for the maximum level and centre frequency has been developed that is a function of, among other parameters, the cavity size. Acceptable results are obtained when comparing predicted and measured hull-pressure and radiated noise spectra for various cases. The comparison also shows differences that require adjustments of parameters that need to be further investigated.

Propeller tip vortex cavitation control and induced noise suppression by water injection

As noise emission from ships garners increasing interest, control of propeller cavitation, which generates vibration and radiates noise, gains critical importance. In the case of full-scale propellers, the tip vortex cavitation typically incepts first, and noise levels rise dramatically thereafter. We propose the mass injection method to reduce noise induced from the tip vortex cavitation and increase the cavity inception speed. Water injected from the propeller tip decreases the rotational speed of the tip flow, and restrains the growth of the tip vortex cavity. For the preliminary test, we investigate the flow around an elliptic wing in a cavitation tunnel, and measured the induced noise levels. The results show a clear decrease in the noise levels in the case of water injection from the wing tip compared to that of without injection. On the basis of a successful preliminary study, we apply this method to the rotating model propeller. Experimental investigations of the model tests in a large cavitation tunnel show that we can control the tip vortex cavitation of the propeller, and hence, increase the cavitation inception speed of the ship using water injection from the propeller tip.

Numerical Prediction of Tip Vortex Cavitation for Marine Propellers in Non-uniform Wake

Tip vortex cavitation is the first type of cavitation to take place around most marine propellers. But the numerical prediction of tip vortex cavitation is one of the challenges for propeller wake because of turbulence dissipation during the numerical simulation. Several parameters of computational mesh and numerical algorithm are tested by mean of the predicted length of tip vortex cavtiation to validate a developed method. The predicted length of tip vortex cavtiation is on the increase about 0.4 propeller diameters using the developed numerical method. The predicted length of tip vortex cavtiation by RNG k – e model is about 3 times of that by SST k – ω model. Therefore, based on the validation of the present approach, the cavitating flows generated by two rotating propellers under a non-uniform inflow are calculated further. The distributions of axial velocity, total pressure and vapor volume fraction in the transversal planes across tip vortex region are shown to be useful in analyzing the feature of the cavitating flow. The strongest kernel of tip vortex cavitation is not at the position most close to blade tip but slightly far away from the region. During the growth of tip vortex cavitation extension, it appears short and thick, and then it becomes long and thin. The pressure fluctuations at the positions inside tip vortex region also validates the conclusion. A key finding of the study is that the grids constructed especially for tip vortex flows by using separated computational domain is capable of decreasing the turbulence dissipation and correctly capturing the feature of propeller tip vortex cavitation under uniform and non-uniform inflows. The turbulence model and advanced grids is important to predict tip vortex cavitation.

Compressive spherical beamforming for localization of incipient tip vortex cavitation.

Noises by incipient propeller tip vortex cavitation (TVC) are generally generated at regions near the propeller tip. Localization of these sparse noises is performed using compressive sensing (CS) with measurement data from cavitation tunnel experiments. Since initial TVC sound radiates in all directions as a monopole source, a sensing matrix for CS is formulated by adopting spherical beamforming. CS localization is examined with known source acoustic measurements, where the CS estimated source position coincides with the known source position. Afterwards, CS is applied to initial cavitation noise cases. The result of cavitation localization was detected near the upper downstream area of the propeller and showed less ambiguity compared to Bartlett spherical beamforming. Standard constraint in CS was modified by exploiting the physical features of cavitation to suppress remaining ambiguity. CS localization of TVC using the modified constraint is shown according to cavitation numbers and compared to high-speed camera images.

Study on Prediction of Underwater Radiated Noise from Propeller Tip Vortex Cavitation

The method to predict underwater radiated noise from tip vortex cavitation was studied. The growth of a single cavitation bubble in tip vortex was estimated by substituting the tip vortex to Rankine combined vortex. The ideal spectrum function for the sound pressure generated by a single cavitation bubble was used, also the empirical factor for the number of collapsed bubbles per unit time was introduced. The estimated noise data were compared with measured ship's ones and it was found out that this method can estimate noise data within 3dB difference.

The singing vortex.

Marine propellers display several forms of cavitation. Of these, propeller-tip vortex cavitation is one of the important factors in propeller design. The dynamic behaviour of the tip vortex is responsible for hull vibration and noise. Thus, cavitation in the vortices trailing from tips of propeller blades has been studied extensively. Under certain circumstances cavitating vortices have been observed to have wave-like disturbances on the surfaces of vapour cores. Intense sound at discrete frequencies can result from a coupling between tip vortex disturbances and oscillating sheet cavitation on the surfaces of the propeller blades. This research article focuses on the dynamics of vortex cavitation and more in particular on the energy and frequency content of the radiated pressures.

On the mechanism of the bursting phenomena of propeller tip vortex cavitation

The bursting phenomenon of tip vortex cavitation of a propeller sometimes causes severe high-frequency vibration, but its mechanism has not yet been elucidated. In this study, we carried out model experiments by changing the propellers, wake distributions, thrust coefficients, and cavitation numbers parametrically, examined the bursting phenomenon with a high-speed video camera, and measured the pressure fluctuations caused by the phenomenon. We also measured flow distribution around the tip vortex. As a result, we found that in the bursting phenomenon, large pressure fluctuations occurred twice, and that they strongly depended on the wake distribution. Two means were suggested to suppress the bursting phenomenon, other than changing the wake distribution: stabilizing tip vortex cavitation or reducing the cavity volume. Numerical fluid simulations around a propeller in noncavitating, unsteady conditions were also conducted, and the strength of the tip vortex along the circumference and its derivative were examined. As a result, the phenomena were parameterized by the time derivative of the strength of the tip vortex, and if it was higher than a threshold value, the tip vortex cavitation burst. Therefore, it is possible to predict the occurrence of the bursting phenomenon by numerical analysis.

Bursting Phenomenon of Propeller Tip Vortex Cavitation and its Relationship with the Result of Numerical Simulation of Propeller in Unsteady Condition

Bursting phenomenon of tip vortex cavitation causes severe high-frequency vibration, but estimation of it by numerical computation is not yet realized. In this study the authors made model experiments to observe the bursting phenomena and measured the pressure fluctuations caused by that, changing two propellers, wake distributions, thrust coefficients and cavitation numbers parametrically. After analyzing frequency of it we found that there would be two methods to suppress the bursting phenomenon : stabilizing the tip vortex cavitation or reducing the volume of cavitation. We also did numerical fluid simulations around a propeller in non-cavitating, unsteady condition and compared the strength of tip vortex along the circumference and its derivatives. As a result the phenomena were parameterized by the time derivative of the strength of tip vortex, and if it was higher than a certain value, tip vortex cavitation bursted. Therefore there is the possibility to predict the occurrence of bursting phenomenon by numerical analysis.

Higher Order Pressure Fluctuation of Propeller for Container Ship

Three (3) propellers were newly designed for the container ship on which comparatively large pressure fluctuation of higher order blade frequencies (2nd5th) was observed. 1) The following concepts were applied in the new propeller design in order to decrease the propeller tip-vortex cavitation, consequently to decrease the pressure fluctuation of higher order blade frequencies, without changing the number of propeller blades.(1) Tip-unload(2) Smaller diameter(3) Increase of blade area(4) Increase of skewModel experiments were performed in the cavitation tunnel by using these propeller models. These propellers were evaluated after investigating the repeatability of the fluctuating pressure and the effects of the number of propeller revolutions (at the test) and the cavitation number on the fluctuating pressure. The fluctuating pressure of one (1) of these propellers was found to have decreases by about 40 % on average.